Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2019	Oct. 31, 2019
Entity Information [Line Items]
Entity Central Index Key	0001002910
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Amendment Flag	false
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2019
Document Transition Report	false
Entity Registrant Name	Ameren Corporation
Entity File Number	1-14756
Entity Tax Identification Number	43-1723446
Entity Incorporation, State or Country Code	MO
Entity Address, Address Line One	1901 Chouteau Avenue
Entity Address, City or Town	St. Louis
Entity Address, State or Province	MO
Entity Address, Postal Zip Code	63103
City Area Code	(314)
Local Phone Number	621-3222
Title of 12(b) Security	Common Stock, $0.01 par value per share
Trading Symbol	AEE
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		246,029,792
Union Electric Company
Entity Information [Line Items]
Entity Central Index Key	0000100826
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Amendment Flag	false
Entity Registrant Name	Union Electric Company
Entity File Number	1-2967
Entity Tax Identification Number	43-0559760
Entity Incorporation, State or Country Code	MO
Entity Address, Address Line One	1901 Chouteau Avenue
Entity Address, City or Town	St. Louis
Entity Address, State or Province	MO
Entity Address, Postal Zip Code	63103
City Area Code	(314)
Local Phone Number	621-3222
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		102,123,834
Ameren Illinois Company
Entity Information [Line Items]
Entity Central Index Key	0000018654
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Amendment Flag	false
Entity Registrant Name	Ameren Illinois Company
Entity File Number	1-3672
Entity Tax Identification Number	37-0211380
Entity Incorporation, State or Country Code	IL
Entity Address, Address Line One	10 Executive Drive
Entity Address, City or Town	Collinsville
Entity Address, State or Province	IL
Entity Address, Postal Zip Code	62234
City Area Code	(618)
Local Phone Number	343-8150
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		25,452,373

Consolidated Statement of Income (Loss) and Comprehensive Income (Loss) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,659	$ 1,724	$ 4,594	$ 4,872
Operating Expenses:
Fuel	147	216	409	590
Purchased Power	148	148	440	453
Natural gas purchased for resale	31	30	236	252
Other operations and maintenance	434	429	1,301	1,299
Depreciation and amortization	248	241	745	713
Taxes other than income taxes	131	127	375	374
Total operating expenses	1,139	1,191	3,506	3,681
Operating Income	520	533	1,088	1,191
Other Income (Expense), Net	34	32	99	84
Interest Charges	96	101	290	302
Income Before Income Taxes	458	464	897	973
Income Taxes	92	105	158	221
Net income	366	359	739	752
Less: Net Income Attributable to Noncontrolling Interests	2	2	5	5
Net income attributable to Ameren common shareholders	364	357	734	747
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	0	2	1	1
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	366	361	740	753
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	2	2	5	5
Comprehensive Income	$ 364	$ 359	$ 735	$ 748
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic	$ 1.48	$ 1.46	$ 2.99	$ 3.06
Earnings Per Share, Diluted	$ 1.47	$ 1.45	$ 2.97	$ 3.04
Average Common Shares Outstanding - Basic (in shares)	245.9	244.1	245.5	243.6
Weighted Average Number of Shares Outstanding, Diluted (in shares)	247.5	246.3	247.0	245.5
Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,528	$ 1,590	$ 3,928	$ 4,209
Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	131	134	666	663
Union Electric Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	1,059	1,129	2,615	2,876
Operating Expenses:
Fuel	147	216	409	590
Purchased Power	49	49	160	131
Natural gas purchased for resale	6	5	41	37
Other operations and maintenance	242	234	720	707
Depreciation and amortization	138	137	417	411
Taxes other than income taxes	96	94	256	258
Total operating expenses	678	735	2,003	2,134
Operating Income	381	394	612	742
Other Income (Expense), Net	15	16	43	45
Interest Charges	44	50	136	152
Income Before Income Taxes	352	360	519	635
Income Taxes	51	65	70	132
Net income	301	295	449	503
Preferred Stock Dividends	1	1	3	3
Net Income (Loss) Attributable to Parent	300	294	446	500
Union Electric Company \| Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	1,040	1,111	2,517	2,782
Union Electric Company \| Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	19	18	98	94
Ameren Illinois Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	564	564	1,873	1,902
Operating Expenses:
Purchased Power	101	105	284	334
Natural gas purchased for resale	25	25	195	215
Other operations and maintenance	193	195	580	590
Depreciation and amortization	102	94	304	278
Taxes other than income taxes	33	32	110	108
Total operating expenses	454	451	1,473	1,525
Operating Income	110	113	400	377
Other Income (Expense), Net	13	11	39	30
Interest Charges	38	38	111	112
Income Before Income Taxes	85	86	328	295
Income Taxes	20	23	79	73
Net income	65	63	249	222
Preferred Stock Dividends	0	0	2	2
Net Income (Loss) Attributable to Parent	65	63	247	220
Ameren Illinois Company \| Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	452	448	1,305	1,333
Ameren Illinois Company \| Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 112	$ 116	$ 568	$ 569

Consolidated Statement of Income (Loss) and Comprehensive Income (Loss) (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Pension and other postretirement benefit plan activity, tax expense (benefit)	$ 0	$ 0	$ 0	$ 0

Consolidated Balance Sheet - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Current Assets:
Cash and cash equivalents	$ 20	$ 16
Accounts receivable - trade (less allowance for doubtful accounts)	478	463
Unbilled revenue	273	295
Miscellaneous accounts receivable	56	79
Inventories	488	483
Current regulatory assets	74	134
Other current assets	106	63
Total current assets	1,495	1,533
Property, Plant, and Equipment, Net	23,894	22,810
Investments and Other Assets:
Nuclear decommissioning trust fund	798	684
Goodwill	411	411
Regulatory Assets, Noncurrent	1,168	1,127
Other assets	780	650
Total investments and other assets	3,157	2,872
TOTAL ASSETS	28,546	27,215
Current Liabilities:
Current maturities of long-term debt	336	580
Short-term Debt	544	597
Accounts and wages payable	598	817
Taxes accrued	164	53
Current regulatory liabilities	121	149
Other current liabilities	522	491
Total current liabilities	2,285	2,687
Long-term Debt, Net	8,651	7,859
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	2,902	2,666
Regulatory Liability, Noncurrent	4,845	4,637
Asset retirement obligations	671	627
Pension and other postretirement benefits	522	558
Other deferred credits and liabilities	466	408
Total deferred credits and other liabilities	9,406	8,896
Commitments and Contingencies
Stockholders' Equity:
Common Stock	2	2
Other paid-in capital	5,673	5,627
Retained earnings	2,408	2,024
Accumulated other comprehensive income (loss)	(21)	(22)
Shareholder's equity	8,062	7,631
Noncontrolling Interest	142	142
Total equity	8,204	7,773
TOTAL LIABILITIES AND EQUITY	28,546	27,215
Union Electric Company
Current Assets:
Cash and cash equivalents	0	0
Accounts receivable - trade (less allowance for doubtful accounts)	243	223
Accounts receivable - affiliates	20	14
Unbilled revenue	156	155
Miscellaneous accounts receivable	41	42
Inventories	356	358
Other current assets	66	40
Total current assets	882	832
Property, Plant, and Equipment, Net	12,452	12,103
Investments and Other Assets:
Nuclear decommissioning trust fund	798	684
Regulatory Assets, Noncurrent	358	366
Other assets	370	306
Total investments and other assets	1,526	1,356
TOTAL ASSETS	14,860	14,291
Current Liabilities:
Current maturities of long-term debt	336	580
Short-term Debt	144	55
Accounts and wages payable	261	428
Accounts payable - affiliates	126	69
Taxes accrued	148	27
Other current liabilities	240	243
Total current liabilities	1,255	1,402
Long-term Debt, Net	3,779	3,418
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,619	1,576
Regulatory Liability, Noncurrent	2,860	2,799
Asset retirement obligations	667	623
Pension and other postretirement benefits	211	228
Other deferred credits and liabilities	44	16
Total deferred credits and other liabilities	5,401	5,242
Commitments and Contingencies
Stockholders' Equity:
Common Stock	511	511
Other paid-in capital	1,903	1,903
Preferred stock	80	80
Retained earnings	1,931	1,735
Shareholder's equity	4,425	4,229
TOTAL LIABILITIES AND EQUITY	14,860	14,291
Ameren Illinois Company
Current Assets:
Cash and cash equivalents	0	0
Accounts receivable - trade (less allowance for doubtful accounts)	222	224
Accounts receivable - affiliates	38	21
Unbilled revenue	117	140
Miscellaneous accounts receivable	16	40
Inventories	132	125
Current regulatory assets	58	110
Other current assets	26	16
Total current assets	609	676
Property, Plant, and Equipment, Net	9,819	9,198
Investments and Other Assets:
Goodwill	411	411
Regulatory Assets, Noncurrent	796	759
Other assets	311	275
Total investments and other assets	1,518	1,445
TOTAL ASSETS	11,946	11,319
Current Liabilities:
Short-term Debt	310	72
Accounts and wages payable	247	302
Accounts payable - affiliates	67	58
Customer deposits	71	76
Current environmental remediation	56	42
Current regulatory liabilities	49	62
Other current liabilities	176	184
Total current liabilities	976	796
Long-term Debt, Net	3,279	3,296
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,180	1,119
Regulatory Liability, Noncurrent	1,884	1,741
Pension and other postretirement benefits	261	280
Environmental remediation	85	109
Other deferred credits and liabilities	260	204
Total deferred credits and other liabilities	3,670	3,453
Commitments and Contingencies
Stockholders' Equity:
Common Stock	0	0
Other paid-in capital	2,173	2,173
Preferred stock	62	62
Retained earnings	1,786	1,539
Shareholder's equity	4,021	3,774
TOTAL LIABILITIES AND EQUITY	$ 11,946	$ 11,319

Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Accounts receivable - trade allowance for doubtful accounts	$ 19	$ 18
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	400,000,000	400,000,000
Common Stock, Shares, Outstanding	246,000,000	244,500,000
Union Electric Company
Accounts receivable - trade allowance for doubtful accounts	$ 8	$ 7
Common stock, par value (in dollars per share)	$ 5	$ 5
Common stock, shares authorized (in shares)	150,000,000	150,000,000
Common Stock, Shares, Outstanding	102,100,000	102,100,000
Ameren Illinois Company
Accounts receivable - trade allowance for doubtful accounts	$ 11	$ 11
Common stock, no par value (in dollars per share)	$ 0	$ 0
Common stock, shares authorized (in shares)	45,000,000	45,000,000
Common Stock, Shares, Outstanding	25,500,000	25,500,000

Consolidated Statement of Cash Flows - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash Flows From Operating Activities:
Net income	$ 739	$ 752
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	745	699
Amortization of nuclear fuel	56	71
Amortization of debt issuance costs and premium/discounts	14	16
Deferred income taxes and investment tax credits, net	144	212
Allowance for equity funds used during construction	(20)	(25)
Stock-based compensation costs	15	15
Other	(11)	21
Changes in assets and liabilities:
Receivables	10	(129)
Inventories	(4)	(4)
Accounts and wages payable	(205)	(198)
Taxes accrued	118	92
Regulatory assets and liabilities	147	213
Assets, other	(56)	(2)
Liabilities, other	11	(45)
Pension and other postretirement benefits	(35)	(2)
Net cash provided by operating activities	1,668	1,686
Cash Flows From Investing Activities:
Capital expenditures	(1,761)	(1,689)
Nuclear fuel expenditures	(26)	(30)
Purchases of securities – nuclear decommissioning trust fund	(192)	(172)
Sales and maturities of securities – nuclear decommissioning trust fund	184	159
Purchase of bonds	(207)	0
Proceeds from sale of remarketed bonds	207	0
Other	(3)	13
Net cash used in investing activities	(1,798)	(1,719)
Cash Flows From Financing Activities:
Dividends on common stock	(350)	(334)
Dividends paid to noncontrolling interest holders	(5)	(5)
Short-term debt, net	(53)	36
Maturities of long-term debt	(329)	(522)
Issuances of long-term debt	900	853
Issuances of common stock	54	56
Employee payroll taxes related to stock-based compensation	(29)	(19)
Debt issuance costs	(10)	(9)
Other	0	1
Net cash provided by financing activities	178	57
Net change in cash, cash equivalents, and restricted cash	48	24
Cash, cash equivalents, and restricted cash at beginning of year	107	68
Cash, cash equivalents, and restricted cash at end of period	155	92
Union Electric Company
Cash Flows From Operating Activities:
Net income	449	503
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	419	398
Amortization of nuclear fuel	56	71
Amortization of debt issuance costs and premium/discounts	4	4
Deferred income taxes and investment tax credits, net	(9)	4
Allowance for equity funds used during construction	(14)	(19)
Other	10	14
Changes in assets and liabilities:
Receivables	(32)	(156)
Inventories	3	3
Accounts and wages payable	(153)	(168)
Taxes accrued	148	148
Regulatory assets and liabilities	5	149
Assets, other	(37)	0
Liabilities, other	(4)	7
Pension and other postretirement benefits	(5)	3
Net cash provided by operating activities	840	961
Cash Flows From Investing Activities:
Capital expenditures	(751)	(664)
Nuclear fuel expenditures	(26)	(30)
Purchases of securities – nuclear decommissioning trust fund	(192)	(172)
Sales and maturities of securities – nuclear decommissioning trust fund	184	159
Purchase of bonds	(207)	0
Proceeds from sale of remarketed bonds	207	0
Money pool advances, net	0	(28)
Net cash used in investing activities	(785)	(735)
Cash Flows From Financing Activities:
Dividends on common stock	(250)	(225)
Dividends on preferred stock	(3)	(3)
Short-term debt, net	89	(39)
Maturities of long-term debt	(329)	(378)
Issuances of long-term debt	450	423
Debt issuance costs	(6)	(4)
Net cash provided by financing activities	(49)	(226)
Net change in cash, cash equivalents, and restricted cash	6	0
Cash, cash equivalents, and restricted cash at beginning of year	8	7
Cash, cash equivalents, and restricted cash at end of period	14	7
Ameren Illinois Company
Cash Flows From Operating Activities:
Net income	249	222
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	303	278
Amortization of debt issuance costs and premium/discounts	9	10
Deferred income taxes and investment tax credits, net	42	56
Allowance for equity funds used during construction	(6)	(6)
Other	8	5
Changes in assets and liabilities:
Receivables	18	21
Inventories	(7)	(7)
Accounts and wages payable	(48)	(44)
Taxes accrued	14	(40)
Regulatory assets and liabilities	147	63
Assets, other	(15)	0
Liabilities, other	13	(40)
Pension and other postretirement benefits	(27)	(8)
Net cash provided by operating activities	706	516
Cash Flows From Investing Activities:
Capital expenditures	(900)	(947)
Other	(3)	10
Net cash used in investing activities	(903)	(937)
Cash Flows From Financing Activities:
Dividends on preferred stock	(2)	(2)
Short-term debt, net	237	46
Money pool borrowings, net	0	45
Maturities of long-term debt	0	(144)
Issuances of long-term debt	0	430
Debt issuance costs	0	(5)
Capital contribution from parent	0	80
Other	(1)	1
Net cash provided by financing activities	234	451
Net change in cash, cash equivalents, and restricted cash	37	30
Cash, cash equivalents, and restricted cash at beginning of year	80	41
Cash, cash equivalents, and restricted cash at end of period	$ 117	$ 71

Consolidated Statement of Stockholders' Equity - USD ($) $ in Millions	Total	Common Stock	Other Paid-in Capital	Retained Earnings	Deferred Retirement Benefit Costs	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interests	Total Ameren Corporation Shareholders' Equity	Union Electric Company	Union Electric Company Common Stock	Union Electric Company Other Paid-in Capital	Union Electric Company Preferred Stock	Union Electric Company Retained Earnings	Ameren Illinois Company	Ameren Illinois Company Common Stock	Ameren Illinois Company Other Paid-in Capital	Ameren Illinois Company Preferred Stock	Ameren Illinois Company Retained Earnings
Beginning of period at Dec. 31, 2017			$ 5,540	$ 1,660	$ (18)		$ 142						$ 1,632			$ 2,013		$ 1,235
Shares issued under the DRPlus and 401(k) plan			56
Stock-based compensation activity			2
Capital contribution from parent														$ (80)		80
Net income	$ 752								$ 503				503	222				222
Net income attributable to Ameren common shareholders	747			747
Common stock dividends				(334)									(225)
Preferred stock dividends													(3)					(2)
Change in deferred retirement benefit costs	1				1
Net income attributable to noncontrolling interest holders	$ (5)						5
Dividends paid to noncontrolling interest holders							(5)
Common stock shares outstanding at beginning of period at Dec. 31, 2017	242,600,000
Stock Issued During Period, Shares, New Issues	900,000
Stock Issued During Period, Shares, Other	700,000
Common stock shares outstanding at end of period at Sep. 30, 2018	244,200,000
End of period at Sep. 30, 2018	$ 7,798	$ 2	5,598	2,073	(17)	$ (17)	142			$ 511	$ 1,858	$ 80	1,907		$ 0	2,093	$ 62	1,455
Dividends per Common Share (in dollars per share)	$ 1.3725
Shareholders' equity, end of year at Sep. 30, 2018								$ 7,656	4,356					3,610
Beginning of period at Jun. 30, 2018			5,576	1,827	(19)		142						1,788			2,093		1,392
Shares issued under the DRPlus and 401(k) plan			16
Stock-based compensation activity			6
Capital contribution from parent																0
Net income	$ 359								295				295	63				63
Net income attributable to Ameren common shareholders	357			357
Common stock dividends				(111)									(175)
Preferred stock dividends													(1)					0
Change in deferred retirement benefit costs	2				2
Net income attributable to noncontrolling interest holders	$ (2)						2
Dividends paid to noncontrolling interest holders							(2)
Common stock shares outstanding at beginning of period at Jun. 30, 2018	244,000,000.0
Stock Issued During Period, Shares, New Issues	200,000
Stock Issued During Period, Shares, Other	0
Common stock shares outstanding at end of period at Sep. 30, 2018	244,200,000
End of period at Sep. 30, 2018	$ 7,798	2	5,598	2,073	(17)	(17)	142			511	1,858	80	1,907		0	2,093	62	1,455
Dividends per Common Share (in dollars per share)	$ 0.4575
Shareholders' equity, end of year at Sep. 30, 2018								7,656	4,356					3,610
Beginning of period at Dec. 31, 2018	$ 7,773		5,627	2,024	(22)		142						1,735			2,173		1,539
Shares issued under the DRPlus and 401(k) plan			54
Stock-based compensation activity			(8)
Capital contribution from parent														0		0
Net income	739								$ 449				449	$ 249				249
Net income attributable to Ameren common shareholders	734			734
Common stock dividends				(350)									(250)
Preferred stock dividends													(3)					(2)
Change in deferred retirement benefit costs	1				1
Net income attributable to noncontrolling interest holders	$ (5)						5
Dividends paid to noncontrolling interest holders							(5)
Common stock shares outstanding at beginning of period at Dec. 31, 2018	244,500,000								102,100,000					25,500,000
Stock Issued During Period, Shares, New Issues	700,000
Stock Issued During Period, Shares, Other	800,000
Common stock shares outstanding at end of period at Sep. 30, 2019	246,000,000								102,100,000					25,500,000
End of period at Sep. 30, 2019	$ 8,204	2	5,673	2,408	(21)	(21)	142			511	1,903	80	1,931		0	2,173	62	1,786
Dividends per Common Share (in dollars per share)	$ 1.4250
Shareholders' equity, end of year at Sep. 30, 2019	$ 8,062							8,062	$ 4,425					$ 4,021
Beginning of period at Jun. 30, 2019			5,649	2,161	(21)		142						1,781			2,173		1,721
Shares issued under the DRPlus and 401(k) plan			17
Stock-based compensation activity			7
Capital contribution from parent																0
Net income	366								$ 301				301	$ 65				65
Net income attributable to Ameren common shareholders	364			364
Common stock dividends				(117)									(150)
Preferred stock dividends													(1)					0
Change in deferred retirement benefit costs	0				0
Net income attributable to noncontrolling interest holders	$ (2)						2
Dividends paid to noncontrolling interest holders							(2)
Common stock shares outstanding at beginning of period at Jun. 30, 2019	245,800,000
Stock Issued During Period, Shares, New Issues	200,000
Stock Issued During Period, Shares, Other	0
Common stock shares outstanding at end of period at Sep. 30, 2019	246,000,000								102,100,000					25,500,000
End of period at Sep. 30, 2019	$ 8,204	$ 2	$ 5,673	$ 2,408	$ (21)	$ (21)	$ 142			$ 511	$ 1,903	$ 80	$ 1,931		$ 0	$ 2,173	$ 62	$ 1,786
Dividends per Common Share (in dollars per share)	$ 0.4750
Shareholders' equity, end of year at Sep. 30, 2019	$ 8,062							$ 8,062	$ 4,425					$ 4,021

Summary Of Significant Accounting Policies

9 Months Ended

Sep. 30, 2019

Accounting Policies [Abstract]

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

General

Ameren, headquartered in St. Louis, Missouri, is a public utility holding company whose primary assets are its equity interests in its subsidiaries. Ameren’s subsidiaries are separate, independent legal entities with separate businesses, assets, and liabilities. Dividends on Ameren’s common stock and the payment of expenses by Ameren depend on distributions made to it by its subsidiaries. Ameren’s principal subsidiaries are listed below. Ameren has other subsidiaries that conduct other activities, such as providing shared services.


•	Union Electric Company, doing business as Ameren Missouri, operates a rate-regulated electric generation, transmission, and distribution business and a rate-regulated natural gas distribution business in Missouri.


•	Ameren Illinois Company, doing business as Ameren Illinois, operates rate-regulated electric transmission, electric distribution, and natural gas distribution businesses in Illinois.


•	ATXI operates a FERC rate-regulated electric transmission business. ATXI is developing the MISO-approved Illinois Rivers and Mark Twain electric transmission projects.

Ameren’s financial statements are prepared on a consolidated basis and therefore include the accounts of its majority-owned subsidiaries. All intercompany transactions have been eliminated, except as disclosed in Note 8 – Related-party Transactions. Ameren Missouri and Ameren Illinois have no subsidiaries. All tabular dollar amounts are in millions, unless otherwise indicated.

Our accounting policies conform to GAAP. Our financial statements reflect all adjustments (which include normal, recurring adjustments) that are necessary, in our opinion, for a fair statement of our results. The preparation of financial statements in conformity with GAAP requires management to make certain estimates and assumptions. Such estimates and assumptions affect reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the dates of financial statements, and the reported amounts of revenues and expenses during the reported periods. Actual results could differ from those estimates. The results of operations of an interim period may not give a true indication of results that may be expected for a full year. These financial statements should be read in conjunction with the financial statements and accompanying notes included in the Form 10-K.

Variable Interest Entities

As of September 30, 2019, Ameren and Ameren Missouri had interests in unconsolidated variable interest entities that were established to construct wind generation facilities and, ultimately, sell those constructed facilities to Ameren Missouri. Neither Ameren nor Ameren Missouri are the primary beneficiary of these variable interest entities because neither has the power to direct matters that most significantly affect the entities' activities, which include designing, financing, and constructing the wind generation facilities. As a result, these variable interest entities have not been consolidated. As of September 30, 2019, the maximum exposure to loss related to these variable interest entities was approximately $12 million, which primarily represents legal costs incurred. The risk of a loss was assessed to be remote and, accordingly, Ameren and Ameren Missouri have not recognized a liability associated with any portion of the maximum exposure to loss. See Note 2 – Rate and Regulatory Matters for additional information on the agreements to acquire these wind generation facilities.

As of September 30, 2019, and December 31, 2018, Ameren had unconsolidated variable interests as a limited partner in various equity method investments, totaling $27 million and $22 million, respectively, included in “Other assets” on Ameren’s consolidated balance sheet. Ameren is not the primary beneficiary of these investments because it does not have the power to direct matters that most significantly affect the activities of these variable interest entities. As of September 30, 2019, the maximum exposure to loss related to these variable interests is limited to the investment in these partnerships of $27 million plus associated outstanding funding commitments of $37 million.

Company-owned Life Insurance

Ameren and Ameren Illinois have company-owned life insurance, which is recorded at the net cash surrender value. The net cash surrender value is the amount that can be realized under the insurance policies at the balance sheet date. As of September 30, 2019, the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $258 million (December 31, 2018 – $244 million) and $121 million (December 31, 2018 – $122 million), respectively, while total borrowings against the policies were $114 million (December 31, 2018 – $113 million) at both Ameren and Ameren Illinois. Ameren and Ameren Illinois have the right to offset the borrowings

against the cash surrender value of the policies and, consequently, present the net asset in “Other assets” on their respective balance sheets.

Accounting and Reporting Developments

See Note 13 – Supplemental Information for additional information on our adoption of authoritative accounting guidance related to leases. See Note 1 – Summary of Significant Accounting Policies under Part II, Item 8, of the Form 10-K for additional information about recently issued authoritative accounting standards relating to the measurement of credit losses on financial instruments, fair value measurement disclosures, and defined benefit plan disclosures.

Rate And Regulatory Matters

9 Months Ended

Sep. 30, 2019

Public Utilities, General Disclosures [Abstract]

RATE AND REGULATORY MATTERS

Below is a summary of updates to significant regulatory proceedings and related lawsuits. See also Note 2 – Rate and Regulatory Matters under Part II, Item 8, of the Form 10-K. We are unable to predict the ultimate outcome of these matters, the timing of the final decisions of the various agencies and courts, or the impact on our results of operations, financial position, or liquidity.

Missouri

2019 Electric Service Regulatory Rate Review

In July 2019, Ameren Missouri filed a request with the MoPSC seeking approval to decrease its annual revenues for electric service by $1 million. The electric rate decrease request is based on a 9.95% return on common equity, a capital structure composed of 51.9% common equity, a rate base of $8.0 billion, and a test year ended December 31, 2018, with certain pro-forma adjustments expected through an anticipated true-up date of December 31, 2019. Pro-forma adjustments are also expected for fuel costs, transportation costs, MISO multi-value transmission project expenses, and payroll costs effective as of January 1, 2020. The electric rate decrease request reflects the following:


•	decreased net energy costs of approximately $100 million otherwise subject to FAC recovery;


•	higher weather-normalized customer sales volumes, which reduced the rate request by approximately $55 million;


•	decreased expenses, other than net energy costs, of approximately $20 million, which includes a decrease to those expenses subject to regulatory recovery mechanisms and changes in amortization of regulatory assets and liabilities of approximately $80 million;


•	increased depreciation and amortization expense of approximately $115 million for new electric infrastructure investments, of which approximately $35 million reflects higher depreciation rates and another $35 million would otherwise be deferred under PISA; and


•	an increase of approximately $60 million of pre-tax return on rate base, which includes both the debt and equity components, of which approximately $30 million would otherwise be deferred under PISA.

Ameren Missouri’s base rates for electric service, which were last reset on April 1, 2017, and adjusted by a July 2018 MoPSC order, are required to be reset at least every four years to allow for continued use of the FAC. This filing, which includes a request for continued use of the FAC, allows Ameren Missouri to meet that requirement while providing flexibility to time its next regulatory rate review to include wind generation investments expected to be made by the end of 2020.

Ameren Missouri also requested continued use of the regulatory recovery mechanisms for pension and postretirement benefits, uncertain income tax positions and certain excess deferred taxes that the MoPSC previously authorized in earlier electric rate orders.

The MoPSC proceeding relating to the proposed electric service rate changes will take place over a period of up to 11 months, with a decision by the MoPSC expected by late April 2020 and new rates effective by late May 2020. Ameren Missouri cannot predict the level of any electric service rate change the MoPSC may approve, when any rate change may go into effect, whether the requested regulatory recovery mechanisms will be approved, or whether any rate change that may eventually be approved will be sufficient for Ameren Missouri to recover its costs and earn a reasonable return on its investments when the rate change goes into effect.

Wind Generation Facilities and RESRAM

In May 2019, Ameren Missouri entered into a build-transfer agreement to acquire, after construction, an up-to 300-megawatt wind generation facility. In 2018, Ameren Missouri entered into a build-transfer agreement to acquire, after construction, an up-to 400-megawatt wind generation facility. The two build-transfer agreements, which are subject to customary contract terms and conditions, collectively represent approximately $1.2 billion of capital expenditures, are expected to be completed by the end of 2020, and would support Ameren Missouri’s compliance with the Missouri renewable energy standard. Both acquisitions have received all regulatory approvals, and both projects have received all applicable zoning approvals, have entered into RTO interconnection agreements, and have begun construction activities. The county zoning approval process for the Schuyler County portion of the 400-megawatt project is subject to litigation filed in August 2019, which is not expected to affect the completion of the project by the end of 2020. The following table provides information with respect to each build-transfer agreement:



	Up-to 400-Megawatt Facility	Up-to 300-Megawatt Facility
Build-transfer agreement date	May 2018	May 2019
Wind facility developer	Terra-Gen, LLC	Invenergy Renewables, LLC^(a)
Location	Northeastern Missouri	Northwestern Missouri
Status of certificate of convenience and necessity from the MoPSC	Approved October 2018	Approved August 2019
Status of final interconnection costs	Received July 2019	Received July 2019
Status of RTO transmission interconnection agreement	Executed August 2019	Executed October 2019
Status of FERC approval	Received December 2018	Received October 2019
Expected completion date	By the end of 2020	By the end of 2020


(a)	In October 2019, Invenergy Renewables, LLC acquired the project from Enel North America, Inc.

In 2018, Ameren Missouri entered into a build-transfer agreement to acquire, after construction, a 157-megawatt wind generation facility. In July 2019, Ameren Missouri and the developer mutually agreed to terminate the project due to unacceptable interconnection costs, which made the project uneconomic and not in the best interest of Ameren Missouri’s customers. Abandonment costs incurred as a result of terminating the project were immaterial to Ameren Missouri.

In January 2019, the MoOPC filed an appeal with the Missouri Court of Appeals, Western District, challenging the MoPSC’s December 2018 order allowing Ameren Missouri to recover, through the RESRAM, the 15% of depreciation expense and weighted average cost of capital return not recovered under PISA. In October 2019, the Missouri Court of Appeals, Western District upheld the MoPSC’s order. In November 2019, the MoOPC filed a request for appeal of the MoPSC’s order to the Missouri Supreme Court. The RESRAM is designed to mitigate the impacts of regulatory lag for the cost of compliance with renewable energy standards, including recovery of investments in wind and other renewable energy generation, by providing more timely recovery of costs and a return on investments not already provided for in customer rates or recovered under PISA.

MEEIA

As a result of MoPSC orders issued in September 2017, October 2018, January 2019, and September 2019 related to performance incentives for the MEEIA 2013 and MEEIA 2016 programs, Ameren Missouri recognized revenues of $20 million and $5 million during the first quarter of 2019 and 2018, respectively, and $18 million in the third quarter of 2019.

Request for Deferral of Maintenance Expenses Related to Scheduled Callaway Refueling and Maintenance Outages

In October 2019, Ameren Missouri filed a request with the MoPSC for deferral accounting treatment that would allow Ameren Missouri to defer and amortize maintenance expenses related to scheduled refueling and maintenance outages at its Callaway nuclear energy center. These expenses would be amortized over the period between refueling and maintenance outages, which is approximately 18 months. Ameren Missouri cannot predict the ultimate outcome of this regulatory proceeding. If the request is approved prior to the fall 2020 refueling and maintenance outage, Ameren Missouri would defer the maintenance expenses incurred related to the outage as a regulatory asset and begin to amortize those expenses after completion of the outage.

2018 Natural Gas Delivery Service Regulatory Rate Review

In December 2018, Ameren Missouri filed a request with the MoPSC to increase its annual revenues for natural gas delivery service. In August 2019, the MoPSC issued an order approving a stipulation and agreement to decrease Ameren Missouri’s annual revenues for natural gas delivery service by $1 million. The decrease in annual rates is based on a return on common equity range of 9.4% to 9.95% and a capital structure composed of 52.0% common equity, which was Ameren Missouri’s capital structure as of May 31, 2019. This order allows for the use of ISRS, which will be calculated using an ROE of 9.725%. The order represents a $1 million increase to Ameren Missouri’s annual revenues for natural gas delivery service from interim rates, which were approved by the MoPSC in December 2018. The new rates became effective September 1, 2019.

Illinois

Electric Distribution Service Rates

In April 2019, Ameren Illinois filed its annual electric distribution service formula rate update to establish the revenue requirement to be used for 2020 rates with the ICC. Pending ICC approval, this update filing will result in a $7 million decrease in Ameren Illinois’ electric distribution service rates, beginning in January 2020. This update reflects a decrease for the conclusion of the 2017 revenue requirement reconciliation adjustment, which will be fully collected from customers in 2019, consistent with the ICC’s November 2018 annual update filing order. It also reflects an increase to the annual formula rate based on 2018 actual costs and expected net plant additions for 2019, and an increase to include the 2018 revenue requirement reconciliation adjustment. In August 2019, the ICC staff submitted an updated calculation

of the revenue requirement included in Ameren Illinois’ filing, recommending an amount comparable to that included in Ameren Illinois’ filing. In October 2019, the administrative law judges issued a proposed order consistent with Ameren Illinois’ filing. An ICC decision in this proceeding is expected by December 2019.

Electric Customer Energy-Efficiency Investments

In May 2019, Ameren Illinois filed its annual electric customer energy-efficiency formula rate update to establish the revenue requirement to be used for 2020 rates with the ICC. This rate update is based on an 8.9% return on common equity, a capital structure composed of 50% common equity, and $205 million of net electric customer energy-efficiency investments. Pending ICC approval, this update filing will result in 2020 electric customer energy-efficiency rates of $44 million, which represents an increase of $10 million from 2019 rates. In September 2019, the ICC staff submitted an updated calculation of the revenue requirement included in Ameren Illinois’ filing, recommending an amount comparable to that included in Ameren Illinois’ filing. An ICC decision in this proceeding is expected by December 2019.

ATXI’s Illinois Rivers Project

In August 2017, the Illinois Circuit Court for Edgar County dismissed several of ATXI’s condemnation cases related to the one remaining line segment to be completed in the Illinois Rivers project. These cases had been filed to obtain easements and rights of way necessary to complete the line segment. The court found that required notice was not given to the relevant landowners during the underlying ICC proceeding. Upon appeal, in October 2018, the Illinois Supreme Court reversed the Illinois Circuit Court for Edgar County’s decision and remanded the case for further proceedings. In February 2019, the landowners filed an appeal with the United States Supreme Court, which was denied in April 2019. In the second quarter of 2019, at ATXI’s request, the Illinois Circuit Court for Edgar County reinstated the condemnation cases that were previously dismissed. ATXI expects to complete the line segment in 2020. The estimated line segment capital expenditure investment is approximately $81 million, of which $39 million was invested as of September 30, 2019.

Federal

FERC Complaint Cases

In November 2013, a customer group filed a complaint case with the FERC seeking a reduction in the allowed base return on common equity for FERC-regulated transmission rate base under the MISO tariff from 12.38% to 9.15%. In September 2016, the FERC issued an order in the November 2013 complaint case, which lowered the allowed base return on common equity to 10.32%, or a 10.82% total allowed return on common equity with the inclusion of a 50 basis point incentive adder for participation in an RTO, effective since September 2016. The 10.82% allowed return on common equity may be replaced prospectively after the FERC issues a final order in the February 2015 complaint case, discussed below.

Since the maximum FERC-allowed refund period for the November 2013 complaint case ended in February 2015, another customer complaint case was filed in February 2015. MISO transmission owners subsequently filed a motion to dismiss the February 2015 complaint, as discussed below. The February 2015 complaint case seeks a further reduction in the allowed base return on common equity for FERC-regulated transmission rate base under the MISO tariff. In June 2016, an administrative law judge issued an initial decision in the February 2015 complaint case. If approved by the FERC, it would lower the allowed base return on common equity for the 15-month period of February 2015 to May 2016 to 9.70%, or a 10.20% total allowed return on equity with the inclusion of a 50 basis point incentive adder for participation in an RTO. It would also require customer refunds, with interest, for that 15-month period. A final FERC order would also establish the allowed return on common equity that will apply prospectively from the effective date of such order, replacing the current 10.82% total return on common equity. In April 2017, the United States Court of Appeals for the District of Columbia Circuit vacated and remanded to the FERC an order in an unrelated case in which the FERC established the allowed base return on common equity methodology subsequently used in the two MISO complaint cases described above. In October 2018, the FERC issued an order in the unrelated case that proposed a new methodology for determining the base return on equity, which required further briefs from the participants. In November 2018, the FERC issued an order related to the February 2015 complaint case and the September 2016 order, which required participants to file briefs in February 2019 regarding the FERC’s proposed methodology for determining the base return on common equity, including whether and how to apply the proposed methodology to the two MISO complaint cases. In March 2019, the FERC issued separate Notices of Inquiry regarding its allowed base return on common equity policy and its transmission incentives policy. Initial comments were due by June 2019, and reply comments were due by late August 2019. The Notice of Inquiry addressing the FERC’s return on common equity policy, among other things, broadened the ability to comment on the new methodology beyond electric utilities that are participants in the complaint cases. The transmission incentives Notice of Inquiry was open for comment on the FERC’s transmission incentive policy, including incentive adders to the return on common equity. Ameren is unable to predict the ultimate impact of the proposed methodology on these complaint cases or the Notices of Inquiry at this time. As the FERC is under no deadline to issue a final order, the timing of the final order in the February 2015 complaint case and any potential impact to the amounts refunded as a result of the September 2016 order is uncertain.

In September 2017, MISO transmission owners, including Ameren Missouri, Ameren Illinois, and ATXI, filed a motion to dismiss the February 2015 complaint case with the FERC. The MISO transmission owners maintain that the February 2015 complaint was predicated on

the now superseded 12.38% allowed base return on common equity and is therefore inapplicable given the current 10.32% allowed base return on common equity. The MISO transmission owners further maintain that the current 10.32% allowed base return on common equity has not been proven to be unjust and unreasonable based on information provided, including the base return on common equity methodology ranges set forth in the February 2015 complaint case and in the initial decision issued by an administrative law judge in June 2016. Additionally, the MISO transmission owners maintain that the February 2015 complaint should be dismissed because the approach utilized in the case to assert that a return on common equity was unjust and unreasonable was insufficient. That same approach was rejected by the United States Court of Appeals for the District of Columbia Circuit in an unrelated case, as discussed above. The FERC is under no deadline to issue an order on this motion.

As of September 30, 2019, Ameren and Ameren Illinois had recorded current regulatory liabilities of $46 million and $27 million, respectively, to reflect the expected refunds, including interest, associated with the reduced allowed return on common equity in the initial decision in the February 2015 complaint case. Ameren Missouri does not expect that a reduction in the FERC-allowed base return on common equity would be material to its results of operations, financial position, or liquidity.

Short-Term Debt And Liquidity

9 Months Ended

Sep. 30, 2019

Debt Disclosure [Abstract]

SHORT-TERM DEBT AND LIQUIDITY

The liquidity needs of the Ameren Companies are typically supported through the use of available cash, drawings under committed credit agreements, commercial paper issuances, and, in the case of Ameren Missouri and Ameren Illinois, short-term affiliate borrowings. See Note 4 – Short-term Debt and Liquidity under Part II, Item 8, in the Form 10-K for a description of our indebtedness provisions and other covenants as well as a description of money pool arrangements.

The Missouri Credit Agreement and the Illinois Credit Agreement were not utilized for direct borrowings during the nine months ended September 30, 2019, but were used to support commercial paper issuances and to issue letters of credit. Based on commercial paper outstanding and letters of credit issued under the Credit Agreements, the aggregate credit capacity available under the Credit Agreements to Ameren (parent), Ameren Missouri, and Ameren Illinois, collectively, at September 30, 2019, was $1.6 billion. The Ameren Companies were in compliance with the covenants in their Credit Agreements as of September 30, 2019. As of September 30, 2019, the ratios of consolidated indebtedness to consolidated total capitalization, calculated in accordance with the provisions of the Credit Agreements, were 53%, 48%, and 47% for Ameren, Ameren Missouri, and Ameren Illinois, respectively.

Commercial Paper

The following table presents commercial paper outstanding, net of issuance discounts, as of September 30, 2019, and December 31, 2018:



	September 30, 2019		December 31, 2018
Ameren (parent)	$	90	$	470
Ameren Missouri	144		55
Ameren Illinois	310		72
Ameren consolidated	$	544	$	597

The following table summarizes the borrowing activity and relevant interest rates under Ameren (parent)’s, Ameren Missouri’s, and Ameren Illinois’ commercial paper programs for the nine months ended September 30, 2019 and 2018:



	Ameren (parent)			Ameren Missouri			Ameren Illinois				Ameren Consolidated
2019
Average daily commercial paper outstanding at par value	$	532		$	141		$	147			$	821
Weighted-average interest rate	2.70		%	2.73		%	2.58		%		2.68		%
Peak commercial paper during period at par value^(a)	$	651		$	549		$	310			$	1,113
Peak interest rate	3.80		%	2.97		%	5.00		%	^(b)	5.00		%	^(b)
2018
Average daily commercial paper outstanding at par value	$	431		$	81		$	117			$	629
Weighted-average interest rate	2.23		%	1.94		%	2.21		%		2.18		%
Peak commercial paper during period at par value^(a)	$	543		$	481		$	442			$	1,295
Peak interest rate	2.45		%	2.42		%	2.55		%		2.55		%


(a)	The timing of peak outstanding commercial paper issuances varies by company. Therefore, the sum of individual company peak amounts may not equal the Ameren consolidated peak commercial paper issuances for the period.


(b)	In the third quarter of 2019, Ameren’s and Ameren Illinois’ peak interest rate was affected by temporary disruptions in the commercial paper market.

Money Pools

Ameren has money pool agreements with and among its subsidiaries to coordinate and provide for certain short-term cash and working capital requirements. The average interest rate for borrowings under the money pool for the three and nine months ended September 30, 2019, was 2.40% and 2.67%, respectively (2018 – 2.00% and 2.02%, respectively). See Note 8 – Related-party Transactions for the amount of interest income and expense from the money pool arrangements recorded by the Ameren Companies for the three and nine months ended September 30, 2019 and 2018.

Long-Term Debt And Equity Financings

9 Months Ended

Sep. 30, 2019

Long-Term Debt And Equity Financings [Abstract]

LONG-TERM DEBT AND EQUITY FINANCINGS

Ameren

For the three and nine months ended September 30, 2019, Ameren issued a total of 0.2 million and 0.7 million shares of common stock under its DRPlus and 401(k) plan, and received proceeds of $17 million and $54 million, respectively. In addition, in the first quarter of 2019, Ameren issued 0.8 million shares of common stock valued at $54 million upon the vesting of stock-based compensation.

In August 2019, Ameren entered into a forward sale agreement with a counterparty relating to 7.5 million shares of common stock. The forward sale agreement can be settled at Ameren’s discretion on or prior to March 31, 2021. On a settlement date or dates, if Ameren elects to physically settle the forward sale agreement, Ameren will issue shares of common stock to the counterparty at the then-applicable forward sale price. The forward sale price was initially $74.18 per share. The initial forward sale price is subject to adjustment based on a floating interest rate factor equal to the overnight bank funding rate less a spread of 75 basis points, and will be subject to decrease on certain dates specified in the forward sale agreement by specified amounts related to expected dividends on shares of the common stock during the term of the forward sale agreement. If the overnight bank funding rate is less than the spread on any day, the interest rate factor will result in a reduction of the forward sale price.

The forward sale agreement will be physically settled unless Ameren elects to settle in cash or to net share settle. At September 30, 2019, Ameren could have settled the forward sale agreement with physical delivery of 7.5 million shares of common stock to the counterparty in exchange for cash of $557 million. The forward sale agreement could also have been settled at September 30, 2019, with delivery of approximately $47 million of cash or approximately 0.6 million shares of common stock to the counterparty, if Ameren had elected to net cash or net share settle, respectively.

The forward sale agreement has been classified as an equity transaction because it is indexed to Ameren’s common stock, physical settlement is within Ameren’s control, and the other requirements necessary for equity classification were met. As a result of the equity classification, no gain or loss will be recognized within earnings due to subsequent changes in the fair value of the forward sale agreement. If the average price of Ameren’s common stock exceeds the adjusted forward sale price during a quarterly period, the forward sale agreement could have a dilutive effect on earnings per share.

In September 2019, Ameren issued $450 million of 2.50% senior unsecured notes due September 2024, with interest payable semiannually on March 15 and September 15, beginning March 15, 2020. Ameren received net proceeds of $447 million, which were used to repay outstanding short-term debt.

Ameren Missouri

In March 2019, Ameren Missouri issued $450 million of 3.50% first mortgage bonds due March 2029, with interest payable semiannually on March 15 and September 15 of each year, beginning September 15, 2019. Ameren Missouri received net proceeds of $447 million, which were used to repay outstanding short-term debt, including short-term debt that Ameren Missouri incurred in connection with the repayment of $329 million of its 6.70% senior secured notes that matured February 1, 2019.

In June and July 2019, all of the 1992 Series bonds, 1998 Series A bonds, 1998 Series B bonds, and 1998 Series C bonds issued by the Missouri Environmental Improvement and Energy Resources Authority on behalf of Ameren Missouri were subject to purchase in lieu of redemption or a mandatory tender as a result of a change in the method of determining the interest rates on the bonds. The interest rate method of each of the series of bonds, as well as Ameren Missouri’s first mortgage bonds that collaterally secure each of the series of bonds, was changed from a variable rate to a fixed rate. Upon the change in the method of determining the interest rate, the bonds, totaling $207 million, were remarketed to new investors. The following table provides additional information on the bonds:



	1992 Series	1998 Series A	1998 Series B	1998 Series C
Transaction month	June 2019	July 2019	July 2019	June 2019
Principal amount	$47	$60	$50	$50
Fixed interest rate	1.60%	2.90%	2.90%	2.75%
Variable interest rate ^(a)	2.36%	3.35%	3.34%	3.83%
Maturity	December 2022	September 2033	September 2033	September 2033
Interest payment dates	June 1 and December 1	March 1 and September 1	March 1 and September 1	March 1 and September 1
Initial interest payment date	December 2019	September 2019	September 2019	September 2019


(a)	Represents the variable interest rate of the bonds effective prior to the change in method of determining the interest rate.

In October 2019, Ameren Missouri issued $330 million of 3.25% first mortgage bonds due October 2049, with interest payable semiannually on April 1 and October 1 of each year, beginning April 1, 2020. Ameren Missouri received net proceeds of $326 million, which were used to repay $244 million of its 5.10% senior unsecured notes due October 1, 2019, with the remaining proceeds used to repay a portion of its short-term debt.

In October 2019, Ameren Missouri redeemed the remaining amount outstanding of its 5.45% first mortgage bonds due 2028 for less than $1 million.

Ameren Illinois

In 2006, Ameren Illinois purchased all $17 million of the 1993 Series B-1 bonds due 2028 issued by the Illinois Finance Authority on behalf of Ameren Illinois pursuant to a mandatory tender. Ameren Illinois’ 1993 Series B-1 senior unsecured notes due 2028 were not extinguished and remained as “Long-term debt, net” on Ameren’s and Ameren Illinois’ balance sheets. In September 2019, Ameren Illinois exchanged its bond investments for the extinguishment of its senior unsecured notes.

In September 2019, Ameren Illinois redeemed the remaining amount outstanding of its 5.70% first mortgage bonds due 2024 for less than $1 million. Additionally, in October 2019, Ameren Illinois redeemed the remaining amount outstanding of its 5.90% first mortgage bonds due 2023 for less than $1 million. Following the redemption of the 5.90% first mortgage bonds, Ameren Illinois collaterally secured its 6.70% senior secured notes due 2036 with first mortgage bonds issued under its 1992 mortgage indenture.

Indenture Provisions and Other Covenants

See Note 5 – Long-Term Debt and Equity Financings under Part II, Item 8, in the Form 10-K for a description of our indenture provisions and other covenants, as well as restrictions on the payment of dividends. At September 30, 2019, the Ameren Companies were in compliance with the provisions and covenants contained in their indentures and articles of incorporation, as applicable, and ATXI was in compliance with the provisions and covenants contained in its note purchase agreement.

Off-balance-sheet Arrangements

At September 30, 2019, none of the Ameren Companies had any significant off-balance-sheet financing arrangements, other than variable interest entities, letters of credit, and Ameren (parent) guarantee arrangements on behalf of its subsidiaries. See Note 1 – Summary of Significant Accounting Policies for further detail concerning variable interest entities.

Other Income and Expenses

9 Months Ended

Sep. 30, 2019

Other Nonoperating Income (Expense) [Abstract]

OTHER INCOME AND EXPENSES

OTHER INCOME, NET

The following table presents the components of “Other Income, Net” in the Ameren Companies’ statements of income for the three and nine months ended September 30, 2019 and 2018:



	Three Months						Nine Months
	2019			2018			2019			2018
Ameren:
Allowance for equity funds used during construction	$	7		$	11		$	20		$	25
Interest income on industrial development revenue bonds	6			6			19			19
Other interest income	2			2			6			6
Non-service cost components of net periodic benefit income^(a)	23			17			67			52
Miscellaneous income	2			2			6			5
Donations	(1		)	(4		)	(8		)	(15		)
Miscellaneous expense	(5		)	(2		)	(11		)	(8		)
Total Other Income, Net	$	34		$	32		$	99		$	84



	Three Months						Nine Months
	2019			2018			2019			2018
Ameren Missouri:
Allowance for equity funds used during construction	$	6		$	8		$	14		$	19
Interest income on industrial development revenue bonds	6			6			19			19
Other interest income	—			1			—			2
Non-service cost components of net periodic benefit income^(a)	4			4			13			13
Miscellaneous income	2			2			4			3
Donations	(1		)	(3		)	(3		)	(6		)
Miscellaneous expense	(2		)	(2		)	(4		)	(5		)
Total Other Income, Net	$	15		$	16		$	43		$	45
Ameren Illinois:
Allowance for equity funds used during construction	$	1		$	3		$	6		$	6
Interest income	1			1			5			4
Non-service cost components of net periodic benefit income	12			8			36			25
Miscellaneous income	1			1			3			3
Donations	—			—			(5		)	(5		)
Miscellaneous expense	(2		)	(2		)	(6		)	(3		)
Total Other Income, Net	$	13		$	11		$	39		$	30

(a)

For the three and nine months ended September 30, 2019, the non-service cost components of net periodic benefit income were partially offset by a $7 million and $22 million deferral, respectively, due to a regulatory tracking mechanism for the difference between the level of such costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates (2018 – $5 million and $13 million, respectively).

Derivative Financial Instruments

9 Months Ended

Sep. 30, 2019

Derivative Instruments and Hedging Activities Disclosure [Abstract]

DERIVATIVE FINANCIAL INSTRUMENTS

We use derivatives to manage the risk of changes in market prices for natural gas and power, as well as the risk of changes in rail transportation surcharges through fuel oil hedges. Such price fluctuations may cause the following:


•	an unrealized appreciation or depreciation of our contracted commitments to purchase or sell when purchase or sale prices under the commitments are compared with current commodity prices;


•	market values of natural gas inventories that differ from the cost of those commodities in inventory; and


•	actual cash outlays for the purchase of these commodities that differ from anticipated cash outlays.

The derivatives that we use to hedge these risks are governed by our risk management policies for forward contracts, futures, options, and swaps. Our net positions are continually assessed within our structured hedging programs to determine whether new or offsetting transactions are required. The goal of the hedging program is generally to mitigate financial risks while ensuring that sufficient volumes are available to meet our requirements. Contracts we enter into as part of our risk management program may be settled financially, settled by physical delivery, or net settled with the counterparty.

All contracts considered to be derivative instruments are required to be recorded on the balance sheet at their fair values, unless the NPNS exception applies. Many of our physical contracts, such as our purchased power contracts, qualify for the NPNS exception to derivative accounting rules. The revenue or expense on NPNS contracts is recognized at the contract price upon physical delivery. The following disclosures exclude NPNS contracts and other non-derivative commodity contracts that are accounted for under the accrual method of accounting.

If we determine that a contract meets the definition of a derivative and is not eligible for the NPNS exception, we review the contract to determine whether the resulting gains or losses qualify for regulatory deferral. Derivative contracts that qualify for regulatory deferral are recorded at fair value, with changes in fair value recorded as regulatory assets or liabilities in the period in which the change occurs. We believe derivative losses and gains deferred as regulatory assets and liabilities are probable of recovery, or refund, through future rates charged to customers. Regulatory assets and liabilities are amortized to operating income as related losses and gains are reflected in rates charged to customers. Therefore, gains and losses on these derivatives have no effect on operating income. As of September 30, 2019, and December 31, 2018, all contracts that met the definition of a derivative and were not eligible for the NPNS exception received regulatory deferral.

The following table presents open gross commodity contract volumes by commodity type for derivative assets and liabilities as of September 30, 2019, and December 31, 2018. As of September 30, 2019, these contracts extended through October 2022, March 2024, and May 2032 for fuel oils, natural gas, and power, respectively.



	Quantity (in millions)
	2019			2018
Commodity	Ameren Missouri	Ameren Illinois	Ameren	Ameren Missouri	Ameren Illinois	Ameren
Fuel oils (in gallons)^(a)	63	—	63	66	—	66
Natural gas (in mmbtu)	20	142	162	19	154	173
Power (in megawatthours)	4	8	12	1	8	9


(a)	Consists of ultra-low-sulfur diesel products.

The following table presents the carrying value and balance sheet location of all derivative commodity contracts, none of which were designated as hedging instruments, as of September 30, 2019, and December 31, 2018:



			September 30, 2019					December 31, 2018
	Balance Sheet Location		Ameren Missouri		Ameren Illinois		Ameren			Ameren Missouri		Ameren Illinois		Ameren
Fuel oils	Other current assets	$	4	$	—	$	4		$	3	$	—	$	3
	Other assets		2		—		2			5		—		5
Natural gas	Other current assets		—		2		2			—		1		1
	Other assets		—		1		1			—		2		2
Power	Other current assets		8		—		8			4		—		4
	Other assets		4		—		4			—		—		—
	Total assets	$	18	$	3	$	21		$	12	$	3	$	15
Fuel oils	Other current liabilities	$	7	$	—	$	7		$	4	$	—	$	4
	Other deferred credits and liabilities		5		—		5			9		—		9
Natural gas	Other current liabilities		3		12		15			4		8		12
	Other deferred credits and liabilities		1		7		8			1		6		7
Power	Other current liabilities		3		15		18			4		14		18
	Other deferred credits and liabilities		1		189		190			—		169		169
	Total liabilities	$	20	$	223	$	243		$	22	$	197	$	219

The Ameren Companies elect to present the fair value amounts of derivative assets and derivative liabilities subject to an enforceable master netting arrangement or similar agreement at the gross amounts on the balance sheet. However, if the gross amounts recognized on the balance sheet were netted with derivative instruments and cash collateral received or posted, the net amounts would not be materially different from the gross amounts at September 30, 2019, and December 31, 2018.

Credit Risk

In determining our concentrations of credit risk related to derivative instruments, we review our individual counterparties and categorize each counterparty into groupings according to the primary business in which each engages. As of September 30, 2019, if counterparty groups were to fail completely to perform on contracts, the Ameren Companies’ maximum exposure related to derivative assets would have been immaterial with or without consideration of the application of master netting arrangements or similar agreements and collateral held.

Certain of our derivative instruments contain collateral provisions tied to the Ameren Companies’ credit ratings. If our credit ratings were downgraded below investment grade, or if a counterparty with reasonable grounds for uncertainty regarding our ability to satisfy an obligation requested adequate assurance of performance, additional collateral postings might be required. The additional collateral required is the net liability position allowed under master netting arrangements or similar agreements, assuming (1) the credit risk-related contingent features underlying these arrangements were triggered and, (2) those counterparties with rights to do so requested collateral. As of September 30, 2019, the aggregate fair value of derivative instruments with credit risk-related contingent features in a gross liability position, the cash collateral posted, and the aggregate amount of additional collateral that counterparties could require were each immaterial to Ameren, Ameren Missouri, and Ameren Illinois.

Fair Value Measurements

9 Months Ended

Sep. 30, 2019

Fair Value Disclosures [Abstract]

Fair Value Disclosures

FAIR VALUE MEASUREMENTS

Fair value is defined as the price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Fair value measurements are classified in three levels based on the fair value hierarchy as defined by GAAP. See Note 8 – Fair Value Measurements

under Part II, Item 8, of the Form 10-K for information related to hierarchy levels and valuation techniques.

We consider nonperformance risk in our valuation of derivative instruments by analyzing our own credit standing and the credit standing of our counterparties, and by considering any credit enhancements (e.g., collateral). Included in our valuation, and based on current market conditions, is a valuation adjustment for counterparty default derived from market data such as the price of credit default swaps, bond yields, and credit ratings. No material gains or losses related to valuation adjustments for counterparty default risk were recorded at Ameren, Ameren Missouri, or Ameren Illinois in the three and nine months ended September 30, 2019 or 2018. At September 30, 2019, and December 31, 2018, the counterparty default risk valuation adjustment related to derivative contracts was immaterial for Ameren, Ameren Missouri, and Ameren Illinois.

The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of September 30, 2019, and December 31, 2018:



		September 30, 2019									December 31, 2018
		Level 1		Level 2		Level 3		Total			Level 1		Level 2		Level 3		Total
Assets:
Ameren
	Derivative assets – commodity contracts^(a):
	Fuel oils	$	—	$	—	$	6	$	6		$	1	$	—	$	7	$	8
	Natural gas	—		—		3		3			—		2		1		3
	Power	1		—		11		12			—		1		3		4
	Total derivative assets – commodity contracts	$	1	$	—	$	20	$	21		$	1	$	3	$	11	$	15
	Nuclear decommissioning trust fund:
	Equity securities:
	U.S. large capitalization	$	518	$	—	$	—	$	518		$	427	$	—	$	—	$	427
	Debt securities:
	U.S. Treasury and agency securities	—		136		—		136			—		148		—		148
	Corporate bonds	—		86		—		86			—		72		—		72
	Other	—		50		—		50			—		32		—		32
	Total nuclear decommissioning trust fund	$	518	$	272	$	—	$	790	^(b)	$	427	$	252	$	—	$	679	^(b)
	Total Ameren	$	519	$	272	$	20	$	811		$	428	$	255	$	11	$	694
Ameren Missouri
	Derivative assets – commodity contracts^(a):
	Fuel oils	$	—	$	—	$	6	$	6		$	1	$	—	$	7	$	8
	Power	1		—		11		12			—		1		3		4
	Total derivative assets – commodity contracts	$	1	$	—	$	17	$	18		$	1	$	1	$	10	$	12
	Nuclear decommissioning trust fund:
	Equity securities:
	U.S. large capitalization	$	518	$	—	$	—	$	518		$	427	$	—	$	—	$	427
	Debt securities:
	U.S. Treasury and agency securities	—		136		—		136			—		148		—		148
	Corporate bonds	—		86		—		86			—		72		—		72
	Other	—		50		—		50			—		32		—		32
	Total nuclear decommissioning trust fund	$	518	$	272	$	—	$	790	^(b)	$	427	$	252	$	—	$	679	^(b)
	Total Ameren Missouri	$	519	$	272	$	17	$	808		$	428	$	253	$	10	$	691
Ameren Illinois
	Derivative assets – commodity contracts^(a):
	Natural gas	$	—	$	—	$	3	$	3		$	—	$	2	$	1	$	3
Liabilities:
Ameren
	Derivative liabilities – commodity contracts^(a):
	Fuel oils	$	4	$	—	$	8	$	12		$	2	$	—	$	11	$	13
	Natural gas	2		17		4		23			—		15		4		19
	Power	—		1		207		208			—		1		186		187
	Total Ameren	$	6	$	18	$	219	$	243		$	2	$	16	$	201	$	219



		September 30, 2019								December 31, 2018
		Level 1		Level 2		Level 3		Total		Level 1		Level 2		Level 3		Total
Ameren Missouri
	Derivative liabilities – commodity contracts^(a):
	Fuel oils	$	4	$	—	$	8	$	12	$	2	$	—	$	11	$	13
	Natural gas	—		3		1		4		—		5		—		5
	Power	—		1		3		4		—		1		3		4
	Total Ameren Missouri	$	4	$	4	$	12	$	20	$	2	$	6	$	14	$	22
Ameren Illinois
	Derivative liabilities – commodity contracts^(a):
	Natural gas	$	2	$	14	$	3	$	19	$	—	$	10	$	4	$	14
	Power	—		—		204		204		—		—		183		183
	Total Ameren Illinois	$	2	$	14	$	207	$	223	$	—	$	10	$	187	$	197


(a)	The derivative asset and liability balances are presented net of registrant and counterparty credit considerations.


(b)	Balance excludes $8 million and $5 million of cash and cash equivalents, receivables, payables, and accrued income, net, for September 30, 2019, and December 31, 2018, respectively.

Level 3 fuel oils and natural gas derivative contract assets and liabilities measured at fair value on a recurring basis were immaterial for all periods presented. The following table presents the fair value reconciliation of Level 3 power derivative contract assets and liabilities measured at fair value on a recurring basis for the three and nine months ended September 30, 2019 and 2018:



	2019									2018
	Ameren Missouri			Ameren Illinois			Ameren			Ameren Missouri			Ameren Illinois			Ameren
For the three months ended September 30:
Beginning balance at July 1	$	15		$	(191	)	$	(176	)	$	5		$	(190	)	$	(185	)
Realized and unrealized gains/(losses) included in regulatory assets/liabilities	(4		)	(17		)	(21		)	(4		)	—			(4		)
Purchases	—			—			—			1			—			1
Settlements	(1		)	4			3			(1		)	3			2
Transfers out of Level 3	(2		)	—			(2		)	(1		)	—			(1		)
Ending balance at September 30	$	8		$	(204	)	$	(196	)	$	—		$	(187	)	$	(187	)
Change in unrealized gains/(losses) related to assets/liabilities held at September 30	$	(4	)	$	(17	)	$	(21	)	$	—		$	—		$	—
For the nine months ended September 30:
Beginning balance at January 1	$	—		$	(183	)	$	(183	)	$	7		$	(195	)	$	(188	)
Realized and unrealized gains/(losses) included in regulatory assets/liabilities	12			(32		)	(20		)	(7		)	(1		)	(8		)
Purchases	—			—			—			5			—			5
Settlements	(2		)	11			9			(4		)	9			5
Transfers out of Level 3	(2		)	—			(2		)	(1		)	—			(1		)
Ending balance at September 30	8			(204		)	(196		)	—			(187		)	(187		)
Change in unrealized gains/(losses) related to assets/liabilities held at September 30	$	8		$	(31	)	$	(23	)	$	(1	)	$	(2	)	$	(3	)

For the three and nine months ended September 30, 2019 and 2018, there were no material transfers between fair value hierarchy levels.

All gains or losses related to our Level 3 derivative commodity contracts are expected to be recovered or returned through customer rates; therefore, there is no impact to net income resulting from changes in the fair value of these instruments.

The following table describes the valuation techniques and significant unobservable inputs utilized for the fair value of our Level 3 power derivative contract assets and liabilities as of September 30, 2019, and December 31, 2018:



			Fair Value^(a)						Weighted Average
	Commodity		Assets		Liabilities	Valuation Technique(s)	Unobservable Input	Range	Weighted Average
2019	Power^(b)	$	11	$	(207)	Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps ($/MWh)^(c)	22 – 37	25
							Nodal basis ($/MWh)^(c)	(7) – 0	(3)
						Fundamental energy production model	Estimated future natural gas prices ($/mmbtu)^(c)	3 – 3	3
2018	Power^(d)	$	3	$	(186)	Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps ($/MWh)^(c)	23 – 39	28
							Nodal basis ($/MWh)^(c)	(9) – 0	(2)
						Fundamental energy production model	Estimated future natural gas prices ($/mmbtu)^(c)	3 – 4	3


(a)	The derivative asset and liability balances are presented net of registrant and counterparty credit considerations.


(b)	Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2024. Valuations beyond 2024 use fundamentally modeled pricing by month for peak and off-peak demand.


(c)	Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.


(d)	Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2022. Valuations beyond 2022 use fundamentally modeled pricing by month for peak and off-peak demand.

The following table sets forth, by level within the fair value hierarchy, the carrying amount and fair value of financial assets and liabilities disclosed, but not carried, at fair value as of September 30, 2019, and December 31, 2018:



	September 30, 2019
	Carrying Amount			Fair Value
	Carrying Amount			Level 1		Level 2		Level 3			Total
Ameren:
Cash, cash equivalents, and restricted cash	$	155		$	155	$	—	$	—		$	155
Investments in industrial development revenue bonds^(a)	270			—		270		—			270
Short-term debt	544			—		544		—			544
Long-term debt (including current portion)^(a)	8,987		^(b)	—		9,727		491		^(c)	10,218
Ameren Missouri:
Cash, cash equivalents, and restricted cash	$	14		$	14	$	—	$	—		$	14
Investments in industrial development revenue bonds^(a)	270			—		270		—			270
Short-term debt	144			—		144		—			144
Long-term debt (including current portion)^(a)	4,115		^(b)	—		4,766		—			4,766
Ameren Illinois:
Cash, cash equivalents, and restricted cash	$	117		$	117	$	—	$	—		$	117
Short-term debt	310			—		310		—			310
Long-term debt (including current portion)	3,279		^(b)	—		3,788		—			3,788
	December 31, 2018
Ameren:
Cash, cash equivalents, and restricted cash	$	107		$	107	$	—	$	—		$	107
Investments in industrial development revenue bonds^(a)	270			—		270		—			270
Short-term debt	597			—		597		—			597
Long-term debt (including current portion)^(a)	8,439		^(b)	—		8,240		429		^(c)	8,669
Ameren Missouri:
Cash, cash equivalents, and restricted cash	$	8		$	8	$	—	$	—		$	8
Investments in industrial development revenue bonds^(a)	270			—		270		—			270
Short-term debt	55			—		55		—			55
Long-term debt (including current portion)^(a)	3,998		^(b)	—		4,156		—			4,156
Ameren Illinois:
Cash, cash equivalents, and restricted cash	$	80		$	80	$	—	$	—		$	80
Short-term debt	72			—		72		—			72
Long-term debt (including current portion)	3,296		^(b)	—		3,391		—			3,391

(a)

Ameren and Ameren Missouri have investments in industrial development revenue bonds, classified as held-to-maturity and recorded in “Other Assets,” that are equal to the finance obligations for the Peno Creek and Audrain CT energy centers. As of September 30, 2019, and December 31, 2018, the carrying amount of both the investments in industrial development revenue bonds and the finance obligations approximated fair value.

(b)

Includes unamortized debt issuance costs, which were excluded from the fair value measurement, of $65 million, $27 million, and $31 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of September 30, 2019. Includes unamortized debt issuance costs, which were excluded from the fair value measurement, of $58 million, $22 million, and $31 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2018.

(c)

The Level 3 fair value amount consists of ATXI’s senior unsecured notes.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Related Party Transactions

9 Months Ended

Sep. 30, 2019

RELATED PARTY TRANSACTIONS

RELATED-PARTY TRANSACTIONS

In the normal course of business, Ameren Missouri and Ameren Illinois have engaged in, and may in the future engage in, affiliate transactions. These transactions primarily consist of natural gas and power purchases and sales, services received or rendered, and borrowings and lendings. Transactions between Ameren’s subsidiaries are reported as affiliate transactions on their individual financial statements, but those transactions are eliminated in consolidation for Ameren’s consolidated financial statements. For a discussion of material related-party agreements and money pool arrangements, see Note 13 – Related-party Transactions and Note 4 – Short-term Debt and Liquidity under Part II, Item 8, of the Form 10-K.

Electric Power Supply Agreement

In April and September 2019, Ameren Illinois conducted procurement events, administered by the IPA, to purchase energy products. Ameren Missouri was among the winning suppliers in these events. As a result, in April 2019, Ameren Missouri and Ameren Illinois entered into an energy product agreement by which Ameren Missouri agreed to sell, and Ameren Illinois agreed to purchase, 288,000 megawatthours at an average price of $35 per megawatthour during the period of January 2020 through December 2021. In September 2019, Ameren Missouri and Ameren Illinois entered into an energy product agreement by which Ameren Missouri agreed to sell, and Ameren Illinois agreed to purchase, 170,800 megawatthours at an average price of $29 per megawatthour during the period of April 2020 through November 2021.

Software Licensing Agreement

In September 2019, Ameren Missouri purchased a license for advanced metering infrastructure software from Ameren Illinois. The amount of the $24 million cost-based transaction price over the $5 million remaining carrying value of the software was recorded as revenue by Ameren Illinois, with $14 million of revenue recorded at Ameren Illinois Electric Distribution and $5 million recorded at Ameren Illinois Natural Gas. The revenue recorded at Ameren Illinois Electric Distribution was reflected in formula ratemaking, which resulted in no impact to net income. Per authoritative accounting guidance for sales to rate-regulated entities, the revenue recognized by Ameren Illinois was not eliminated upon consolidation by Ameren. Ameren Missouri's $24 million software investment is included in "Property, Plant, and Equipment, Net.” Ameren Missouri and Ameren Illinois included $24 million in "Accounts payable – affiliates" and "Accounts receivable – affiliates," respectively, as of September 30, 2019, as a result of this transaction.

Tax Allocation Agreement

See Note 1 – Summary of Significant Accounting Policies under Part II, Item 8, of the Form 10-K for a discussion of the tax allocation agreement. The following table presents the affiliate balances related to income taxes for Ameren Missouri and Ameren Illinois as of September 30, 2019, and December 31, 2018:



	September 30, 2019				December 31, 2018
	Ameren Missouri		Ameren Illinois		Ameren Missouri		Ameren Illinois
Income taxes payable to parent^(a)	$	58	$	24	$	16	$	7
Income taxes receivable from parent^(b)	—		—		—		6


(a)	Included in “Accounts payable – affiliates” on the balance sheet.


(b)	Included in “Accounts receivable – affiliates” on the balance sheet.

Effects of Related-party Transactions on the Statement of Income

The following table presents the effect on Ameren Missouri and Ameren Illinois of related-party transactions for the three and nine months ended September 30, 2019 and 2018:



				Three Months				Nine Months
Agreement	Income Statement Line Item			Ameren Missouri		Ameren Illinois		Ameren Missouri		Ameren Illinois
Ameren Missouri power supply	Operating Revenues	2019	$	1	$	(a)	$	3	$	(a)
agreements with Ameren Illinois		2018		5		(a)		11		(a)
Ameren Missouri and Ameren Illinois	Operating Revenues	2019	$	7	$	1	$	20	$	2
rent and facility services		2018		6		(b)		17		2
Ameren Missouri and Ameren Illinois	Operating Revenues	2019	$	1	$	(b)	$	1	$	1
miscellaneous support services		2018		(b)		(b)		(b)		(b)



				Three Months				Nine Months
Agreement	Income Statement Line Item			Ameren Missouri		Ameren Illinois		Ameren Missouri		Ameren Illinois
Ameren Missouri software licensing	Operating Revenues	2019	$	(a)	$	19	$	(a)	$	19
with Ameren Illinois		2018		(a)		(a)		(a)		(a)
Total Operating Revenues		2019	$	9	$	20	$	24	$	22
		2018		11		(b)		28		2
Ameren Illinois power supply	Purchased Power	2019	$	(a)	$	1	$	(a)	$	3
agreements with Ameren Missouri		2018		(a)		5		(a)		11
Ameren Illinois transmission	Purchased Power	2019	$	(a)	$	1	$	(a)	$	1
services with ATXI		2018		(a)		(b)		(a)		1
Total Purchased Power		2019	$	(a)	$	2	$	(a)	$	4
		2018		(a)		5		(a)		12
Ameren Missouri and Ameren Illinois	Other Operations and Maintenance	2019	$	(b)	$	1	$	1	$	4
rent and facility services		2018		1		1		2		4
Ameren Services support services	Other Operations and Maintenance	2019	$	34	$	30	$	98	$	91
agreement		2018		36		33		101		93
Total Other Operations and		2019	$	34	$	31	$	99	$	95
Maintenance		2018		37		34		103		97
Money pool borrowings (advances)	Interest Charges/Other Income, Net	2019	$	(b)	$	(b)	$	(b)	$	(b)
		2018		(b)		(b)		(b)		(b)


(a)	Not applicable.

(b)

Amount less than $1 million.

Commitments And Contingencies

9 Months Ended

Sep. 30, 2019

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

COMMITMENTS AND CONTINGENCIES

We are involved in legal, tax, and regulatory proceedings before various courts, regulatory commissions, authorities, and governmental agencies with respect to matters that arise in the ordinary course of business, some of which involve substantial amounts of money. We believe that the final disposition of these proceedings, except as otherwise disclosed in the notes to our financial statements in this report and in the Form 10-K, will not have a material adverse effect on our results of operations, financial position, or liquidity.

Reference is made to Note 1 – Summary of Significant Accounting Policies, Note 2 – Rate and Regulatory Matters, Note 13 – Related-party Transactions, and Note 14 – Commitments and Contingencies under Part II, Item 8, of the Form 10-K. See also Note 1 – Summary of Significant Accounting Policies, Note 2 – Rate and Regulatory Matters, Note 8 – Related-party Transactions, and Note 10 – Callaway Energy Center of this report.

Other Obligations

To supply a portion of the fuel requirements of Ameren Missouri’s energy centers, Ameren Missouri has entered into various long-term commitments for the procurement of coal, natural gas, nuclear fuel, and methane gas. Ameren Missouri and Ameren Illinois also have entered into various long-term commitments for purchased power and natural gas for distribution. The table below presents our estimated minimum fuel, purchased power, and other commitments at September 30, 2019. Ameren’s and Ameren Illinois’ purchased power commitments include the Ameren Illinois agreements entered into as part of the IPA-administered power procurement process. Included in the Other column are minimum purchase commitments under contracts for equipment, design and construction, and meter reading services, among other agreements, at September 30, 2019.



	Coal		Natural Gas^(a)		Nuclear Fuel		Purchased Power^(b)(c)			Methane Gas		Other		Total
Ameren:
2019	$	142	$	51	$	21	$	56	^(d)	$	1	$	20	$	291
2020	226		172		43		146		^(d)	3		40		630
2021	195		105		59		50			3		26		438
2022	137		49		12		12			3		22		235
2023	46		25		42		2			3		22		140
Thereafter	—		48		29		—			26		85		188
Total	$	746	$	450	$	206	$	266		$	39	$	215	$	1,922
Ameren Missouri:
2019	$	142	$	10	$	21	$	—		$	1	$	16	$	190
2020	226		39		43		—			3		29		340
2021	195		23		59		—			3		22		302
2022	137		11		12		—			3		22		185
2023	46		10		42		—			3		22		123
Thereafter	—		17		29		—			26		48		120
Total	$	746	$	110	$	206	$	—		$	39	$	159	$	1,260
Ameren Illinois:
2019	$	—	$	41	$	—	$	56	^(d)	$	—	$	2	$	99
2020	—		133		—		146		^(d)	—		1		280
2021	—		82		—		50			—		—		132
2022	—		38		—		12			—		—		50
2023	—		15		—		2			—		—		17
Thereafter	—		31		—		—			—		—		31
Total	$	—	$	340	$	—	$	266		$	—	$	3	$	609


(a)	Includes amounts for generation and for distribution.


(b)	The purchased power amounts for Ameren and Ameren Illinois exclude agreements for renewable energy credits through 2035 with various renewable energy suppliers due to the contingent nature of the payment amounts, with the exception of expected payments of $11 million through 2023.


(c)	The purchased power amounts for Ameren and Ameren Missouri exclude a 102-megawatt power purchase agreement with a wind farm operator, which expires in 2024, due to the contingent nature of the payment amounts.


(d)	In January 2018, as required by the FEJA, Ameren Illinois entered into 10-year agreements to acquire zero emission credits. Annual zero emission credit commitment amounts will be published by the IPA each May prior to the start of the subsequent planning year. The amounts above reflect Ameren Illinois’ commitment to acquire approximately $44 million of zero emission credits through May 2020.

Environmental Matters

We are subject to various environmental laws, including statutes and regulations, enforced by federal, state, and local authorities. The development and operation of electric generation, transmission, and distribution facilities and natural gas storage, transmission, and distribution facilities can trigger compliance obligations with respect to environmental laws. These laws address emissions, discharges to water, water intake, impacts to air, land, and water, and chemical and waste handling. Complex and lengthy processes are required to obtain and renew approvals, permits, and licenses for new, existing or modified facilities. Additionally, the use and handling of various chemicals or hazardous materials require release prevention plans and emergency response procedures.

The EPA has promulgated environmental regulations that have a significant impact on the electric utility industry. Over time, compliance with these regulations could be costly for Ameren Missouri, which operates coal-fired power plants. As of December 31, 2018, Ameren Missouri’s fossil fuel-fired energy centers represented 16% and 32% of Ameren’s and Ameren Missouri’s rate base, respectively. Regulations that apply to air emissions from the electric utility industry include the NSPS, the CSAPR, the MATS, and the National Ambient Air Quality Standards, which are subject to periodic review for certain pollutants. Collectively, these regulations cover a variety of pollutants, such as SO₂, particulate matter, NO_x,mercury, toxic metals, and acid gases, and CO₂ emissions from new power plants. Water intake and discharges from power plants are regulated under the Clean Water Act. Such regulation could require modifications to water intake structures or more stringent limitations on wastewater discharges at Ameren Missouri’s energy centers, either of which could result in significant capital expenditures. The management and disposal of coal ash is regulated under the CCR rule, which will require the closure of surface impoundments and the installations of dry ash handling systems at several of Ameren Missouri’s energy centers. The individual or combined effects of existing environmental regulations could result in significant capital expenditures, increased operating costs, or the closure or alteration of operations at some of Ameren Missouri’s energy centers. Ameren and Ameren Missouri expect that such compliance costs would be recoverable through rates, subject to MoPSC prudence review, but the timing of costs and their recovery could be subject to regulatory lag.

Ameren and Ameren Missouri estimate that they will need to make capital expenditures of $300 million to $400 million from 2019 through 2023 in order to comply with existing environmental regulations. Additional environmental controls beyond 2023 could be required. This estimate of capital expenditures includes expenditures required by the CCR regulations, by the Clean Water Act rule applicable to cooling water intake structures at existing power plants, and by effluent limitation guidelines applicable to steam electric generating units, all of which are discussed below. This estimate does not include capital expenditures that may be required as a result of the NSR and Clean Air litigation discussed below. Ameren Missouri’s current plan for compliance with existing air emission regulations includes burning ultra-low-sulfur coal and installing new or optimizing existing pollution control equipment. The actual amount of capital expenditures required to comply with existing environmental regulations may vary substantially from the above estimate because of uncertainty as to whether the EPA will substantially revise regulatory obligations, exactly which compliance strategies will be used and their ultimate cost, among other things.

The following sections describe the more significant environmental laws and rules and environmental enforcement and remediation matters that affect or could affect our operations. The EPA has initiated an administrative review of several regulations and proposed amendments to regulations and guidelines, including to the effluent limitation guidelines and the CCR Rule, which could ultimately result in the revision of all or part of such rules.

Clean Air Act

Federal and state laws, including CSAPR, regulate emissions of SO₂ and NO_x through the reduction of emissions at their source and the use and retirement of emission allowances. The first phase of the CSAPR emission reduction requirements became effective in 2015. The second phase of emission reduction requirements, which were revised by the EPA in 2016, became effective in 2017; additional emission reduction requirements may apply in subsequent years. To achieve compliance with the CSAPR, Ameren Missouri burns ultra-low-sulfur coal, operates two scrubbers at its Sioux energy center, and optimizes other existing pollution control equipment. Ameren Missouri expects to incur additional costs to lower its emissions at one or more of its energy centers to comply with the CSAPR in future years. These higher costs are expected to be recovered from customers through the FAC or higher base rates.

CO₂ Emissions Standards

In 2015, the EPA issued the Clean Power Plan, which would have established CO₂ emissions standards applicable to existing power plants. The United States Supreme Court stayed the rule in February 2016. In July 2019, the EPA finalized regulations that repealed the Clean Power Plan and replaced it with the Affordable Clean Energy Rule, which establishes emission guidelines for states to follow in developing plans to limit CO₂ emissions from coal-fired electric generating units. The EPA has identified certain efficiency measures as the best system of emission reduction for coal-fired electric generating units. The Affordable Clean Energy Rule went into effect on September 6, 2019. The rule requires the state of Missouri to develop a compliance plan and submit it to the EPA for approval by September 2022. The plan is expected to include a standard of performance for each affected generating unit. We are evaluating the impact of the adoption and implementation of the Affordable Clean Energy Rule and, along with other stakeholders, will be working with the state of Missouri to develop the compliance plan submitted to the EPA. At this time, we cannot predict the outcome of Missouri’s compliance plan development process. As such, the impact on the results of operations, financial position, and liquidity of Ameren and Ameren Missouri is uncertain. We also cannot predict the outcome of any potential legal challenges to the rule.

NSR and Clean Air Litigation

In January 2011, the Department of Justice, on behalf of the EPA, filed a complaint against Ameren Missouri in the United States District Court for the Eastern District of Missouri alleging that in performing projects at its Rush Island coal-fired energy center in 2007 and 2010, Ameren Missouri violated provisions of the Clean Air Act and Missouri law. In January 2017, the district court issued a liability ruling and in September 2019 entered a final judgment that ordered Ameren Missouri to install a flue gas desulfurization system at the Rush Island energy center and a dry sorbent injection system at the Labadie energy center. There were no fines in the order. In October 2019, Ameren Missouri appealed the district court’s ruling to the United States Court of Appeals for the Eighth Circuit. Additionally, in October 2019, following a request by Ameren Missouri, the district court stayed the majority of its order while the case is appealed. Ameren Missouri believes that the district court both misinterpreted and misapplied the law in its ruling. We are unable to predict the ultimate resolution of this matter. Based on the initial procedural schedule, the Court of Appeals for the Eighth Circuit is expected to hear oral arguments in 2020; however, it is under no deadline to issue a ruling in this case.

The ultimate resolution of this matter could have a material adverse effect on the results of operations, financial position, and liquidity of Ameren and Ameren Missouri. Among other things and subject to economic and regulatory considerations, resolution of this matter could result in increased capital expenditures for the installation of pollution control equipment, as well as increased operations and maintenance expenses. Capital expenditures to comply with the district court’s order for installation of a flue gas desulfurization system at the Rush Island energy center are estimated at approximately $1 billion. Further, the flue gas desulfurization system would result in additional operation and maintenance expenses of $30 million to $50 million annually for the life of the energy center. Estimates for the additional capital expenditures and operation and maintenance expenses for the Labadie energy center to comply with the district court’s order are under development. As a

result of the district court’s stay, Ameren Missouri does not expect to make significant capital expenditures or incur operations and maintenance expenses related to the district court’s order while the case is under appeal.

Clean Water Act

In July 2018, the United States Court of Appeals for the Second Circuit upheld the EPA’s Section 316(b) Rule applicable to cooling water intake structures at existing power plants. The rule requires a case-by-case evaluation and plan for reducing the number of aquatic organisms impinged on a power plant’s cooling water intake screens or entrained through the plant’s cooling water system. All of Ameren Missouri’s coal-fired and nuclear energy centers are subject to the cooling water intake structures rule. Requirements of the rule are being implemented by Ameren Missouri during the permit renewal process of each energy center’s water discharge permit, which is expected to be completed by 2023.

In 2015, the EPA issued a rule to revise the effluent limitation guidelines applicable to steam electric generating units. These guidelines established national standards for water discharges that are based on the effectiveness of available control technology. The EPA’s 2015 rule prohibits effluent discharges of certain waste streams and imposes more stringent limitations on certain water discharges from power plants. In September 2017, the EPA published a rule that postponed the compliance dates by two years for the limitations applicable to two specific waste streams so that it could potentially revise those standards. To meet the requirements of the guidelines, Ameren Missouri is constructing wastewater treatment facilities and dry ash handling systems at three of its energy centers and is scheduled to complete the projects in 2020. Estimated capital expenditures to complete these projects are included in the CCR management compliance plan, discussed below.

CCR Management

In 2015, the EPA issued the CCR rule, which established requirements for the management and disposal of CCR from coal-fired power plants. These regulations affect CCR disposal and handling costs at Ameren Missouri’s energy centers. Ameren Missouri is in the process of closing its surface impoundments, with the last of such closures scheduled for 2023. In July 2018, the EPA issued revisions to the CCR rule, proposed additional revisions, and indicated that additional revisions to the CCR rule are likely. Ameren and Ameren Missouri have AROs of $160 million recorded on their respective balance sheets as of September 30, 2019, associated with CCR storage facilities. Ameren Missouri estimates it will need to make capital expenditures of $150 million to $200 million from 2019 through 2023 to implement its CCR management compliance plan, which includes installation of dry ash handling systems, wastewater treatment facilities, and groundwater monitoring equipment.

Remediation

The Ameren Companies are involved in a number of remediation actions to clean up sites impacted by the use or disposal of materials containing hazardous substances. Federal and state laws can require responsible parties to fund remediation regardless of their degree of fault, the legality of original disposal, or the ownership of a disposal site. Ameren Missouri and Ameren Illinois have each been identified as a potentially responsible party at several contaminated sites.

As of September 30, 2019, Ameren Illinois has remediated the majority of the 44 former MGP sites in Illinois it owned or for which it was otherwise responsible. Ameren Illinois estimates it could substantially conclude remediation efforts at the remaining sites by 2023. The ICC allows Ameren Illinois to recover such remediation and related litigation costs from its electric and natural gas utility customers through environmental cost riders. Costs are subject to annual prudence review by the ICC. As of September 30, 2019, Ameren Illinois estimated the remaining obligation related to these former MGP sites at $139 million to $209 million. Ameren and Ameren Illinois recorded a liability of $139 million to represent the estimated minimum obligation for these sites, as no other amount within the range was a better estimate.

The scope of the remediation activities at these former MGP sites may increase as remediation efforts continue. Considerable uncertainty remains in these estimates because many site-specific factors can influence the ultimate actual costs, including unanticipated underground structures, the degree to which groundwater is encountered, regulatory changes, local ordinances, and site accessibility. The actual costs and timing of completion may vary substantially from these estimates.

Our operations or those of our predecessor companies involve the use of, disposal of, and, in appropriate circumstances, the cleanup of substances regulated under environmental laws. We are unable to determine whether such practices will result in future environmental commitments or will affect our results of operations, financial position, or liquidity.

Callaway Energy Center

9 Months Ended

Sep. 30, 2019

Nuclear Waste Matters [Abstract]

CALLAWAY ENERGY CENTER

See Note 9 – Callaway Energy Center under Part II, Item 8, of the Form 10-K for information regarding spent nuclear fuel recovery, recovery of decommissioning costs, and the nuclear decommissioning trust fund. The fair value of the trust fund for Ameren Missouri’s Callaway energy center is reported as “Nuclear decommissioning trust fund” in Ameren’s and Ameren Missouri’s balance sheets. This amount is legally restricted and may be used only to fund the costs of nuclear decommissioning. Changes in the fair value of the trust fund are

recorded as an increase or decrease to the nuclear decommissioning trust fund, with an offsetting adjustment to the related regulatory liability.

Insurance

The following table presents insurance coverage at Ameren Missouri’s Callaway energy center as of September 30, 2019. The property coverage and the nuclear liability coverage renewal dates are April 1 and January 1, respectively, of each year. Both coverages were renewed in 2019.



Type and Source of Coverage	Maximum Coverages			Maximum Assessments for Single Incidents
Public liability and nuclear worker liability:
American Nuclear Insurers	$	450		$	—
Pool participation	13,486		^(a)	138		^(b)
	$	13,936	^(c)	$	138
Property damage:
NEIL and EMANI	$	3,200	^(d)	$	28	^(e)
Replacement power:
NEIL	$	490	^(f)	$	7	^(e)


(a)	Provided through mandatory participation in an industrywide retrospective premium assessment program. The maximum coverage available is dependent on the number of United States commercial reactors participating in the program.


(b)	Retrospective premium under the Price-Anderson Act. This is subject to retrospective assessment with respect to a covered loss in excess of $450 million in the event of an incident at any licensed United States commercial reactor, payable at $21 million per year.


(c)	Limit of liability for each incident under the Price-Anderson liability provisions of the Atomic Energy Act of 1954, as amended. This limit is subject to change to account for the effects of inflation and changes in the number of licensed reactors.


(d)	NEIL provides $2.7 billion in property damage, stabilization, decontamination, and premature decommissioning insurance for radiation events and $2.3 billion in property damage insurance for nonradiation events. EMANI provides $490 million in property damage insurance for both radiation and nonradiation events.


(e)	All NEIL insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL.


(f)	Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity up to $4.5 million for 52 weeks, which commences after the first twelve weeks of an outage, plus up to $3.6 million per week for a minimum of 71 weeks thereafter for a total not exceeding the policy limit of $490 million. Nonradiation events are limited to $328 million.

The Price-Anderson Act is a federal law that limits the liability for claims from an incident involving any licensed United States commercial nuclear energy center. The limit is based on the number of licensed reactors. The limit of liability and the maximum potential annual payments are adjusted at least every five years for inflation to reflect changes in the Consumer Price Index. The most recent five-year inflationary adjustment became effective in November 2018. Owners of nuclear reactors cover this exposure through a combination of private insurance and mandatory participation in a financial protection pool, as established by the Price-Anderson Act.

Losses resulting from terrorist attacks on nuclear facilities insured by NEIL are subject to industrywide aggregates, such that terrorist acts against one or more commercial nuclear power plants within a stated time period would be treated as a single event, and the owners of the nuclear power plants would share the limit of liability. NEIL policies have an aggregate limit of $3.2 billion within a 12-month period for radiation events, or $1.8 billion for events not involving radiation contamination. The EMANI policies are not subject to industrywide aggregates in the event of terrorist attacks on nuclear facilities.

If losses from a nuclear incident at the Callaway energy center exceed the limits of, or are not covered by insurance, or if coverage is unavailable, Ameren Missouri is at risk for any uninsured losses. If a serious nuclear incident were to occur, it could have a material adverse effect on Ameren’s and Ameren Missouri’s results of operations, financial position, or liquidity.

Retirement Benefits

9 Months Ended

Sep. 30, 2019

Defined Benefit Plan [Abstract]

RETIREMENT BENEFITS

The following table presents the components of the net periodic benefit cost (income) incurred for Ameren’s pension and postretirement benefit plans for the three and nine months ended September 30, 2019 and 2018:



	Pension Benefits												Postretirement Benefits
	Three Months						Nine Months						Three Months						Nine Months
	2019			2018			2019			2018			2019			2018			2019			2018
Service cost^(a)	$	22		$	25		$	66		$	75		$	4		$	6		$	13		$	16
Non-service cost components:
Interest cost	46			42			139			126			11			10			32			30
Expected return on plan assets	(69		)	(68		)	(207		)	(206		)	(19		)	(20		)	(57		)	(58		)
Amortization of:
Prior service benefit	—			—			—			—			(1		)	(1		)	(4		)	(3		)
Actuarial loss (gain)	6			17			19			51			(4		)	(2		)	(11		)	(5		)
Total non-service cost components^(b)	$	(17	)	$	(9	)	$	(49	)	$	(29	)	$	(13	)	$	(13	)	$	(40	)	$	(36	)
Net periodic benefit cost (income)	$	5		$	16		$	17		$	46		$	(9	)	$	(7	)	$	(27	)	$	(20	)


(a)	Service cost, net of capitalization, is reflected in “Operating Expenses – Other operations and maintenance” on Ameren’s statement of income.


(b)	Non-service cost components are reflected in “Other Income, Net” on Ameren’s statement of income. See Note 5 – Other Income, Net for additional information.

Ameren Missouri and Ameren Illinois are responsible for their respective shares of Ameren’s pension and postretirement costs. The following table presents the respective share of net periodic pension and other postretirement benefit costs (income) incurred for the three and nine months ended September 30, 2019 and 2018:



	Pension Benefits												Postretirement Benefits
	Three Months						Nine Months						Three Months						Nine Months
	2019			2018			2019			2018			2019			2018			2019			2018
Ameren Missouri^(a)	$	1		$	6		$	3		$	17		$	(1	)	$	(1	)	$	(4	)	$	(1	)
Ameren Illinois	5			11			15			30			(8		)	(6		)	(23		)	(19		)
Other	(1		)	(1		)	(1		)	(1		)	—			—			—			—
Ameren^(a)	$	5		$	16		$	17		$	46		$	(9	)	$	(7	)	$	(27	)	$	(20	)

(a)

Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates.

Income Taxes

9 Months Ended

Sep. 30, 2019

Income Tax Disclosure [Abstract]

Income Taxes

INCOME TAXES

The following table presents a reconciliation of the federal statutory corporate income tax rate to the effective income tax rate for the three and nine months ended September 30, 2019 and 2018:



	Ameren			Ameren Missouri			Ameren Illinois
	2019	2018		2019	2018		2019	2018
Three Months
Federal statutory corporate income tax rate:	21%	21%		21%	21%		21%	21%
Increases (decreases) from:
Amortization of excess deferred taxes	(7)	(6)	^(a)	(11)	(7)	^(a)	(3)	(3)
Depreciation differences	—	—		—	—		—	(1)
Amortization of deferred investment tax credit	(1)	(1)		(1)	—		—	—
State tax	6	6		5	4		6	5
Stock-based compensation	1	—		—	—		—	—
TCJA	—	3	^(b)	—	—		—	4	^(b)
Effective income tax rate	20%	23%		14%	18%		24%	26%



	Ameren			Ameren Missouri			Ameren Illinois
	2019	2018		2019	2018		2019	2018
Nine Months
Federal statutory corporate income tax rate:	21%	21%		21%	21%		21%	21%
Increases (decreases) from:
Amortization of excess deferred taxes	(7)	(3)	^(a)	(12)	(4)	^(a)	(4)	(4)
Amortization of deferred investment tax credit	(1)	(1)		(1)	—		—	—
State tax	6	6		5	4		7	7
Stock-based compensation	(1)	—		—	—		—	—
TCJA	—	1	^(b)	—	—		—	1	^(b)
Other	—	(1)		—	—		—	—
Effective income tax rate	18%	23%		13%	21%		24%	25%


(a)	Based on an order by the MoPSC in July 2018, Ameren Missouri began amortizing excess deferred taxes in August 2018.


(b)	The Ameren Companies updated their respective provisional estimates recorded related to TCJA, as discussed below.

Federal Tax Reform

As of December 31, 2017, the Ameren Companies made provisional estimates for the measurement and accounting of certain effects of the TCJA in accordance with SEC guidance, which provides for a one-year period in which to complete the required analysis and update provisional estimates. During the three and nine months ended September 30, 2018, Ameren, Ameren Missouri, and Ameren Illinois updated their respective provisional estimates and recorded $13 million, $4 million, and $4 million, respectively, of income tax expense, primarily due to the application of proposed IRS regulations on depreciation transition rules. As of December 31, 2018, Ameren, Ameren Missouri, and Ameren Illinois completed their accounting for certain effects of the TCJA.

Supplemental Information

9 Months Ended

Sep. 30, 2019

Supplemental Information [Abstract]

Supplemental Information

SUPPLEMENTAL INFORMATION

Cash, Cash Equivalents, and Restricted Cash

The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets and the statements of cash flows as of September 30, 2019, and December 31, 2018:



	September 30, 2019						December 31, 2018
	Ameren		Ameren Missouri		Ameren Illinois		Ameren		Ameren Missouri		Ameren Illinois
Cash and cash equivalents	$	20	$	—	$	—	$	16	$	—	$	—
Restricted cash included in “Other current assets”	14		4		6		13		4		6
Restricted cash included in “Other assets”	111		—		111		74		—		74
Restricted cash included in “Nuclear decommissioning trust fund”	10		10		—		4		4		—
Total cash, cash equivalents, and restricted cash	$	155	$	14	$	117	$	107	$	8	$	80

Restricted cash included in “Other current assets” primarily represents funds held by an irrevocable Voluntary Employee Beneficiary Association (VEBA) trust, which provides health care benefits for active employees. Restricted cash included in “Other assets” on Ameren’s and Ameren Illinois’ balance sheets primarily represents amounts collected under a cost recovery rider restricted for use in the procurement of renewable energy credits and amounts in a trust fund restricted for the use of funding certain asbestos-related claims.

Accounts Receivable

“Accounts receivable – trade” on Ameren’s and Ameren Illinois’ balance sheets include certain receivables purchased at a discount from alternative retail electric suppliers that elect to participate in the utility consolidated billing program. At September 30, 2019, and December 31, 2018, “Other current liabilities” on Ameren’s and Ameren Illinois’ balance sheets included payables for purchased receivables of $37 million and $33 million, respectively.

For the three and nine months ended September 30, 2019 and 2018, the Ameren Companies recorded immaterial bad debt expense.

Leases

In the first quarter of 2019, we adopted authoritative accounting guidance related to leases, which affected our financial position, but did not materially affect our results of operations or liquidity. The most significant impact for us was the recognition of right-of-use assets and lease liabilities for operating leases, while the accounting for our finance leases remained substantially unchanged. Ameren and Ameren Missouri recognized right-of-use assets and offsetting lease liabilities of $38 million and $36 million at January 1, 2019, respectively, primarily

related to rail car leases. The effect of the adoption was immaterial at Ameren Illinois. No adjustment to comparative periods was made. We elected the available practical expedients upon adoption.

Ameren Missouri primarily leases rail cars under operating lease arrangements for the transportation of coal inventory to its energy centers. Although Ameren Missouri has options to renew a portion of these arrangements for up to five years on similar terms, the exercise of these options was not assumed in the recognition of right-of-use assets and lease obligations. For rail car leases, we account for the lease and non-lease components as a single lease component.

The operating lease expense and the cash paid for amounts included in the measurement of operating lease liabilities at Ameren and Ameren Missouri were immaterial for the three and nine months ended September 30, 2019 and 2018.

The following table provides supplemental balance sheet information related to operating leases as of September 30, 2019:



	Ameren			Ameren Missouri
Other assets	$	38		$	36
Other current liabilities	8			7
Other deferred credits and liabilities	30			29
Weighted average remaining operating lease term	6 years			6 years
Weighted average discount rate^(a)	3.5		%	3.4		%


(a)	As an implicit rate is not readily determinable under most of our lease agreements, we use our incremental borrowing rate based on the information available at commencement date in determining the present value of lease payments. We use an implicit rate when readily determinable.

The following table presents Ameren’s and Ameren Missouri’s remaining maturities of operating lease liabilities as of September 30, 2019:



	Ameren		Ameren Missouri
2019	$	2	$	2
2020	8		8
2021	8		7
2022	7		6
2023	6		6
Thereafter	11		10
Total lease payments	42		39
Less imputed interest	4		3
Total^(a)	$	38	$	36

(a)

The amount of remaining maturities of operating lease liabilities under previous authoritative accounting guidance as of December 31, 2018, is materially consistent with the amount as of September 30, 2019. Maturities of certain financing arrangements, including the Peno Creek and Audrain energy centers' long-term agreements, are no longer required to be disclosed as lease-related maturities. See Note 5 – Long-Term Debt and Equity Financings under Part II, Item 8, in the Form 10-K for further information on financing arrangements.

Supplemental Cash Flow Information

The following table provides noncash financing and investing activity excluded from the statements of cash flows for the nine months ended September 30, 2019 and 2018:



	September 30, 2019								September 30, 2018
	Ameren			Ameren Missouri		Ameren Illinois			Ameren		Ameren Missouri		Ameren Illinois
Investing
Exchange of bond investments for the extinguishment of senior unsecured notes^(a)	$	17		$	—	$	17		$	—	$	—	$	—
Accrued capital expenditures	273			138		128			240		94		133
Net realized and unrealized gain – nuclear decommissioning trust fund	100			100		—			33		33		—
Financing
Exchange of bond investments for the extinguishment of senior unsecured notes^(a)	$	(17	)	$	—	$	(17	)	$	—	$	—	$	—
Issuance of common stock for stock-based compensation	54			—		—			35		—		—


(a)	See Note 4 – Long-term Debt and Equity Financings for additional information.

Asset Retirement Obligations

The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the nine months ended September 30, 2019:



	Ameren Missouri				Ameren Illinois			Ameren
Balance at December 31, 2018	$	646		^(a)	$	4	^(b)	$	650		^(a)
Liabilities settled	(10		)		—			(10		)
Accretion	21			^(c)	—			21			^(c)
Change in estimates	33			^(d)	—			33			^(d)
Balance at September 30, 2019	$	690		^(a)	$	4	^(b)	$	694		^(a)


(a)	Balance included $23 million in “Other current liabilities” on the balance sheet as of both December 31, 2018, and September 30, 2019.


(b)	Included in “Other deferred credits and liabilities” on the balance sheet.


(c)	Accretion expense attributable to Ameren Missouri was recorded as a decrease to regulatory liabilities.


(d)	Ameren Missouri changed its fair value estimate primarily due to an increase in the cost estimate for closure of certain CCR storage facilities.

Stock-based Compensation

The following table summarizes Ameren's nonvested performance share unit and restricted stock unit activity for the nine months ended September 30, 2019:



	Performance Share Units					Restricted Stock Units
	Share Units		Weighted-average Fair Value per Share Unit			Stock Units		Weighted-average Fair Value per Stock Unit
Nonvested at January 1, 2019^(a)	682,811		$	56.58		155,253		$	57.38
Granted	297,728		67.42		^(b)	128,883		65.49
Forfeitures	(33,195	)	64.34			(11,028	)	62.74
Vested and undistributed^(c)	(180,823	)	62.24			(40,616	)	61.91
Vested and distributed	(176,923	)	44.13			(2,403	)	54.30
Nonvested at September 30, 2019^(d)	589,598		$	63.62		230,089		$	60.90


(a)	Does not include 619,783 performance share units and 26,557 restricted stock units that were vested and undistributed.


(b)	Significant inputs to the Monte Carlo simulation model used to calculate the fair value of performance share units granted include Ameren’s closing common share price of $65.23 at December 31, 2018, Ameren’s common stock volatility of 17%, a volatility range for the peer group of 15% to 25%, and a three-year risk-free rate of 2.46%.


(c)	Vested and undistributed units are awards that vest on a pro-rata basis due to attainment of retirement eligibility by certain employees, but have not yet been distributed. For vested and undistributed performance share units, the number of shares issued for retirement-eligible employees will vary depending on actual performance over the three year performance period.


(d)	Does not include 448,831 performance share units and 67,173 restricted stock units that were vested and undistributed.

For the nine months ended September 30, 2019 and 2018, excess tax benefits associated with the settlement of stock-based compensation awards reduced income tax expense by $14 million and $6 million, respectively.

Deferred Compensation

As of September 30, 2019, and December 31, 2018, “Other deferred credits and liabilities” on Ameren’s balance sheet included deferred compensation obligations of $78 million and $80 million, respectively, recorded at the present value of future benefits to be paid.

Operating Revenues

As of September 30, 2019 and 2018, our remaining performance obligations for contracts with a term greater than one year were immaterial. The Ameren Companies elected not to disclose the aggregate amount of the transaction price allocated to the performance obligations that are unsatisfied as of the end of the reporting period for contracts with an initial expected term of one year or less.

See Note 14 – Segment Information for disaggregated revenue information.

Excise Taxes

Ameren Missouri and Ameren Illinois collect from their customers excise taxes, including municipal and state excise taxes and gross receipts taxes, that are levied on the sale or distribution of natural gas and electricity. The following table presents the excise taxes recorded on a gross basis in “Operating Revenues – Electric,” “Operating Revenues – Natural gas” and “Operating Expenses – Taxes other than income taxes” on the statements of income for the three and nine months ended September 30, 2019 and 2018:



	Three Months				Nine Months
	2019		2018		2019		2018
Ameren Missouri	$	49	$	52	$	118	$	133
Ameren Illinois	27		26		91		89
Ameren	$	76	$	78	$	209	$	222

Earnings per Share

Earnings per basic and diluted share are computed by dividing “Net Income Attributable to Ameren Common Shareholders” by the weighted-average number of basic and diluted common shares outstanding, respectively, during the applicable period. The weighted-average shares outstanding for earnings per diluted share includes the incremental effects of stock-based performance share units, restricted stock units, and the forward sale agreement when the impact would be dilutive, as calculated using the treasury stock method.

The following table presents Ameren’s basic and diluted earnings per share calculations and reconciles the weighted-average number of common shares outstanding to the diluted weighted-average number of common shares outstanding for the three and nine months ended September 30, 2019 and 2018:



	Three Months		Nine Months
	2019	2018	2019	2018
Weighted-average Common Shares Outstanding – Basic	245.9	244.1	245.5	243.6
Assumed settlement of performance share units and restricted stock units	1.4	2.2	1.4	1.9
Dilutive effect of forward sale agreement	0.2	—	0.1	—
Weighted-average Common Shares Outstanding – Diluted^(a)	247.5	246.3	247.0	245.5

(a)

There were no potentially dilutive securities excluded from the earnings per diluted share calculations for the three and nine months ended September 30, 2019 and 2018.

Segment Information

9 Months Ended

Sep. 30, 2019

Segment Reporting [Abstract]

SEGMENT INFORMATION

Ameren has four segments: Ameren Missouri, Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Transmission. The Ameren Missouri segment includes all of the operations of Ameren Missouri. Ameren Illinois Electric Distribution consists of the electric distribution business of Ameren Illinois. Ameren Illinois Natural Gas consists of the natural gas business of Ameren Illinois. Ameren Transmission primarily consists of the aggregated electric transmission businesses of Ameren Illinois and ATXI. The category called Other primarily includes Ameren (parent) activities and Ameren Services.

Ameren Missouri has one segment. Ameren Illinois has three segments: Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Illinois Transmission. See Note 1 – Summary of Significant Accounting Policies for additional information regarding the operations of Ameren Missouri, Ameren Illinois, and ATXI.

Segment operating revenues and a majority of operating expenses are directly recognized and incurred by Ameren Illinois at each Ameren Illinois segment. Common operating expenses, miscellaneous income and expenses, interest charges, and income tax expense are allocated by Ameren Illinois to each Ameren Illinois segment based on factors that primarily relate to the nature of the cost. Additionally, Ameren Illinois Transmission earns revenue from transmission services provided to Ameren Illinois Electric Distribution, other retail electric suppliers, and wholesale customers. The transmission expense for Illinois customers who have elected to purchase their power from Ameren Illinois is recovered through a cost recovery mechanism with no net effect on Ameren Illinois Electric Distribution earnings, as costs are offset by corresponding revenues. Transmission revenues from these transactions are reflected in Ameren Transmission’s and Ameren Illinois Transmission’s operating revenues. An intersegment elimination at Ameren and Ameren Illinois occurs to eliminate these transmission revenues and expenses.

The following tables present revenues, net income attributable to common shareholders, and capital expenditures by segment at Ameren and Ameren Illinois for the three and nine months ended September 30, 2019 and 2018. Ameren, Ameren Missouri, and Ameren Illinois management review segment capital expenditure information rather than any individual or total asset amount.

Ameren



	Ameren Missouri		Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas			Ameren Transmission			Other			Intersegment Eliminations			Ameren
Three Months 2019:
External revenues	$	1,050	$	374	$	107		$	109		$	—		$	—		$	1,640
Intersegment revenues	9		15		5			19			—			(29		)	19		^(b)
Net income (loss) attributable to Ameren common shareholders	300		32		(1		)	53		^(a)	(20		)	—			364
Capital expenditures	256		139		113			129			(7		)	6			636
Three Months 2018:
External revenues	$	1,118	$	392	$	116		$	98		$	—		$	—		$	1,724
Intersegment revenues	11		—		—			15			—			(26		)	—
Net income (loss) attributable to Ameren common shareholders	294		35		—			48		^(a)	(20		)	—			357
Capital expenditures	210		135		111			124			1			(4		)	577
Nine Months 2019:
External revenues	$	2,591	$	1,118	$	563		$	303		$	—		$	—		$	4,575
Intersegment revenues	24		17		5			48			—			(75		)	19		^(b)
Net income (loss) attributable to Ameren common shareholders	446		105		57			139		^(a)	(13		)	—			734
Capital expenditures	751		390		241			377			3			(1		)	1,761
Nine Months 2018:
External revenues	$	2,848	$	1,177	$	569		$	278		$	—		$	—		$	4,872
Intersegment revenues	28		2		—			42			—			(72		)	—
Net income (loss) attributable to Ameren common shareholders	500		101		49			121		^(a)	(24		)	—			747
Capital expenditures	664		389		237			399			6			(6		)	1,689


(a)	Ameren Transmission earnings include an allocation of financing costs from Ameren (parent).

(b)

Intersegment revenues at Ameren include $14 million and $5 million of revenue from Ameren Illinois Electric Distribution and Ameren Illinois Natural Gas, respectively, for the three and nine months ended September 30, 2019, for a software licensing agreement with Ameren Missouri. Under authoritative accounting guidance for rate-regulated entities, the revenue recognized by Ameren Illinois was not eliminated upon consolidation. See Note 8 – Related-party Transactions under Part I, Item 1, of this report for additional information.

Ameren Illinois



	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas			Ameren Illinois Transmission		Intersegment Eliminations			Ameren Illinois
Three Months 2019:
External revenues	$	389	$	112		$	63	$	—		$	564
Intersegment revenues	—		—			18		(18		)	—
Net income (loss) available to common shareholder	32		(1		)	34		—			65
Capital expenditures	139		113			92		—			344
Three Months 2018:
External revenues	$	392	$	116		$	56	$	—		$	564
Intersegment revenues	—		—			15		(15		)	—
Net income available to common shareholder	35		—			28		—			63
Capital expenditures	135		111			99		—			345
Nine Months 2019:
External revenues	$	1,135	$	568		$	170	$	—		$	1,873
Intersegment revenues	—		—			47		(47		)	—
Net income available to common shareholder	105		57			85		—			247
Capital expenditures	390		241			269		—			900
Nine Months 2018:
External revenues	$	1,179	$	569		$	154	$	—		$	1,902
Intersegment revenues	—		—			41		(41		)	—
Net income available to common shareholder	101		49			70		—			220
Capital expenditures	389		237			321		—			947

The following tables present disaggregated revenues by segment at Ameren and Ameren Illinois for the three and nine months ended September 30, 2019 and 2018. Economic factors affect the nature, timing, amount, and uncertainty of revenues and cash flows in a similar manner across customer classes. Revenues from alternative revenue programs have a similar distribution among customer classes as revenues from contracts with customers. Other revenues not associated with contracts with customers are presented in the Other customer classification, along with electric transmission and off-system revenues.

Ameren



	Ameren Missouri		Ameren Illinois Electric Distribution			Ameren Illinois Natural Gas			Ameren Transmission		Intersegment Eliminations			Ameren
Three Months 2019:
Residential	$	489	$	224		$	—		$	—	$	—		$	713
Commercial	394		123			—			—		—			517
Industrial	94		27			—			—		—			121
Other	63		15		^(a)	—			128		(29		)	177
Total electric revenues	$	1,040	$	389		$	—		$	128	$	(29	)	$	1,528
Residential	$	8	$	—		$	65		$	—	$	—		$	73
Commercial	4		—			17			—		—			21
Industrial	1		—			2			—		—			3
Other	6		—			28		^(a)	—		—			34
Total gas revenues	$	19	$	—		$	112		$	—	$	—		$	131
Total revenues^(c)	$	1,059	$	389		$	112		$	128	$	(29	)	$	1,659



	Ameren Missouri			Ameren Illinois Electric Distribution			Ameren Illinois Natural Gas			Ameren Transmission		Intersegment Eliminations			Ameren
Three Months 2018:
Residential	$	508		$	223		$	—		$	—	$	—		$	731
Commercial	417			131			—			—		—			548
Industrial	101			28			—			—		—			129
Other	85		^(b)	10			—			113		(26		)	182		^(b)
Total electric revenues	$	1,111		$	392		$	—		$	113	$	(26	)	$	1,590
Residential	$	8		$	—		$	68		$	—	$	—		$	76
Commercial	3			—			20			—		—			23
Industrial	1			—			1			—		—			2
Other	6			—			27			—		—			33
Total gas revenues	$	18		$	—		$	116		$	—	$	—		$	134
Total revenues^(c)	$	1,129		$	392		$	116		$	113	$	(26	)	$	1,724
Nine Months 2019:
Residential	$	1,134		$	640		$	—		$	—	$	—		$	1,774
Commercial	943			370			—			—		—			1,313
Industrial	226			94			—			—		—			320
Other	214			31		^(a)	—			351		(75		)	521
Total electric revenues	$	2,517		$	1,135		$	—		$	351	$	(75	)	$	3,928
Residential	$	56		$	—		$	399		$	—	$	—		$	455
Commercial	24			—			105			—		—			129
Industrial	3			—			9			—		—			12
Other	15			—			55		^(a)	—		—			70
Total gas revenues	$	98		$	—		$	568		$	—	$	—		$	666
Total revenues^(c)	$	2,615		$	1,135		$	568		$	351	$	(75	)	$	4,594
Nine Months 2018:
Residential	$	1,272		$	663		$	—		$	—	$	—		$	1,935
Commercial	1,033			381			—			—		—			1,414
Industrial	249			96			—			—		—			345
Other	228		^(b)	39			—			320		(72		)	515		^(b)
Total electric revenues	$	2,782		$	1,179		$	—		$	320	$	(72	)	$	4,209
Residential	$	62		$	—		$	408		$	—	$	—		$	470
Commercial	25			—			113			—		—			138
Industrial	3			—			12			—		—			15
Other	4			—			36			—		—			40
Total gas revenues	$	94		$	—		$	569		$	—	$	—		$	663
Total revenues^(c)	$	2,876		$	1,179		$	569		$	320	$	(72	)	$	4,872


(a)	Includes $14 million and $5 million for Ameren Illinois Electric Distribution and Ameren Illinois Natural Gas, respectively, for the three and nine months ended September 30, 2019, for a software licensing agreement with Ameren Missouri. See Note 8 – Related-party Transactions for additional information.


(b)	Includes $13 million and $60 million for the three and nine months ended September 30, 2018, respectively, for the reduction to revenue for the excess amounts collected in rates to be refunded related to the TCJA from January 1, 2018, through July 31, 2018. See Note 2 – Rate and Regulatory Matters under Part II, Item 8, of the Form 10-K for additional information.


(c)	The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the three and nine months ended September 30, 2019 and 2018:



	Ameren Missouri		Ameren Illinois Electric Distribution			Ameren Illinois Natural Gas			Ameren Transmission			Ameren
Three Months 2019:
Revenues from alternative revenue programs	$	26	$	(145	)	$	1		$	(12	)	$	(130	)
Other revenues not from contracts with customers	5		1			1			—			7
Three Months 2018:
Revenues from alternative revenue programs	$	1	$	(98	)	$	(2	)	$	(12	)	$	(111	)
Other revenues not from contracts with customers	3		1			1			—			5



	Ameren Missouri			Ameren Illinois Electric Distribution			Ameren Illinois Natural Gas			Ameren Transmission			Ameren
Nine Months 2019:
Revenues from alternative revenue programs	$	41		$	(111	)	$	2		$	(25	)	$	(93	)
Other revenues not from contracts with customers	14			5			2			—			21
Nine Months 2018:
Revenues from alternative revenue programs	$	(8	)	$	(52	)	$	(10	)	$	(21	)	$	(91	)
Other revenues not from contracts with customers	22			14			2			—			38

Ameren Illinois



	Ameren Illinois Electric Distribution			Ameren Illinois Natural Gas			Ameren Illinois Transmission		Intersegment Eliminations			Ameren Illinois
Three Months 2019:
Residential	$	224		$	65		$	—	$	—		$	289
Commercial	123			17			—		—			140
Industrial	27			2			—		—			29
Other	15		^(a)	28		^(a)	81		(18		)	106
Total revenues^(b)	$	389		$	112		$	81	$	(18	)	$	564
Three Months 2018:
Residential	$	223		$	68		$	—	$	—		$	291
Commercial	131			20			—		—			151
Industrial	28			1			—		—			29
Other	10			27			71		(15		)	93
Total revenues^(b)	$	392		$	116		$	71	$	(15	)	$	564
Nine Months 2019:
Residential	$	640		$	399		$	—	$	—		$	1,039
Commercial	370			105			—		—			475
Industrial	94			9			—		—			103
Other	31		^(a)	55		^(a)	217		(47		)	256
Total revenues^(b)	$	1,135		$	568		$	217	$	(47	)	$	1,873
Nine Months 2018:
Residential	$	663		$	408		$	—	$	—		$	1,071
Commercial	381			113			—		—			494
Industrial	96			12			—		—			108
Other	39			36			195		(41		)	229
Total revenues^(b)	$	1,179		$	569		$	195	$	(41	)	$	1,902


(a)	Includes $14 million and $5 million for Ameren Illinois Electric Distribution and Ameren Illinois Natural Gas, respectively, for the three and nine months ended September 30, 2019, for a software licensing agreement with Ameren Missouri. See Note 8 – Related-party Transactions for additional information.


(b)	The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the Ameren Illinois segments for the three and nine months ended September 30, 2019 and 2018:



	Ameren Illinois Electric Distribution			Ameren Illinois Natural Gas			Ameren Illinois Transmission			Ameren Illinois
Three Months 2019:
Revenues from alternative revenue programs	$	(145	)	$	1		$	(12	)	$	(156	)
Other revenues not from contracts with customers	1			1			—			2
Three Months 2018:
Revenues from alternative revenue programs	$	(98	)	$	(2	)	$	(10	)	$	(110	)
Other revenues not from contracts with customers	1			1			—			2



	Ameren Illinois Electric Distribution			Ameren Illinois Natural Gas			Ameren Illinois Transmission			Ameren Illinois
Nine Months 2019:
Revenues from alternative revenue programs	$	(111	)	$	2		$	(26	)	$	(135	)
Other revenues not from contracts with customers	5			2			—			7
Nine Months 2018:
Revenues from alternative revenue programs	$	(52	)	$	(10	)	$	(19	)	$	(81	)
Other revenues not from contracts with customers	14			2			—			16

Summary Of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2019

Accounting Policies [Abstract]

Nature of Operations

Ameren, headquartered in St. Louis, Missouri, is a public utility holding company whose primary assets are its equity interests in its subsidiaries. Ameren’s subsidiaries are separate, independent legal entities with separate businesses, assets, and liabilities. Dividends on Ameren’s common stock and the payment of expenses by Ameren depend on distributions made to it by its subsidiaries. Ameren’s principal subsidiaries are listed below. Ameren has other subsidiaries that conduct other activities, such as providing shared services.


•	Union Electric Company, doing business as Ameren Missouri, operates a rate-regulated electric generation, transmission, and distribution business and a rate-regulated natural gas distribution business in Missouri.


•	Ameren Illinois Company, doing business as Ameren Illinois, operates rate-regulated electric transmission, electric distribution, and natural gas distribution businesses in Illinois.

•

ATXI operates a FERC rate-regulated electric transmission business. ATXI is developing the MISO-approved Illinois Rivers and Mark Twain electric transmission projects.

Consolidation

Our accounting policies conform to GAAP. Our financial statements reflect all adjustments (which include normal, recurring adjustments) that are necessary, in our opinion, for a fair statement of our results. The preparation of financial statements in conformity with GAAP requires management to make certain estimates and assumptions. Such estimates and assumptions affect reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the dates of financial statements, and the reported amounts of revenues and expenses during the reported periods. Actual results could differ from those estimates. The results of operations of an interim period may not give a true indication of results that may be expected for a full year. These financial statements should be read in conjunction with the financial statements and accompanying notes included in the Form 10-K.

Variable Interest Entities

As of September 30, 2019, Ameren and Ameren Missouri had interests in unconsolidated variable interest entities that were established to construct wind generation facilities and, ultimately, sell those constructed facilities to Ameren Missouri. Neither Ameren nor Ameren Missouri are the primary beneficiary of these variable interest entities because neither has the power to direct matters that most significantly affect the entities' activities, which include designing, financing, and constructing the wind generation facilities. As a result, these variable interest entities have not been consolidated. As of September 30, 2019, the maximum exposure to loss related to these variable interest entities was approximately $12 million, which primarily represents legal costs incurred. The risk of a loss was assessed to be remote and, accordingly, Ameren and Ameren Missouri have not recognized a liability associated with any portion of the maximum exposure to loss. See Note 2 – Rate and Regulatory Matters for additional information on the agreements to acquire these wind generation facilities.

As of September 30, 2019, and December 31, 2018, Ameren had unconsolidated variable interests as a limited partner in various equity method investments, totaling $27 million and $22 million, respectively, included in “Other assets” on Ameren’s consolidated balance sheet. Ameren is not the primary beneficiary of these investments because it does not have the power to direct matters that most significantly affect the activities of these variable interest entities. As of September 30, 2019, the maximum exposure to loss related to these variable interests is limited to the investment in these partnerships of $27 million plus associated outstanding funding commitments of $37 million.

Company-owned Life Insurance

Ameren and Ameren Illinois have company-owned life insurance, which is recorded at the net cash surrender value. The net cash surrender value is the amount that can be realized under the insurance policies at the balance sheet date. As of September 30, 2019, the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $258 million (December 31, 2018 – $244 million) and $121 million (December 31, 2018 – $122 million), respectively, while total borrowings against the policies were $114 million (December 31, 2018 – $113 million) at both Ameren and Ameren Illinois. Ameren and Ameren Illinois have the right to offset the borrowings

against the cash surrender value of the policies and, consequently, present the net asset in “Other assets” on their respective balance sheets.

Accounting and Reporting Developments

See Note 13 – Supplemental Information for additional information on our adoption of authoritative accounting guidance related to leases. See Note 1 – Summary of Significant Accounting Policies under Part II, Item 8, of the Form 10-K for additional information about recently issued authoritative accounting standards relating to the measurement of credit losses on financial instruments, fair value measurement disclosures, and defined benefit plan disclosures.

Derivatives, Policy

The Ameren Companies elect to present the fair value amounts of derivative assets and derivative liabilities subject to an enforceable master netting arrangement or similar agreement at the gross amounts on the balance sheet.

We use derivatives to manage the risk of changes in market prices for natural gas and power, as well as the risk of changes in rail transportation surcharges through fuel oil hedges. Such price fluctuations may cause the following:


•	an unrealized appreciation or depreciation of our contracted commitments to purchase or sell when purchase or sale prices under the commitments are compared with current commodity prices;


•	market values of natural gas inventories that differ from the cost of those commodities in inventory; and


•	actual cash outlays for the purchase of these commodities that differ from anticipated cash outlays.

The derivatives that we use to hedge these risks are governed by our risk management policies for forward contracts, futures, options, and swaps. Our net positions are continually assessed within our structured hedging programs to determine whether new or offsetting transactions are required. The goal of the hedging program is generally to mitigate financial risks while ensuring that sufficient volumes are available to meet our requirements. Contracts we enter into as part of our risk management program may be settled financially, settled by physical delivery, or net settled with the counterparty.

Lease Accounting Policy

In the first quarter of 2019, we adopted authoritative accounting guidance related to leases, which affected our financial position, but did not materially affect our results of operations or liquidity. The most significant impact for us was the recognition of right-of-use assets and lease liabilities for operating leases, while the accounting for our finance leases remained substantially unchanged. Ameren and Ameren Missouri recognized right-of-use assets and offsetting lease liabilities of $38 million and $36 million at January 1, 2019, respectively, primarily

related to rail car leases. The effect of the adoption was immaterial at Ameren Illinois. No adjustment to comparative periods was made. We elected the available practical expedients upon adoption.

Ameren Missouri primarily leases rail cars under operating lease arrangements for the transportation of coal inventory to its energy centers. Although Ameren Missouri has options to renew a portion of these arrangements for up to five years on similar terms, the exercise of these options was not assumed in the recognition of right-of-use assets and lease obligations. For rail car leases, we account for the lease and non-lease components as a single lease component.

Deferred Compensation

As of September 30, 2019, and December 31, 2018, “Other deferred credits and liabilities” on Ameren’s balance sheet included deferred compensation obligations of $78 million and $80 million, respectively, recorded at the present value of future benefits to be paid.

Revenue from Contract with Customer

Operating Revenues

As of September 30, 2019 and 2018, our remaining performance obligations for contracts with a term greater than one year were immaterial. The Ameren Companies elected not to disclose the aggregate amount of the transaction price allocated to the performance obligations that are unsatisfied as of the end of the reporting period for contracts with an initial expected term of one year or less.

See Note 14 – Segment Information for disaggregated revenue information.

Excise Taxes

Ameren Missouri and Ameren Illinois collect from their customers excise taxes, including municipal and state excise taxes and gross receipts taxes, that are levied on the sale or distribution of natural gas and electricity.

Rate And Regulatory Matters (Tables)

9 Months Ended

Sep. 30, 2019

Public Utilities, General Disclosures [Abstract]

Schedule of Wind Generation Facilities

The following table provides information with respect to each build-transfer agreement:



	Up-to 400-Megawatt Facility	Up-to 300-Megawatt Facility
Build-transfer agreement date	May 2018	May 2019
Wind facility developer	Terra-Gen, LLC	Invenergy Renewables, LLC^(a)
Location	Northeastern Missouri	Northwestern Missouri
Status of certificate of convenience and necessity from the MoPSC	Approved October 2018	Approved August 2019
Status of final interconnection costs	Received July 2019	Received July 2019
Status of RTO transmission interconnection agreement	Executed August 2019	Executed October 2019
Status of FERC approval	Received December 2018	Received October 2019
Expected completion date	By the end of 2020	By the end of 2020

(a)

In October 2019, Invenergy Renewables, LLC acquired the project from Enel North America, Inc.

Short-Term Debt and Liquidity (Tables)

9 Months Ended

Sep. 30, 2019

Debt Disclosure [Abstract]

Schedule of Short-term Debt

The following table presents commercial paper outstanding, net of issuance discounts, as of September 30, 2019, and December 31, 2018:



	September 30, 2019		December 31, 2018
Ameren (parent)	$	90	$	470
Ameren Missouri	144		55
Ameren Illinois	310		72
Ameren consolidated	$	544	$	597

The following table summarizes the borrowing activity and relevant interest rates under Ameren (parent)’s, Ameren Missouri’s, and Ameren Illinois’ commercial paper programs for the nine months ended September 30, 2019 and 2018:



	Ameren (parent)			Ameren Missouri			Ameren Illinois				Ameren Consolidated
2019
Average daily commercial paper outstanding at par value	$	532		$	141		$	147			$	821
Weighted-average interest rate	2.70		%	2.73		%	2.58		%		2.68		%
Peak commercial paper during period at par value^(a)	$	651		$	549		$	310			$	1,113
Peak interest rate	3.80		%	2.97		%	5.00		%	^(b)	5.00		%	^(b)
2018
Average daily commercial paper outstanding at par value	$	431		$	81		$	117			$	629
Weighted-average interest rate	2.23		%	1.94		%	2.21		%		2.18		%
Peak commercial paper during period at par value^(a)	$	543		$	481		$	442			$	1,295
Peak interest rate	2.45		%	2.42		%	2.55		%		2.55		%


(a)	The timing of peak outstanding commercial paper issuances varies by company. Therefore, the sum of individual company peak amounts may not equal the Ameren consolidated peak commercial paper issuances for the period.

(b)

In the third quarter of 2019, Ameren’s and Ameren Illinois’ peak interest rate was affected by temporary disruptions in the commercial paper market.

Long-Term Debt and Equity Financings (Tables)

9 Months Ended

Sep. 30, 2019

Long-Term Debt And Equity Financings [Abstract]

Schedule of Long-term Debt Instruments [Table Text Block]

The following table provides additional information on the bonds:



	1992 Series	1998 Series A	1998 Series B	1998 Series C
Transaction month	June 2019	July 2019	July 2019	June 2019
Principal amount	$47	$60	$50	$50
Fixed interest rate	1.60%	2.90%	2.90%	2.75%
Variable interest rate ^(a)	2.36%	3.35%	3.34%	3.83%
Maturity	December 2022	September 2033	September 2033	September 2033
Interest payment dates	June 1 and December 1	March 1 and September 1	March 1 and September 1	March 1 and September 1
Initial interest payment date	December 2019	September 2019	September 2019	September 2019

(a)

Represents the variable interest rate of the bonds effective prior to the change in method of determining the interest rate.

- Definition

Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28541-108399

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21506-112644

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(e),(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
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-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21521-112644

Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21538-112644

Reference 8: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

Reference 9: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

Reference 10: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21475-112644

+ Details

Other Income and Expenses (Tables)

9 Months Ended

Sep. 30, 2019

Other Nonoperating Income (Expense) [Abstract]

Other Income And Expenses

The following table presents the components of “Other Income, Net” in the Ameren Companies’ statements of income for the three and nine months ended September 30, 2019 and 2018:



	Three Months						Nine Months
	2019			2018			2019			2018
Ameren:
Allowance for equity funds used during construction	$	7		$	11		$	20		$	25
Interest income on industrial development revenue bonds	6			6			19			19
Other interest income	2			2			6			6
Non-service cost components of net periodic benefit income^(a)	23			17			67			52
Miscellaneous income	2			2			6			5
Donations	(1		)	(4		)	(8		)	(15		)
Miscellaneous expense	(5		)	(2		)	(11		)	(8		)
Total Other Income, Net	$	34		$	32		$	99		$	84



	Three Months						Nine Months
	2019			2018			2019			2018
Ameren Missouri:
Allowance for equity funds used during construction	$	6		$	8		$	14		$	19
Interest income on industrial development revenue bonds	6			6			19			19
Other interest income	—			1			—			2
Non-service cost components of net periodic benefit income^(a)	4			4			13			13
Miscellaneous income	2			2			4			3
Donations	(1		)	(3		)	(3		)	(6		)
Miscellaneous expense	(2		)	(2		)	(4		)	(5		)
Total Other Income, Net	$	15		$	16		$	43		$	45
Ameren Illinois:
Allowance for equity funds used during construction	$	1		$	3		$	6		$	6
Interest income	1			1			5			4
Non-service cost components of net periodic benefit income	12			8			36			25
Miscellaneous income	1			1			3			3
Donations	—			—			(5		)	(5		)
Miscellaneous expense	(2		)	(2		)	(6		)	(3		)
Total Other Income, Net	$	13		$	11		$	39		$	30

(a)

For the three and nine months ended September 30, 2019, the non-service cost components of net periodic benefit income were partially offset by a $7 million and $22 million deferral, respectively, due to a regulatory tracking mechanism for the difference between the level of such costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates (2018 – $5 million and $13 million, respectively).

Derivative Financial Instruments (Tables)

9 Months Ended

Sep. 30, 2019

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Open Gross Derivative Volumes By Commodity Type

The following table presents open gross commodity contract volumes by commodity type for derivative assets and liabilities as of September 30, 2019, and December 31, 2018. As of September 30, 2019, these contracts extended through October 2022, March 2024, and May 2032 for fuel oils, natural gas, and power, respectively.



	Quantity (in millions)
	2019			2018
Commodity	Ameren Missouri	Ameren Illinois	Ameren	Ameren Missouri	Ameren Illinois	Ameren
Fuel oils (in gallons)^(a)	63	—	63	66	—	66
Natural gas (in mmbtu)	20	142	162	19	154	173
Power (in megawatthours)	4	8	12	1	8	9

(a)

Consists of ultra-low-sulfur diesel products.

Derivative Instruments Carrying Value

The following table presents the carrying value and balance sheet location of all derivative commodity contracts, none of which were designated as hedging instruments, as of September 30, 2019, and December 31, 2018:



			September 30, 2019					December 31, 2018
	Balance Sheet Location		Ameren Missouri		Ameren Illinois		Ameren			Ameren Missouri		Ameren Illinois		Ameren
Fuel oils	Other current assets	$	4	$	—	$	4		$	3	$	—	$	3
	Other assets		2		—		2			5		—		5
Natural gas	Other current assets		—		2		2			—		1		1
	Other assets		—		1		1			—		2		2
Power	Other current assets		8		—		8			4		—		4
	Other assets		4		—		4			—		—		—
	Total assets	$	18	$	3	$	21		$	12	$	3	$	15
Fuel oils	Other current liabilities	$	7	$	—	$	7		$	4	$	—	$	4
	Other deferred credits and liabilities		5		—		5			9		—		9
Natural gas	Other current liabilities		3		12		15			4		8		12
	Other deferred credits and liabilities		1		7		8			1		6		7
Power	Other current liabilities		3		15		18			4		14		18
	Other deferred credits and liabilities		1		189		190			—		169		169
	Total liabilities	$	20	$	223	$	243		$	22	$	197	$	219

Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2019

Fair Value Disclosures [Abstract]

Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis

The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of September 30, 2019, and December 31, 2018:



		September 30, 2019									December 31, 2018
		Level 1		Level 2		Level 3		Total			Level 1		Level 2		Level 3		Total
Assets:
Ameren
	Derivative assets – commodity contracts^(a):
	Fuel oils	$	—	$	—	$	6	$	6		$	1	$	—	$	7	$	8
	Natural gas	—		—		3		3			—		2		1		3
	Power	1		—		11		12			—		1		3		4
	Total derivative assets – commodity contracts	$	1	$	—	$	20	$	21		$	1	$	3	$	11	$	15
	Nuclear decommissioning trust fund:
	Equity securities:
	U.S. large capitalization	$	518	$	—	$	—	$	518		$	427	$	—	$	—	$	427
	Debt securities:
	U.S. Treasury and agency securities	—		136		—		136			—		148		—		148
	Corporate bonds	—		86		—		86			—		72		—		72
	Other	—		50		—		50			—		32		—		32
	Total nuclear decommissioning trust fund	$	518	$	272	$	—	$	790	^(b)	$	427	$	252	$	—	$	679	^(b)
	Total Ameren	$	519	$	272	$	20	$	811		$	428	$	255	$	11	$	694
Ameren Missouri
	Derivative assets – commodity contracts^(a):
	Fuel oils	$	—	$	—	$	6	$	6		$	1	$	—	$	7	$	8
	Power	1		—		11		12			—		1		3		4
	Total derivative assets – commodity contracts	$	1	$	—	$	17	$	18		$	1	$	1	$	10	$	12
	Nuclear decommissioning trust fund:
	Equity securities:
	U.S. large capitalization	$	518	$	—	$	—	$	518		$	427	$	—	$	—	$	427
	Debt securities:
	U.S. Treasury and agency securities	—		136		—		136			—		148		—		148
	Corporate bonds	—		86		—		86			—		72		—		72
	Other	—		50		—		50			—		32		—		32
	Total nuclear decommissioning trust fund	$	518	$	272	$	—	$	790	^(b)	$	427	$	252	$	—	$	679	^(b)
	Total Ameren Missouri	$	519	$	272	$	17	$	808		$	428	$	253	$	10	$	691
Ameren Illinois
	Derivative assets – commodity contracts^(a):
	Natural gas	$	—	$	—	$	3	$	3		$	—	$	2	$	1	$	3
Liabilities:
Ameren
	Derivative liabilities – commodity contracts^(a):
	Fuel oils	$	4	$	—	$	8	$	12		$	2	$	—	$	11	$	13
	Natural gas	2		17		4		23			—		15		4		19
	Power	—		1		207		208			—		1		186		187
	Total Ameren	$	6	$	18	$	219	$	243		$	2	$	16	$	201	$	219



		September 30, 2019								December 31, 2018
		Level 1		Level 2		Level 3		Total		Level 1		Level 2		Level 3		Total
Ameren Missouri
	Derivative liabilities – commodity contracts^(a):
	Fuel oils	$	4	$	—	$	8	$	12	$	2	$	—	$	11	$	13
	Natural gas	—		3		1		4		—		5		—		5
	Power	—		1		3		4		—		1		3		4
	Total Ameren Missouri	$	4	$	4	$	12	$	20	$	2	$	6	$	14	$	22
Ameren Illinois
	Derivative liabilities – commodity contracts^(a):
	Natural gas	$	2	$	14	$	3	$	19	$	—	$	10	$	4	$	14
	Power	—		—		204		204		—		—		183		183
	Total Ameren Illinois	$	2	$	14	$	207	$	223	$	—	$	10	$	187	$	197


(a)	The derivative asset and liability balances are presented net of registrant and counterparty credit considerations.


(b)	Balance excludes $8 million and $5 million of cash and cash equivalents, receivables, payables, and accrued income, net, for September 30, 2019, and December 31, 2018, respectively.

Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level Three In The Fair Value Hierarchy

The following table presents the fair value reconciliation of Level 3 power derivative contract assets and liabilities measured at fair value on a recurring basis for the three and nine months ended September 30, 2019 and 2018:



	2019									2018
	Ameren Missouri			Ameren Illinois			Ameren			Ameren Missouri			Ameren Illinois			Ameren
For the three months ended September 30:
Beginning balance at July 1	$	15		$	(191	)	$	(176	)	$	5		$	(190	)	$	(185	)
Realized and unrealized gains/(losses) included in regulatory assets/liabilities	(4		)	(17		)	(21		)	(4		)	—			(4		)
Purchases	—			—			—			1			—			1
Settlements	(1		)	4			3			(1		)	3			2
Transfers out of Level 3	(2		)	—			(2		)	(1		)	—			(1		)
Ending balance at September 30	$	8		$	(204	)	$	(196	)	$	—		$	(187	)	$	(187	)
Change in unrealized gains/(losses) related to assets/liabilities held at September 30	$	(4	)	$	(17	)	$	(21	)	$	—		$	—		$	—
For the nine months ended September 30:
Beginning balance at January 1	$	—		$	(183	)	$	(183	)	$	7		$	(195	)	$	(188	)
Realized and unrealized gains/(losses) included in regulatory assets/liabilities	12			(32		)	(20		)	(7		)	(1		)	(8		)
Purchases	—			—			—			5			—			5
Settlements	(2		)	11			9			(4		)	9			5
Transfers out of Level 3	(2		)	—			(2		)	(1		)	—			(1		)
Ending balance at September 30	8			(204		)	(196		)	—			(187		)	(187		)
Change in unrealized gains/(losses) related to assets/liabilities held at September 30	$	8		$	(31	)	$	(23	)	$	(1	)	$	(2	)	$	(3	)

Fair Value, Assets and Liabilities Measured on Nonrecurring Basis, Valuation Techniques

The following table describes the valuation techniques and significant unobservable inputs utilized for the fair value of our Level 3 power derivative contract assets and liabilities as of September 30, 2019, and December 31, 2018:



			Fair Value^(a)						Weighted Average
	Commodity		Assets		Liabilities	Valuation Technique(s)	Unobservable Input	Range	Weighted Average
2019	Power^(b)	$	11	$	(207)	Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps ($/MWh)^(c)	22 – 37	25
							Nodal basis ($/MWh)^(c)	(7) – 0	(3)
						Fundamental energy production model	Estimated future natural gas prices ($/mmbtu)^(c)	3 – 3	3
2018	Power^(d)	$	3	$	(186)	Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps ($/MWh)^(c)	23 – 39	28
							Nodal basis ($/MWh)^(c)	(9) – 0	(2)
						Fundamental energy production model	Estimated future natural gas prices ($/mmbtu)^(c)	3 – 4	3


(a)	The derivative asset and liability balances are presented net of registrant and counterparty credit considerations.


(b)	Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2024. Valuations beyond 2024 use fundamentally modeled pricing by month for peak and off-peak demand.


(c)	Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.

(d)

Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2022. Valuations beyond 2022 use fundamentally modeled pricing by month for peak and off-peak demand.

Schedule of Financial Assets and Liabilities

The following table sets forth, by level within the fair value hierarchy, the carrying amount and fair value of financial assets and liabilities disclosed, but not carried, at fair value as of September 30, 2019, and December 31, 2018:



	September 30, 2019
	Carrying Amount			Fair Value
	Carrying Amount			Level 1		Level 2		Level 3			Total
Ameren:
Cash, cash equivalents, and restricted cash	$	155		$	155	$	—	$	—		$	155
Investments in industrial development revenue bonds^(a)	270			—		270		—			270
Short-term debt	544			—		544		—			544
Long-term debt (including current portion)^(a)	8,987		^(b)	—		9,727		491		^(c)	10,218
Ameren Missouri:
Cash, cash equivalents, and restricted cash	$	14		$	14	$	—	$	—		$	14
Investments in industrial development revenue bonds^(a)	270			—		270		—			270
Short-term debt	144			—		144		—			144
Long-term debt (including current portion)^(a)	4,115		^(b)	—		4,766		—			4,766
Ameren Illinois:
Cash, cash equivalents, and restricted cash	$	117		$	117	$	—	$	—		$	117
Short-term debt	310			—		310		—			310
Long-term debt (including current portion)	3,279		^(b)	—		3,788		—			3,788
	December 31, 2018
Ameren:
Cash, cash equivalents, and restricted cash	$	107		$	107	$	—	$	—		$	107
Investments in industrial development revenue bonds^(a)	270			—		270		—			270
Short-term debt	597			—		597		—			597
Long-term debt (including current portion)^(a)	8,439		^(b)	—		8,240		429		^(c)	8,669
Ameren Missouri:
Cash, cash equivalents, and restricted cash	$	8		$	8	$	—	$	—		$	8
Investments in industrial development revenue bonds^(a)	270			—		270		—			270
Short-term debt	55			—		55		—			55
Long-term debt (including current portion)^(a)	3,998		^(b)	—		4,156		—			4,156
Ameren Illinois:
Cash, cash equivalents, and restricted cash	$	80		$	80	$	—	$	—		$	80
Short-term debt	72			—		72		—			72
Long-term debt (including current portion)	3,296		^(b)	—		3,391		—			3,391

(a)

Ameren and Ameren Missouri have investments in industrial development revenue bonds, classified as held-to-maturity and recorded in “Other Assets,” that are equal to the finance obligations for the Peno Creek and Audrain CT energy centers. As of September 30, 2019, and December 31, 2018, the carrying amount of both the investments in industrial development revenue bonds and the finance obligations approximated fair value.

(b)

Includes unamortized debt issuance costs, which were excluded from the fair value measurement, of $65 million, $27 million, and $31 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of September 30, 2019. Includes unamortized debt issuance costs, which were excluded from the fair value measurement, of $58 million, $22 million, and $31 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2018.

(c)

The Level 3 fair value amount consists of ATXI’s senior unsecured notes.

Related Party Transactions (Tables)

9 Months Ended

Sep. 30, 2019

Schedule of affiliate receivables and payables

The following table presents the affiliate balances related to income taxes for Ameren Missouri and Ameren Illinois as of September 30, 2019, and December 31, 2018:



	September 30, 2019				December 31, 2018
	Ameren Missouri		Ameren Illinois		Ameren Missouri		Ameren Illinois
Income taxes payable to parent^(a)	$	58	$	24	$	16	$	7
Income taxes receivable from parent^(b)	—		—		—		6


(a)	Included in “Accounts payable – affiliates” on the balance sheet.

(b)

Included in “Accounts receivable – affiliates” on the balance sheet.

Schedule of Related Party Transactions

The following table presents the effect on Ameren Missouri and Ameren Illinois of related-party transactions for the three and nine months ended September 30, 2019 and 2018:



				Three Months				Nine Months
Agreement	Income Statement Line Item			Ameren Missouri		Ameren Illinois		Ameren Missouri		Ameren Illinois
Ameren Missouri power supply	Operating Revenues	2019	$	1	$	(a)	$	3	$	(a)
agreements with Ameren Illinois		2018		5		(a)		11		(a)
Ameren Missouri and Ameren Illinois	Operating Revenues	2019	$	7	$	1	$	20	$	2
rent and facility services		2018		6		(b)		17		2
Ameren Missouri and Ameren Illinois	Operating Revenues	2019	$	1	$	(b)	$	1	$	1
miscellaneous support services		2018		(b)		(b)		(b)		(b)



				Three Months				Nine Months
Agreement	Income Statement Line Item			Ameren Missouri		Ameren Illinois		Ameren Missouri		Ameren Illinois
Ameren Missouri software licensing	Operating Revenues	2019	$	(a)	$	19	$	(a)	$	19
with Ameren Illinois		2018		(a)		(a)		(a)		(a)
Total Operating Revenues		2019	$	9	$	20	$	24	$	22
		2018		11		(b)		28		2
Ameren Illinois power supply	Purchased Power	2019	$	(a)	$	1	$	(a)	$	3
agreements with Ameren Missouri		2018		(a)		5		(a)		11
Ameren Illinois transmission	Purchased Power	2019	$	(a)	$	1	$	(a)	$	1
services with ATXI		2018		(a)		(b)		(a)		1
Total Purchased Power		2019	$	(a)	$	2	$	(a)	$	4
		2018		(a)		5		(a)		12
Ameren Missouri and Ameren Illinois	Other Operations and Maintenance	2019	$	(b)	$	1	$	1	$	4
rent and facility services		2018		1		1		2		4
Ameren Services support services	Other Operations and Maintenance	2019	$	34	$	30	$	98	$	91
agreement		2018		36		33		101		93
Total Other Operations and		2019	$	34	$	31	$	99	$	95
Maintenance		2018		37		34		103		97
Money pool borrowings (advances)	Interest Charges/Other Income, Net	2019	$	(b)	$	(b)	$	(b)	$	(b)
		2018		(b)		(b)		(b)		(b)


(a)	Not applicable.

(b)

Amount less than $1 million.

Commitments and Contingencies (Tables)

9 Months Ended

Sep. 30, 2019

Commitments and Contingencies Disclosure [Abstract]

Long-term Purchase Commitment

The table below presents our estimated minimum fuel, purchased power, and other commitments at September 30, 2019. Ameren’s and Ameren Illinois’ purchased power commitments include the Ameren Illinois agreements entered into as part of the IPA-administered power procurement process. Included in the Other column are minimum purchase commitments under contracts for equipment, design and construction, and meter reading services, among other agreements, at September 30, 2019.



	Coal		Natural Gas^(a)		Nuclear Fuel		Purchased Power^(b)(c)			Methane Gas		Other		Total
Ameren:
2019	$	142	$	51	$	21	$	56	^(d)	$	1	$	20	$	291
2020	226		172		43		146		^(d)	3		40		630
2021	195		105		59		50			3		26		438
2022	137		49		12		12			3		22		235
2023	46		25		42		2			3		22		140
Thereafter	—		48		29		—			26		85		188
Total	$	746	$	450	$	206	$	266		$	39	$	215	$	1,922
Ameren Missouri:
2019	$	142	$	10	$	21	$	—		$	1	$	16	$	190
2020	226		39		43		—			3		29		340
2021	195		23		59		—			3		22		302
2022	137		11		12		—			3		22		185
2023	46		10		42		—			3		22		123
Thereafter	—		17		29		—			26		48		120
Total	$	746	$	110	$	206	$	—		$	39	$	159	$	1,260
Ameren Illinois:
2019	$	—	$	41	$	—	$	56	^(d)	$	—	$	2	$	99
2020	—		133		—		146		^(d)	—		1		280
2021	—		82		—		50			—		—		132
2022	—		38		—		12			—		—		50
2023	—		15		—		2			—		—		17
Thereafter	—		31		—		—			—		—		31
Total	$	—	$	340	$	—	$	266		$	—	$	3	$	609


(a)	Includes amounts for generation and for distribution.


(b)	The purchased power amounts for Ameren and Ameren Illinois exclude agreements for renewable energy credits through 2035 with various renewable energy suppliers due to the contingent nature of the payment amounts, with the exception of expected payments of $11 million through 2023.


(c)	The purchased power amounts for Ameren and Ameren Missouri exclude a 102-megawatt power purchase agreement with a wind farm operator, which expires in 2024, due to the contingent nature of the payment amounts.

(d)

In January 2018, as required by the FEJA, Ameren Illinois entered into 10-year agreements to acquire zero emission credits. Annual zero emission credit commitment amounts will be published by the IPA each May prior to the start of the subsequent planning year. The amounts above reflect Ameren Illinois’ commitment to acquire approximately $44 million of zero emission credits through May 2020.

Callaway Energy Center (Tables)

9 Months Ended

Sep. 30, 2019

Nuclear Waste Matters [Abstract]

Schedule of Insurance Coverage at Callaway Energy Center

The following table presents insurance coverage at Ameren Missouri’s Callaway energy center as of September 30, 2019. The property coverage and the nuclear liability coverage renewal dates are April 1 and January 1, respectively, of each year. Both coverages were renewed in 2019.



Type and Source of Coverage	Maximum Coverages			Maximum Assessments for Single Incidents
Public liability and nuclear worker liability:
American Nuclear Insurers	$	450		$	—
Pool participation	13,486		^(a)	138		^(b)
	$	13,936	^(c)	$	138
Property damage:
NEIL and EMANI	$	3,200	^(d)	$	28	^(e)
Replacement power:
NEIL	$	490	^(f)	$	7	^(e)


(a)	Provided through mandatory participation in an industrywide retrospective premium assessment program. The maximum coverage available is dependent on the number of United States commercial reactors participating in the program.


(b)	Retrospective premium under the Price-Anderson Act. This is subject to retrospective assessment with respect to a covered loss in excess of $450 million in the event of an incident at any licensed United States commercial reactor, payable at $21 million per year.


(c)	Limit of liability for each incident under the Price-Anderson liability provisions of the Atomic Energy Act of 1954, as amended. This limit is subject to change to account for the effects of inflation and changes in the number of licensed reactors.


(d)	NEIL provides $2.7 billion in property damage, stabilization, decontamination, and premature decommissioning insurance for radiation events and $2.3 billion in property damage insurance for nonradiation events. EMANI provides $490 million in property damage insurance for both radiation and nonradiation events.


(e)	All NEIL insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL.

(f)

Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity up to $4.5 million for 52 weeks, which commences after the first twelve weeks of an outage, plus up to $3.6 million per week for a minimum of 71 weeks thereafter for a total not exceeding the policy limit of $490 million. Nonradiation events are limited to $328 million.

Retirement Benefits (Tables)

9 Months Ended

Sep. 30, 2019

Defined Benefit Plan [Abstract]

Components Of Net Periodic Benefit Cost

The following table presents the components of the net periodic benefit cost (income) incurred for Ameren’s pension and postretirement benefit plans for the three and nine months ended September 30, 2019 and 2018:



	Pension Benefits												Postretirement Benefits
	Three Months						Nine Months						Three Months						Nine Months
	2019			2018			2019			2018			2019			2018			2019			2018
Service cost^(a)	$	22		$	25		$	66		$	75		$	4		$	6		$	13		$	16
Non-service cost components:
Interest cost	46			42			139			126			11			10			32			30
Expected return on plan assets	(69		)	(68		)	(207		)	(206		)	(19		)	(20		)	(57		)	(58		)
Amortization of:
Prior service benefit	—			—			—			—			(1		)	(1		)	(4		)	(3		)
Actuarial loss (gain)	6			17			19			51			(4		)	(2		)	(11		)	(5		)
Total non-service cost components^(b)	$	(17	)	$	(9	)	$	(49	)	$	(29	)	$	(13	)	$	(13	)	$	(40	)	$	(36	)
Net periodic benefit cost (income)	$	5		$	16		$	17		$	46		$	(9	)	$	(7	)	$	(27	)	$	(20	)


(a)	Service cost, net of capitalization, is reflected in “Operating Expenses – Other operations and maintenance” on Ameren’s statement of income.


(b)	Non-service cost components are reflected in “Other Income, Net” on Ameren’s statement of income. See Note 5 – Other Income, Net for additional information.

Summary Of Benefit Plan Costs Incurred

Ameren Missouri and Ameren Illinois are responsible for their respective shares of Ameren’s pension and postretirement costs. The following table presents the respective share of net periodic pension and other postretirement benefit costs (income) incurred for the three and nine months ended September 30, 2019 and 2018:



	Pension Benefits												Postretirement Benefits
	Three Months						Nine Months						Three Months						Nine Months
	2019			2018			2019			2018			2019			2018			2019			2018
Ameren Missouri^(a)	$	1		$	6		$	3		$	17		$	(1	)	$	(1	)	$	(4	)	$	(1	)
Ameren Illinois	5			11			15			30			(8		)	(6		)	(23		)	(19		)
Other	(1		)	(1		)	(1		)	(1		)	—			—			—			—
Ameren^(a)	$	5		$	16		$	17		$	46		$	(9	)	$	(7	)	$	(27	)	$	(20	)

(a)

Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates.

Income Taxes Income Taxes (Tables)

9 Months Ended

Sep. 30, 2019

Income Tax Disclosure [Abstract]

Schedule of Effective Income Tax Rate Reconciliation

The following table presents a reconciliation of the federal statutory corporate income tax rate to the effective income tax rate for the three and nine months ended September 30, 2019 and 2018:



	Ameren			Ameren Missouri			Ameren Illinois
	2019	2018		2019	2018		2019	2018
Three Months
Federal statutory corporate income tax rate:	21%	21%		21%	21%		21%	21%
Increases (decreases) from:
Amortization of excess deferred taxes	(7)	(6)	^(a)	(11)	(7)	^(a)	(3)	(3)
Depreciation differences	—	—		—	—		—	(1)
Amortization of deferred investment tax credit	(1)	(1)		(1)	—		—	—
State tax	6	6		5	4		6	5
Stock-based compensation	1	—		—	—		—	—
TCJA	—	3	^(b)	—	—		—	4	^(b)
Effective income tax rate	20%	23%		14%	18%		24%	26%



	Ameren			Ameren Missouri			Ameren Illinois
	2019	2018		2019	2018		2019	2018
Nine Months
Federal statutory corporate income tax rate:	21%	21%		21%	21%		21%	21%
Increases (decreases) from:
Amortization of excess deferred taxes	(7)	(3)	^(a)	(12)	(4)	^(a)	(4)	(4)
Amortization of deferred investment tax credit	(1)	(1)		(1)	—		—	—
State tax	6	6		5	4		7	7
Stock-based compensation	(1)	—		—	—		—	—
TCJA	—	1	^(b)	—	—		—	1	^(b)
Other	—	(1)		—	—		—	—
Effective income tax rate	18%	23%		13%	21%		24%	25%


(a)	Based on an order by the MoPSC in July 2018, Ameren Missouri began amortizing excess deferred taxes in August 2018.

(b)

The Ameren Companies updated their respective provisional estimates recorded related to TCJA, as discussed below.

Supplemental Information (Tables)

9 Months Ended

Sep. 30, 2019

Supplemental Information [Abstract]

Schedule of Cash and Cash Equivalents Including Restricted Cash

The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets and the statements of cash flows as of September 30, 2019, and December 31, 2018:



	September 30, 2019						December 31, 2018
	Ameren		Ameren Missouri		Ameren Illinois		Ameren		Ameren Missouri		Ameren Illinois
Cash and cash equivalents	$	20	$	—	$	—	$	16	$	—	$	—
Restricted cash included in “Other current assets”	14		4		6		13		4		6
Restricted cash included in “Other assets”	111		—		111		74		—		74
Restricted cash included in “Nuclear decommissioning trust fund”	10		10		—		4		4		—
Total cash, cash equivalents, and restricted cash	$	155	$	14	$	117	$	107	$	8	$	80

Lease Supplemental Information

The following table provides supplemental balance sheet information related to operating leases as of September 30, 2019:



	Ameren			Ameren Missouri
Other assets	$	38		$	36
Other current liabilities	8			7
Other deferred credits and liabilities	30			29
Weighted average remaining operating lease term	6 years			6 years
Weighted average discount rate^(a)	3.5		%	3.4		%

(a)

As an implicit rate is not readily determinable under most of our lease agreements, we use our incremental borrowing rate based on the information available at commencement date in determining the present value of lease payments. We use an implicit rate when readily determinable.

Schedule of Maturity of Operating Lease Liabilities

The following table presents Ameren’s and Ameren Missouri’s remaining maturities of operating lease liabilities as of September 30, 2019:



	Ameren		Ameren Missouri
2019	$	2	$	2
2020	8		8
2021	8		7
2022	7		6
2023	6		6
Thereafter	11		10
Total lease payments	42		39
Less imputed interest	4		3
Total^(a)	$	38	$	36

(a)

The amount of remaining maturities of operating lease liabilities under previous authoritative accounting guidance as of December 31, 2018, is materially consistent with the amount as of September 30, 2019. Maturities of certain financing arrangements, including the Peno Creek and Audrain energy centers' long-term agreements, are no longer required to be disclosed as lease-related maturities. See Note 5 – Long-Term Debt and Equity Financings under Part II, Item 8, in the Form 10-K for further information on financing arrangements.

Schedule of Cash Flow, Supplemental Disclosures

Supplemental Cash Flow Information

The following table provides noncash financing and investing activity excluded from the statements of cash flows for the nine months ended September 30, 2019 and 2018:



	September 30, 2019								September 30, 2018
	Ameren			Ameren Missouri		Ameren Illinois			Ameren		Ameren Missouri		Ameren Illinois
Investing
Exchange of bond investments for the extinguishment of senior unsecured notes^(a)	$	17		$	—	$	17		$	—	$	—	$	—
Accrued capital expenditures	273			138		128			240		94		133
Net realized and unrealized gain – nuclear decommissioning trust fund	100			100		—			33		33		—
Financing
Exchange of bond investments for the extinguishment of senior unsecured notes^(a)	$	(17	)	$	—	$	(17	)	$	—	$	—	$	—
Issuance of common stock for stock-based compensation	54			—		—			35		—		—

(a)

See Note 4 – Long-term Debt and Equity Financings for additional information.

Asset Retirement Obligation Disclosure

The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the nine months ended September 30, 2019:



	Ameren Missouri				Ameren Illinois			Ameren
Balance at December 31, 2018	$	646		^(a)	$	4	^(b)	$	650		^(a)
Liabilities settled	(10		)		—			(10		)
Accretion	21			^(c)	—			21			^(c)
Change in estimates	33			^(d)	—			33			^(d)
Balance at September 30, 2019	$	690		^(a)	$	4	^(b)	$	694		^(a)


(a)	Balance included $23 million in “Other current liabilities” on the balance sheet as of both December 31, 2018, and September 30, 2019.


(b)	Included in “Other deferred credits and liabilities” on the balance sheet.


(c)	Accretion expense attributable to Ameren Missouri was recorded as a decrease to regulatory liabilities.

(d)

Ameren Missouri changed its fair value estimate primarily due to an increase in the cost estimate for closure of certain CCR storage facilities.

Summary Of Nonvested Shares Related To Long-Term Incentive Plan

The following table summarizes Ameren's nonvested performance share unit and restricted stock unit activity for the nine months ended September 30, 2019:



	Performance Share Units					Restricted Stock Units
	Share Units		Weighted-average Fair Value per Share Unit			Stock Units		Weighted-average Fair Value per Stock Unit
Nonvested at January 1, 2019^(a)	682,811		$	56.58		155,253		$	57.38
Granted	297,728		67.42		^(b)	128,883		65.49
Forfeitures	(33,195	)	64.34			(11,028	)	62.74
Vested and undistributed^(c)	(180,823	)	62.24			(40,616	)	61.91
Vested and distributed	(176,923	)	44.13			(2,403	)	54.30
Nonvested at September 30, 2019^(d)	589,598		$	63.62		230,089		$	60.90


(a)	Does not include 619,783 performance share units and 26,557 restricted stock units that were vested and undistributed.


(b)	Significant inputs to the Monte Carlo simulation model used to calculate the fair value of performance share units granted include Ameren’s closing common share price of $65.23 at December 31, 2018, Ameren’s common stock volatility of 17%, a volatility range for the peer group of 15% to 25%, and a three-year risk-free rate of 2.46%.


(c)	Vested and undistributed units are awards that vest on a pro-rata basis due to attainment of retirement eligibility by certain employees, but have not yet been distributed. For vested and undistributed performance share units, the number of shares issued for retirement-eligible employees will vary depending on actual performance over the three year performance period.

(d)

Does not include 448,831 performance share units and 67,173 restricted stock units that were vested and undistributed.

Schedule of excise taxes

The following table presents the excise taxes recorded on a gross basis in “Operating Revenues – Electric,” “Operating Revenues – Natural gas” and “Operating Expenses – Taxes other than income taxes” on the statements of income for the three and nine months ended September 30, 2019 and 2018:



	Three Months				Nine Months
	2019		2018		2019		2018
Ameren Missouri	$	49	$	52	$	118	$	133
Ameren Illinois	27		26		91		89
Ameren	$	76	$	78	$	209	$	222

Schedule of Earnings Per Share, Basic and Diluted

The following table presents Ameren’s basic and diluted earnings per share calculations and reconciles the weighted-average number of common shares outstanding to the diluted weighted-average number of common shares outstanding for the three and nine months ended September 30, 2019 and 2018:



	Three Months		Nine Months
	2019	2018	2019	2018
Weighted-average Common Shares Outstanding – Basic	245.9	244.1	245.5	243.6
Assumed settlement of performance share units and restricted stock units	1.4	2.2	1.4	1.9
Dilutive effect of forward sale agreement	0.2	—	0.1	—
Weighted-average Common Shares Outstanding – Diluted^(a)	247.5	246.3	247.0	245.5

(a)

There were no potentially dilutive securities excluded from the earnings per diluted share calculations for the three and nine months ended September 30, 2019 and 2018.

Segment Information (Tables)

9 Months Ended

Sep. 30, 2019

Segment Reporting [Abstract]

Schedule Of Segment Reporting Information By Segment

The following tables present revenues, net income attributable to common shareholders, and capital expenditures by segment at Ameren and Ameren Illinois for the three and nine months ended September 30, 2019 and 2018. Ameren, Ameren Missouri, and Ameren Illinois management review segment capital expenditure information rather than any individual or total asset amount.

Ameren



	Ameren Missouri		Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas			Ameren Transmission			Other			Intersegment Eliminations			Ameren
Three Months 2019:
External revenues	$	1,050	$	374	$	107		$	109		$	—		$	—		$	1,640
Intersegment revenues	9		15		5			19			—			(29		)	19		^(b)
Net income (loss) attributable to Ameren common shareholders	300		32		(1		)	53		^(a)	(20		)	—			364
Capital expenditures	256		139		113			129			(7		)	6			636
Three Months 2018:
External revenues	$	1,118	$	392	$	116		$	98		$	—		$	—		$	1,724
Intersegment revenues	11		—		—			15			—			(26		)	—
Net income (loss) attributable to Ameren common shareholders	294		35		—			48		^(a)	(20		)	—			357
Capital expenditures	210		135		111			124			1			(4		)	577
Nine Months 2019:
External revenues	$	2,591	$	1,118	$	563		$	303		$	—		$	—		$	4,575
Intersegment revenues	24		17		5			48			—			(75		)	19		^(b)
Net income (loss) attributable to Ameren common shareholders	446		105		57			139		^(a)	(13		)	—			734
Capital expenditures	751		390		241			377			3			(1		)	1,761
Nine Months 2018:
External revenues	$	2,848	$	1,177	$	569		$	278		$	—		$	—		$	4,872
Intersegment revenues	28		2		—			42			—			(72		)	—
Net income (loss) attributable to Ameren common shareholders	500		101		49			121		^(a)	(24		)	—			747
Capital expenditures	664		389		237			399			6			(6		)	1,689


(a)	Ameren Transmission earnings include an allocation of financing costs from Ameren (parent).

(b)

Intersegment revenues at Ameren include $14 million and $5 million of revenue from Ameren Illinois Electric Distribution and Ameren Illinois Natural Gas, respectively, for the three and nine months ended September 30, 2019, for a software licensing agreement with Ameren Missouri. Under authoritative accounting guidance for rate-regulated entities, the revenue recognized by Ameren Illinois was not eliminated upon consolidation. See Note 8 – Related-party Transactions under Part I, Item 1, of this report for additional information.

Ameren Illinois



	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas			Ameren Illinois Transmission		Intersegment Eliminations			Ameren Illinois
Three Months 2019:
External revenues	$	389	$	112		$	63	$	—		$	564
Intersegment revenues	—		—			18		(18		)	—
Net income (loss) available to common shareholder	32		(1		)	34		—			65
Capital expenditures	139		113			92		—			344
Three Months 2018:
External revenues	$	392	$	116		$	56	$	—		$	564
Intersegment revenues	—		—			15		(15		)	—
Net income available to common shareholder	35		—			28		—			63
Capital expenditures	135		111			99		—			345
Nine Months 2019:
External revenues	$	1,135	$	568		$	170	$	—		$	1,873
Intersegment revenues	—		—			47		(47		)	—
Net income available to common shareholder	105		57			85		—			247
Capital expenditures	390		241			269		—			900
Nine Months 2018:
External revenues	$	1,179	$	569		$	154	$	—		$	1,902
Intersegment revenues	—		—			41		(41		)	—
Net income available to common shareholder	101		49			70		—			220
Capital expenditures	389		237			321		—			947

Disaggregation of Revenue

The following tables present disaggregated revenues by segment at Ameren and Ameren Illinois for the three and nine months ended September 30, 2019 and 2018. Economic factors affect the nature, timing, amount, and uncertainty of revenues and cash flows in a similar manner across customer classes. Revenues from alternative revenue programs have a similar distribution among customer classes as revenues from contracts with customers. Other revenues not associated with contracts with customers are presented in the Other customer classification, along with electric transmission and off-system revenues.

Ameren



	Ameren Missouri		Ameren Illinois Electric Distribution			Ameren Illinois Natural Gas			Ameren Transmission		Intersegment Eliminations			Ameren
Three Months 2019:
Residential	$	489	$	224		$	—		$	—	$	—		$	713
Commercial	394		123			—			—		—			517
Industrial	94		27			—			—		—			121
Other	63		15		^(a)	—			128		(29		)	177
Total electric revenues	$	1,040	$	389		$	—		$	128	$	(29	)	$	1,528
Residential	$	8	$	—		$	65		$	—	$	—		$	73
Commercial	4		—			17			—		—			21
Industrial	1		—			2			—		—			3
Other	6		—			28		^(a)	—		—			34
Total gas revenues	$	19	$	—		$	112		$	—	$	—		$	131
Total revenues^(c)	$	1,059	$	389		$	112		$	128	$	(29	)	$	1,659



	Ameren Missouri			Ameren Illinois Electric Distribution			Ameren Illinois Natural Gas			Ameren Transmission		Intersegment Eliminations			Ameren
Three Months 2018:
Residential	$	508		$	223		$	—		$	—	$	—		$	731
Commercial	417			131			—			—		—			548
Industrial	101			28			—			—		—			129
Other	85		^(b)	10			—			113		(26		)	182		^(b)
Total electric revenues	$	1,111		$	392		$	—		$	113	$	(26	)	$	1,590
Residential	$	8		$	—		$	68		$	—	$	—		$	76
Commercial	3			—			20			—		—			23
Industrial	1			—			1			—		—			2
Other	6			—			27			—		—			33
Total gas revenues	$	18		$	—		$	116		$	—	$	—		$	134
Total revenues^(c)	$	1,129		$	392		$	116		$	113	$	(26	)	$	1,724
Nine Months 2019:
Residential	$	1,134		$	640		$	—		$	—	$	—		$	1,774
Commercial	943			370			—			—		—			1,313
Industrial	226			94			—			—		—			320
Other	214			31		^(a)	—			351		(75		)	521
Total electric revenues	$	2,517		$	1,135		$	—		$	351	$	(75	)	$	3,928
Residential	$	56		$	—		$	399		$	—	$	—		$	455
Commercial	24			—			105			—		—			129
Industrial	3			—			9			—		—			12
Other	15			—			55		^(a)	—		—			70
Total gas revenues	$	98		$	—		$	568		$	—	$	—		$	666
Total revenues^(c)	$	2,615		$	1,135		$	568		$	351	$	(75	)	$	4,594
Nine Months 2018:
Residential	$	1,272		$	663		$	—		$	—	$	—		$	1,935
Commercial	1,033			381			—			—		—			1,414
Industrial	249			96			—			—		—			345
Other	228		^(b)	39			—			320		(72		)	515		^(b)
Total electric revenues	$	2,782		$	1,179		$	—		$	320	$	(72	)	$	4,209
Residential	$	62		$	—		$	408		$	—	$	—		$	470
Commercial	25			—			113			—		—			138
Industrial	3			—			12			—		—			15
Other	4			—			36			—		—			40
Total gas revenues	$	94		$	—		$	569		$	—	$	—		$	663
Total revenues^(c)	$	2,876		$	1,179		$	569		$	320	$	(72	)	$	4,872


(a)	Includes $14 million and $5 million for Ameren Illinois Electric Distribution and Ameren Illinois Natural Gas, respectively, for the three and nine months ended September 30, 2019, for a software licensing agreement with Ameren Missouri. See Note 8 – Related-party Transactions for additional information.


(b)	Includes $13 million and $60 million for the three and nine months ended September 30, 2018, respectively, for the reduction to revenue for the excess amounts collected in rates to be refunded related to the TCJA from January 1, 2018, through July 31, 2018. See Note 2 – Rate and Regulatory Matters under Part II, Item 8, of the Form 10-K for additional information.


(c)	The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the three and nine months ended September 30, 2019 and 2018:



	Ameren Missouri		Ameren Illinois Electric Distribution			Ameren Illinois Natural Gas			Ameren Transmission			Ameren
Three Months 2019:
Revenues from alternative revenue programs	$	26	$	(145	)	$	1		$	(12	)	$	(130	)
Other revenues not from contracts with customers	5		1			1			—			7
Three Months 2018:
Revenues from alternative revenue programs	$	1	$	(98	)	$	(2	)	$	(12	)	$	(111	)
Other revenues not from contracts with customers	3		1			1			—			5



	Ameren Missouri			Ameren Illinois Electric Distribution			Ameren Illinois Natural Gas			Ameren Transmission			Ameren
Nine Months 2019:
Revenues from alternative revenue programs	$	41		$	(111	)	$	2		$	(25	)	$	(93	)
Other revenues not from contracts with customers	14			5			2			—			21
Nine Months 2018:
Revenues from alternative revenue programs	$	(8	)	$	(52	)	$	(10	)	$	(21	)	$	(91	)
Other revenues not from contracts with customers	22			14			2			—			38

Ameren Illinois



	Ameren Illinois Electric Distribution			Ameren Illinois Natural Gas			Ameren Illinois Transmission		Intersegment Eliminations			Ameren Illinois
Three Months 2019:
Residential	$	224		$	65		$	—	$	—		$	289
Commercial	123			17			—		—			140
Industrial	27			2			—		—			29
Other	15		^(a)	28		^(a)	81		(18		)	106
Total revenues^(b)	$	389		$	112		$	81	$	(18	)	$	564
Three Months 2018:
Residential	$	223		$	68		$	—	$	—		$	291
Commercial	131			20			—		—			151
Industrial	28			1			—		—			29
Other	10			27			71		(15		)	93
Total revenues^(b)	$	392		$	116		$	71	$	(15	)	$	564
Nine Months 2019:
Residential	$	640		$	399		$	—	$	—		$	1,039
Commercial	370			105			—		—			475
Industrial	94			9			—		—			103
Other	31		^(a)	55		^(a)	217		(47		)	256
Total revenues^(b)	$	1,135		$	568		$	217	$	(47	)	$	1,873
Nine Months 2018:
Residential	$	663		$	408		$	—	$	—		$	1,071
Commercial	381			113			—		—			494
Industrial	96			12			—		—			108
Other	39			36			195		(41		)	229
Total revenues^(b)	$	1,179		$	569		$	195	$	(41	)	$	1,902


(a)	Includes $14 million and $5 million for Ameren Illinois Electric Distribution and Ameren Illinois Natural Gas, respectively, for the three and nine months ended September 30, 2019, for a software licensing agreement with Ameren Missouri. See Note 8 – Related-party Transactions for additional information.


(b)	The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the Ameren Illinois segments for the three and nine months ended September 30, 2019 and 2018:



	Ameren Illinois Electric Distribution			Ameren Illinois Natural Gas			Ameren Illinois Transmission			Ameren Illinois
Three Months 2019:
Revenues from alternative revenue programs	$	(145	)	$	1		$	(12	)	$	(156	)
Other revenues not from contracts with customers	1			1			—			2
Three Months 2018:
Revenues from alternative revenue programs	$	(98	)	$	(2	)	$	(10	)	$	(110	)
Other revenues not from contracts with customers	1			1			—			2



	Ameren Illinois Electric Distribution			Ameren Illinois Natural Gas			Ameren Illinois Transmission			Ameren Illinois
Nine Months 2019:
Revenues from alternative revenue programs	$	(111	)	$	2		$	(26	)	$	(135	)
Other revenues not from contracts with customers	5			2			—			7
Nine Months 2018:
Revenues from alternative revenue programs	$	(52	)	$	(10	)	$	(19	)	$	(81	)
Other revenues not from contracts with customers	14			2			—			16

Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Basis Of Presentation And Significant Accounting Policies [Line Items]
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets and Liabilities, Net	$ 27	$ 22
Unrecorded Unconditional Purchase Obligation	1,922
Cash Surrender Value of Life Insurance	258	244
Corporate owned life insurance, borrowings	114	113
Ameren Illinois Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Unrecorded Unconditional Purchase Obligation	609
Cash Surrender Value of Life Insurance	121	122
Corporate owned life insurance, borrowings	118	$ 113
Variable Interest Entity, Not Primary Beneficiary [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	12
Partnership Funding Commitment
Basis Of Presentation And Significant Accounting Policies [Line Items]
Unrecorded Unconditional Purchase Obligation	$ 37

Rate And Regulatory Matters (Narrative-Missouri) (Detail) $ in Millions	3 Months Ended					9 Months Ended
	Dec. 31, 2019	Sep. 30, 2019 USD ($) MWh	Mar. 31, 2019 USD ($)	Dec. 31, 2018 MWh	Sep. 30, 2018 USD ($) MWh	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)
Rate And Regulatory Matters [Line Items]
Total operating revenues		$ 1,640			$ 1,724	$ 4,575	$ 4,872
Union Electric Company
Rate And Regulatory Matters [Line Items]
Frequency Rates Must be Reset to Use the FAC		four years
Depreciation Percentage Not Included in PISA Deferral						15.00%
Pending Rate Case \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 1
Public Utilities, Requested Return on Equity, Percentage		9.95%
Public Utilities, Requested Equity Capital Structure, Percentage		51.90%
Rate Base		$ 8,000
Number of Months to Complete MoPSC Electric Service Proceeding		11 months
Pending Rate Case \| Natural Gas \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 1
Public Utilities, Requested Equity Capital Structure, Percentage		52.00%
ISRS Return on Equity Percentage		9.725%
Public Utilities, Requested Rate Increase (Decrease) on Interim Rates, Amount		$ 1
Components of Rate Decrease - Net Energy Costs \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		100
Components of Rate Decrease - Customer Sales Volume \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		55
Components of Rate Decrease - Other Expenses \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		20
Components of Rate Decrease - Other Expenses Subject to Cost Recovery Mechanisms \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		80
Components of Rate Decrease - Depreciation and Amortization \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		115
Components of Rate Decrease - Higher Depreciation Rates \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		35
Components of Rate Decrease - PISA Eligible Depreciation \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		35
Components of Rate Decrease - Return on Rate Base \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		60
Components of Rate Decrease - PISA Eligible Return on Rate Base \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 30
Wind Generation Facility \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Amount of Megawatts \| MWh		300		157	400
Estimated Capital Project Costs						$ 1,200
MEEIA 2013 & 2016 \| Final Rate Order \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Total operating revenues			$ 20
MEEIA 2016 \| Final Rate Order \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Total operating revenues		$ 18	$ 5
Subsequent Event \| Pending Rate Case \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Number of Months Between Callaway Maintenance Outages	18 months
Minimum \| Pending Rate Case \| Natural Gas \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage		9.40%
Maximum \| Pending Rate Case \| Natural Gas \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage		9.95%

Rate And Regulatory Matters (Narrative-Illinois) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($)
Ameren Transmission Company of Illinois
Rate And Regulatory Matters [Line Items]
Estimated Capital Project Costs	$ 81
Property, Plant and Equipment, Additions	$ 39
Pending Rate Case \| Ameren Illinois Company \| Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage	8.90%
Public Utilities, Requested Equity Capital Structure, Percentage	50.00%
Electric Energy-Efficiency Investment	$ 205
Electric Energy-Efficiency Revenue Requirement	44
Pending Rate Case \| IEIMA \| Ameren Illinois Company \| Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	7
Pending Rate Case \| FEJA energy-efficiency rider \| Ameren Illinois Company \| Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 10

Rate And Regulatory Matters (Narrative-Federal) (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 121	$ 149
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 49	$ 62
Midwest Independent Transmission System Operator, Inc
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage	12.38%
Midwest Independent Transmission System Operator, Inc \| Final Rate Order
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage	10.32%
Customer Requested Rate on Equity	9.15%
Incentive adder to FERC allowed base return on common equity	0.50%
Midwest Independent Transmission System Operator, Inc \| Administrative Law Judge
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage	9.70%
Midwest Independent Transmission System Operator, Inc \| Pending Ferc Case
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 46
Midwest Independent Transmission System Operator, Inc \| Pending Ferc Case \| Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 27
Maximum \| Midwest Independent Transmission System Operator, Inc \| Final Rate Order
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage	10.82%
Maximum \| Midwest Independent Transmission System Operator, Inc \| Administrative Law Judge
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage	10.20%

Short-Term Debt And Liquidity (Narrative) (Detail) $ in Billions	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018	Sep. 30, 2019 USD ($)	Sep. 30, 2018
Credit Agreements
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1.6		$ 1.6
Actual debt-to-capital ratio	0.53		0.53
Utilities
Short-term Debt [Line Items]
Short-term Debt, Weighted Average Interest Rate, over Time	2.40%	2.00%	2.67%	2.02%
Union Electric Company \| Missouri Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.48		0.48
Ameren Illinois Company \| Illinois Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.47		0.47

Short-Term Debt and Liquidity (Commercial Paper outstanding) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Commercial paper outstanding	$ 544	$ 597
Ameren (parent)
Short-term Debt [Line Items]
Commercial paper outstanding	90	470
Union Electric Company
Short-term Debt [Line Items]
Commercial paper outstanding	144	55
Ameren Illinois Company
Short-term Debt [Line Items]
Commercial paper outstanding	$ 310	$ 72

Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 821	$ 629
Weighted-average interest rate	2.68%	2.18%
Short-term Debt, Maximum Amount Outstanding During Period	$ 1,113	$ 1,295
Peak interest rate	5.00%	2.55%
Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 532	$ 431
Weighted-average interest rate	2.70%	2.23%
Short-term Debt, Maximum Amount Outstanding During Period	$ 651	$ 543
Peak interest rate	3.80%	2.45%
Union Electric Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 141	$ 81
Weighted-average interest rate	2.73%	1.94%
Short-term Debt, Maximum Amount Outstanding During Period	$ 549	$ 481
Peak interest rate	2.97%	2.42%
Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 147	$ 117
Weighted-average interest rate	2.58%	2.21%
Short-term Debt, Maximum Amount Outstanding During Period	$ 310	$ 442
Peak interest rate	5.00%	2.55%

Long-Term debt and Equity Financings (Narrative) (Details) - USD ($) $ / shares in Units, shares in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2019	Oct. 30, 2019	Oct. 01, 2019	Aug. 05, 2019
Long-Term Debt And Equity Financings [Line Items]
Stock Issued During Period, Shares, New Issues	0.2	0.2	0.7	0.9
Issuance of common stock	$ 17,000,000		$ 54,000,000	$ 56,000,000
Stock Issued During Period, Shares, Other	0.0	0.0	0.8	0.7
Noncash financing activity – Issuance of common stock for stock-based compensation			$ 54,000,000	$ 35,000,000
Forward Contract Indexed to Issuer's Equity, Indexed Shares (in shares)								7.5
Forward Contract Indexed to Issuer's Equity, Initial Forward Rate (in dollars per share)								$ 74.18
Forward Contract Indexed To Issuer's Equity, Basis Spread on Variable Rate			0.75%
Forward Contract Indexed to Issuer's Equity, Settlement Alternatives, Cash, at Fair Value	$ 557,000,000		$ 557,000,000
Period End Net Cash Settlement Price	$ 47,000,000		$ 47,000,000
Period End Net Share Settlement Price (in shares)	0.6		0.6
Unsecured Debt [Member] \| First Mortgage Bonds, 2.50%, Due 2024 - $450 Issuance [Domain]
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 450,000,000		$ 450,000,000
Debt Instrument, Interest Rate, Stated Percentage	2.50%		2.50%
Proceeds from Issuance of Debt			$ 447,000,000
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Noncash financing activity – Issuance of common stock for stock-based compensation			0	0
Union Electric Company \| Secured Debt \| First Mortgage Bonds, 3.50%, Due 2029 - $450 Issuance
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 450,000,000		$ 450,000,000
Debt Instrument, Interest Rate, Stated Percentage	3.50%		3.50%
Proceeds from Issuance of Secured Debt			$ 447,000,000
Union Electric Company \| Secured Debt \| Senior Secured Notes 6.70% Due 2019
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.70%		6.70%
Repayments of other long-term debt			$ 329,000,000
Union Electric Company \| Secured Debt \| 1992 Series Bonds, 1998 Series A Bonds, 1998 Series B Bonds, 1998 Series C Bonds [Domain]
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 207,000,000		207,000,000
Union Electric Company \| Secured Debt \| First Mortgage Bonds, 5.45%, Due 2028 - Less Than $1 Million Outstanding [Domain]
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 1,000,000		$ 1,000,000
Debt Instrument, Interest Rate, Stated Percentage	5.45%		5.45%
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Noncash financing activity – Issuance of common stock for stock-based compensation			$ 0	$ 0
Ameren Illinois Company \| Unsecured Debt [Member] \| Senior Unsecured 1993 Series B-1 [Domain]
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 17,000,000		17,000,000
Ameren Illinois Company \| Secured Debt \| First Mortgage Bonds, 5.70%, Due 2024 - Less Than $1 Million Outstanding [Domain]
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 1,000,000		$ 1,000,000
Debt Instrument, Interest Rate, Stated Percentage	5.70%		5.70%
Subsequent Event \| Union Electric Company \| Unsecured Debt [Member] \| Senior Unsecured Notes 5.10% Due 2019 [Domain]
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Interest Rate, Stated Percentage							5.10%
Repayments of other long-term debt					$ 244,000,000
Subsequent Event \| Union Electric Company \| Secured Debt \| First Mortgage Bonds, 3.25%, Due 2049 - $330 Issuance [Member]
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount							$ 330,000,000
Debt Instrument, Interest Rate, Stated Percentage					3.25%
Proceeds from Issuance of Secured Debt					$ 326,000,000
Subsequent Event \| Ameren Illinois Company \| Secured Debt \| First Mortgage Bonds, 5.90%, Due 2023 - Less Than $1 Million Outstanding [Domain]
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount						$ 1,000,000
Debt Instrument, Interest Rate, Stated Percentage						5.90%
Subsequent Event \| Ameren Illinois Company \| Secured Debt \| Senior Secured Notes. 6.70%, Due 2023 [Domain]
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Interest Rate, Stated Percentage						6.70%

Long-Term Debt and Equity Financings Environmental Bonds (Details) - Union Electric Company - Environmental Improvement And Pollution Control Revenue Bonds [Member]	9 Months Ended
	Sep. 30, 2019 USD ($)
Series 1992 Due 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 47,000,000
Debt Instrument, Interest Rate, Stated Percentage	1.60%
Debt Instrument, Interest Rate During Period	2.36%
Series 1998 Due 2033 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 60,000,000
Debt Instrument, Interest Rate, Stated Percentage	2.90%
Debt Instrument, Interest Rate During Period	3.35%
Series B 1998 Due 2033 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 50,000,000
Debt Instrument, Interest Rate, Stated Percentage	2.90%
Debt Instrument, Interest Rate During Period	3.34%
Series C 1998 Due 2033 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 50,000,000
Debt Instrument, Interest Rate, Stated Percentage	2.75%
Debt Instrument, Interest Rate During Period	3.83%

Other Income, Net (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
Other Income, Net (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 7	$ 11	$ 20	$ 25
Interest income on industrial development revenue bonds	6	6	19	19
Other interest income	2	2	6	6
Non-service cost components of net periodic benefit income	23	17	67	52
Miscellaneous income	2	2	6	5
Donations	(1)	(4)	(8)	(15)
Miscellaneous expense	(5)	(2)	(11)	(8)
Total Other Income, Net	34	32	99	84
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	6	8	14	19
Interest income on industrial development revenue bonds	6	6	19	19
Other interest income	0	1	0	2
Non-service cost components of net periodic benefit income	4	4	13	13
Miscellaneous income	2	2	4	3
Donations	(1)	(3)	(3)	(6)
Miscellaneous expense	(2)	(2)	(4)	(5)
Total Other Income, Net	15	16	43	45
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	7	5	22	13
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	1	3	6	6
Other interest income	1	1	5	4
Non-service cost components of net periodic benefit income	12	8	36	25
Miscellaneous income	1	1	3	3
Donations	0	0	(5)	(5)
Miscellaneous expense	(2)	(2)	(6)	(3)
Total Other Income, Net	$ 13	$ 11	$ 39	$ 30

Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Fuel Oils
Derivative [Line Items]
Derivative Asset, Notional Amount	$ 63	$ 66
Natural Gas
Derivative [Line Items]
Derivative Asset, Notional Amount	162	173
Power
Derivative [Line Items]
Derivative Asset, Notional Amount	12	9
Union Electric Company \| Fuel Oils
Derivative [Line Items]
Derivative Asset, Notional Amount	63	66
Union Electric Company \| Natural Gas
Derivative [Line Items]
Derivative Asset, Notional Amount	20	19
Union Electric Company \| Power
Derivative [Line Items]
Derivative Asset, Notional Amount	4	1
Ameren Illinois Company \| Fuel Oils
Derivative [Line Items]
Derivative Asset, Notional Amount	0	0
Ameren Illinois Company \| Natural Gas
Derivative [Line Items]
Derivative Asset, Notional Amount	142	154
Ameren Illinois Company \| Power
Derivative [Line Items]
Derivative Asset, Notional Amount	$ 8	$ 8

Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative assets	$ 21	$ 15
Derivative liabilities	243	219
Fuel Oils \| Other Current Assets
Derivative [Line Items]
Derivative assets	4	3
Fuel Oils \| Other Assets
Derivative [Line Items]
Derivative assets	2	5
Fuel Oils \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	7	4
Fuel Oils \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	5	9
Natural Gas \| Other Current Assets
Derivative [Line Items]
Derivative assets	2	1
Natural Gas \| Other Assets
Derivative [Line Items]
Derivative assets	1	2
Natural Gas \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	15	12
Natural Gas \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	8	7
Power \| Other Current Assets
Derivative [Line Items]
Derivative assets	8	4
Power \| Other Assets
Derivative [Line Items]
Derivative assets	4	0
Power \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	18	18
Power \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	190	169
Union Electric Company
Derivative [Line Items]
Derivative assets	18	12
Derivative liabilities	20	22
Union Electric Company \| Fuel Oils \| Other Current Assets
Derivative [Line Items]
Derivative assets	4	3
Union Electric Company \| Fuel Oils \| Other Assets
Derivative [Line Items]
Derivative assets	2	5
Union Electric Company \| Fuel Oils \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	7	4
Union Electric Company \| Fuel Oils \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	5	9
Union Electric Company \| Natural Gas \| Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Union Electric Company \| Natural Gas \| Other Assets
Derivative [Line Items]
Derivative assets	0	0
Union Electric Company \| Natural Gas \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	3	4
Union Electric Company \| Natural Gas \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	1	1
Union Electric Company \| Power \| Other Current Assets
Derivative [Line Items]
Derivative assets	8	4
Union Electric Company \| Power \| Other Assets
Derivative [Line Items]
Derivative assets	4	0
Union Electric Company \| Power \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	3	4
Union Electric Company \| Power \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	1	0
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	3	3
Derivative liabilities	223	197
Ameren Illinois Company \| Fuel Oils \| Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company \| Fuel Oils \| Other Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company \| Fuel Oils \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company \| Fuel Oils \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company \| Natural Gas \| Other Current Assets
Derivative [Line Items]
Derivative assets	2	1
Ameren Illinois Company \| Natural Gas \| Other Assets
Derivative [Line Items]
Derivative assets	1	2
Ameren Illinois Company \| Natural Gas \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	12	8
Ameren Illinois Company \| Natural Gas \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	7	6
Ameren Illinois Company \| Power \| Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company \| Power \| Other Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company \| Power \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	15	14
Ameren Illinois Company \| Power \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	$ 189	$ 169

Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	$ 790	$ 679
Assets fair value	811	694
Excluded receivables, payables, and accrued income, net	8	5
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	518	427
Assets fair value	519	428
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	272	252
Assets fair value	272	255
Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Assets fair value	20	11
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	21	15
Derivative liabilities	243	219
Commodity Contract \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	1
Derivative liabilities	6	2
Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	3
Derivative liabilities	18	16
Commodity Contract \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	20	11
Derivative liabilities	219	201
Fuel Oils \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	6	8
Derivative liabilities	12	13
Fuel Oils \| Commodity Contract \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	1
Derivative liabilities	4	2
Fuel Oils \| Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Fuel Oils \| Commodity Contract \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	6	7
Derivative liabilities	8	11
Power \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	12	4
Derivative liabilities	208	187
Power \| Commodity Contract \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	0
Derivative liabilities	0	0
Power \| Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	1
Derivative liabilities	1	1
Power \| Commodity Contract \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	11	3
Derivative liabilities	207	186
Natural Gas \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	3
Derivative liabilities	23	19
Natural Gas \| Commodity Contract \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	2	0
Natural Gas \| Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	2
Derivative liabilities	17	15
Natural Gas \| Commodity Contract \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	1
Derivative liabilities	4	4
Equity Securities \| U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	518	427
Equity Securities \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) \| U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	518	427
Equity Securities \| Significant Other Observable Inputs (Level 2) \| U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Equity Securities \| Significant Other Unobservable Inputs (Level 3) \| U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	86	72
Debt Securities \| US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	136	148
Debt Securities \| Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	50	32
Debt Securities \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) \| US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) \| Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities \| Significant Other Observable Inputs (Level 2) \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	86	72
Debt Securities \| Significant Other Observable Inputs (Level 2) \| US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	136	148
Debt Securities \| Significant Other Observable Inputs (Level 2) \| Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	50	32
Debt Securities \| Significant Other Unobservable Inputs (Level 3) \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities \| Significant Other Unobservable Inputs (Level 3) \| US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities \| Significant Other Unobservable Inputs (Level 3) \| Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	790	679
Assets fair value	808	691
Excluded receivables, payables, and accrued income, net	8	5
Union Electric Company \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	518	427
Assets fair value	519	428
Union Electric Company \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	272	252
Assets fair value	272	253
Union Electric Company \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Assets fair value	17	10
Union Electric Company \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	18	12
Derivative liabilities	20	22
Union Electric Company \| Commodity Contract \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	1
Derivative liabilities	4	2
Union Electric Company \| Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	1
Derivative liabilities	4	6
Union Electric Company \| Commodity Contract \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	17	10
Derivative liabilities	12	14
Union Electric Company \| Fuel Oils \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	6	8
Derivative liabilities	12	13
Union Electric Company \| Fuel Oils \| Commodity Contract \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	1
Derivative liabilities	4	2
Union Electric Company \| Fuel Oils \| Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Union Electric Company \| Fuel Oils \| Commodity Contract \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	6	7
Derivative liabilities	8	11
Union Electric Company \| Power \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	12	4
Derivative liabilities	4	4
Union Electric Company \| Power \| Commodity Contract \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	0
Derivative liabilities	0	0
Union Electric Company \| Power \| Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	1
Derivative liabilities	1	1
Union Electric Company \| Power \| Commodity Contract \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	11	3
Derivative liabilities	3	3
Union Electric Company \| Natural Gas \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	4	5
Union Electric Company \| Natural Gas \| Commodity Contract \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Union Electric Company \| Natural Gas \| Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	3	5
Union Electric Company \| Natural Gas \| Commodity Contract \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	1	0
Union Electric Company \| Equity Securities \| U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	518	427
Union Electric Company \| Equity Securities \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) \| U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	518	427
Union Electric Company \| Equity Securities \| Significant Other Observable Inputs (Level 2) \| U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company \| Equity Securities \| Significant Other Unobservable Inputs (Level 3) \| U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company \| Debt Securities \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	86	72
Union Electric Company \| Debt Securities \| US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	136	148
Union Electric Company \| Debt Securities \| Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	50	32
Union Electric Company \| Debt Securities \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company \| Debt Securities \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) \| US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company \| Debt Securities \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) \| Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company \| Debt Securities \| Significant Other Observable Inputs (Level 2) \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	86	72
Union Electric Company \| Debt Securities \| Significant Other Observable Inputs (Level 2) \| US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	136	148
Union Electric Company \| Debt Securities \| Significant Other Observable Inputs (Level 2) \| Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	50	32
Union Electric Company \| Debt Securities \| Significant Other Unobservable Inputs (Level 3) \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company \| Debt Securities \| Significant Other Unobservable Inputs (Level 3) \| US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company \| Debt Securities \| Significant Other Unobservable Inputs (Level 3) \| Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Ameren Illinois Company \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	223	197
Ameren Illinois Company \| Commodity Contract \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	2	0
Ameren Illinois Company \| Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	14	10
Ameren Illinois Company \| Commodity Contract \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	207	187
Ameren Illinois Company \| Power \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	204	183
Ameren Illinois Company \| Power \| Commodity Contract \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company \| Power \| Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company \| Power \| Commodity Contract \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	204	183
Ameren Illinois Company \| Natural Gas \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	3
Derivative liabilities	19	14
Ameren Illinois Company \| Natural Gas \| Commodity Contract \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	2	0
Ameren Illinois Company \| Natural Gas \| Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	2
Derivative liabilities	14	10
Ameren Illinois Company \| Natural Gas \| Commodity Contract \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	1
Derivative liabilities	$ 3	$ 4

Fair Value Measurements (Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level Three In The Fair Value Hierarchy) (Detail) - Power - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	$ 0	$ 1	$ 0	$ 5
Change in unrealized gains (losses) related to assets/liabilities held at period end	(21)	0	(23)	(3)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	(196)	(187)	(196)	(187)	$ (176)	$ (183)	$ (185)	$ (188)
Included in regulatory assets/liabilities	(21)	(4)	(20)	(8)
Settlements	3	2	9	5
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(2)	(1)	(2)	(1)
Union Electric Company
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	1	0	5
Change in unrealized gains (losses) related to assets/liabilities held at period end	(4)	0	8	(1)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	8	0	8	0	15	0	5	7
Included in regulatory assets/liabilities	(4)	(4)	12	(7)
Settlements	(1)	(1)	(2)	(4)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(2)	(1)	(2)	(1)
Ameren Illinois Company
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0	0	0
Change in unrealized gains (losses) related to assets/liabilities held at period end	(17)	0	(31)	(2)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	(204)	(187)	(204)	(187)	$ (191)	$ (183)	$ (190)	$ (195)
Included in regulatory assets/liabilities	(17)	0	(32)	(1)
Settlements	4	3	$ 11	9
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	$ 0	$ 0		$ 0

Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) - Power $ in Millions	Sep. 30, 2019 USD ($) $ / MWh $ / MMBTU	Sep. 30, 2018 USD ($) $ / MWh $ / MMBTU
Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets \| $	$ 11	$ 3
Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities \| $	$ (207)	$ (186)
Measurement Input, Quoted Price [Member] \| Discounted Cash Flow \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	22
Measurement Input, Quoted Price [Member] \| Discounted Cash Flow \| Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	37
Commodity Forward Price \| Discounted Cash Flow \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input		23
Commodity Forward Price \| Discounted Cash Flow \| Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input		39
Commodity Forward Price \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	25	28
Nodal Basis \| Discounted Cash Flow \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	(7)	(9)
Nodal Basis \| Discounted Cash Flow \| Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0	0
Nodal Basis \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	(3)	(2)
Commodity Future Price \| Fundamental Energy Production Model \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input \| $ / MMBTU	3	3
Commodity Future Price \| Fundamental Energy Production Model \| Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input \| $ / MMBTU	3	4
Commodity Future Price \| Fundamental Energy Production Model \| Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input \| $ / MMBTU	3	3

Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Financial Assets and Liabilities) (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 155	$ 107	$ 92	$ 68
Short-term Debt	544	597
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	14	8	7	7
Short-term Debt	144	55
Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	117	80	$ 71	$ 41
Short-term Debt	310	72
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	155	107
Available-for-sale Securities and Held-to-maturity Securities	270	270
Short-term Debt	544	597
Long-term debt (including current portion)	8,987	8,439
Debt Issuance Costs, Net	65	58
Carrying Amount \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	14	8
Available-for-sale Securities and Held-to-maturity Securities	270	270
Short-term Debt	144	55
Long-term debt (including current portion)	4,115	3,998
Debt Issuance Costs, Net	27	22
Carrying Amount \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	117	80
Short-term Debt	310	72
Long-term debt (including current portion)	3,279	3,296
Debt Issuance Costs, Net	31	31
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	155	107
Short-term Debt, Fair Value	544	597
Investments, Fair Value Disclosure	270	270
Long-term Debt, Fair Value	10,218	8,669
Fair Value \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	155	107
Short-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Fair Value \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Short-term Debt, Fair Value	544	597
Investments, Fair Value Disclosure	270	270
Long-term Debt, Fair Value	9,727	8,240
Fair Value \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Short-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	491	429
Fair Value \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	14	8
Short-term Debt, Fair Value	144	55
Investments, Fair Value Disclosure	270	270
Long-term Debt, Fair Value	4,766	4,156
Fair Value \| Union Electric Company \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	14	8
Short-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Fair Value \| Union Electric Company \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Short-term Debt, Fair Value	144	55
Investments, Fair Value Disclosure	270	270
Long-term Debt, Fair Value	4,766	4,156
Fair Value \| Union Electric Company \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Short-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Fair Value \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	117	80
Short-term Debt, Fair Value	310	72
Long-term Debt, Fair Value	3,788	3,391
Fair Value \| Ameren Illinois Company \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	117	80
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	0	0
Fair Value \| Ameren Illinois Company \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Short-term Debt, Fair Value	310	72
Long-term Debt, Fair Value	3,788	3,391
Fair Value \| Ameren Illinois Company \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	$ 0	$ 0

Related Party Transactions (Schedule of Related Party Transactions) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Related Party Transaction [Line Items]
Operating Revenues	$ 19	$ 0	$ 19	$ 0
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	0	0	0	0
Ameren Missouri Power Supply Agreements with Ameren Illinois \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	1	5	3	11
Ameren Missouri and Ameren Illinois Rent and Facility Services \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	7	6	20	17
Operating Expenses	1	1	1	2
Ameren Missouri and Ameren Illinois Rent and Facility Services \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	1	2	2
Operating Expenses	1	1	4	4
Ameren Missouri and Ameren Illinois Miscellaneous Support Services \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	1
Ameren Missouri and Ameren Illinois Miscellaneous Support Services \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	1
Ameren Missouri Software Licensing with Ameren Illinois \| Ameren Illinois Natural Gas
Related Party Transaction [Line Items]
Operating Revenues	5
Ameren Missouri Software Licensing with Ameren Illinois \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	19		19
Total Related Party Operating Revenues \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	9	11	24	28
Total Related Party Operating Revenues \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	20	1	22	2
Ameren Illinois Power Supply Agreements with Ameren Missouri \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	1	5	3	11
Ameren Illinois Transmission Services With ATXI \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	1	1	1	1
Purchased Power \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	2	5	4	12
Ameren Services Support Services Agreement \| Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	34	36	98	101
Ameren Services Support Services Agreement \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	30	33	91	93
Total Related Party Other Operations and Maintenance \| Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	34	37	99	103
Total Related Party Other Operations and Maintenance \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	31	34	95	97
Money Pool \| Union Electric Company
Related Party Transaction [Line Items]
Interest Charges	1	1	1	1
Money Pool \| Ameren Illinois Company
Related Party Transaction [Line Items]
Interest Charges	$ 1	$ 1	$ 1	$ 1

Related Party Transactions Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) MWh $ / MWh	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Related Party Transaction [Line Items]
Revenue from Related Parties	$ 19	$ 0	$ 19	$ 0
Ameren Illinois Company
Related Party Transaction [Line Items]
Revenue from Related Parties	0	$ 0	0	$ 0
Accounts payable - affiliates	67		67		$ 58
Accounts receivable - affiliates	38		38		21
Ameren Illinois Company \| Ameren Missouri Software Licensing with Ameren Illinois
Related Party Transaction [Line Items]
Related Party Software Licensing Agreement	24
Capitalized Computer Software, Net	5		5
Revenue from Related Parties	19		19
Accounts receivable - affiliates	24		24
Ameren Illinois Electric Distribution \| Ameren Missouri Software Licensing with Ameren Illinois
Related Party Transaction [Line Items]
Revenue from Related Parties	14
Ameren Illinois Natural Gas \| Ameren Missouri Software Licensing with Ameren Illinois
Related Party Transaction [Line Items]
Revenue from Related Parties	5
Union Electric Company
Related Party Transaction [Line Items]
Accounts payable - affiliates	126		126		69
Accounts receivable - affiliates	20		20		$ 14
Union Electric Company \| Ameren Missouri Software Licensing with Ameren Illinois
Related Party Transaction [Line Items]
Capitalized Computer Software, Gross	24		24
Accounts payable - affiliates	$ 24		$ 24
April 2019 Procurement \| Ameren Illinois Company \| Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power \| MWh			288,000
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate \| $ / MWh			35
September 2019 Procurement \| Ameren Illinois Company \| Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power \| MWh			170,800
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate \| $ / MWh			29

Related Party Transactions (Schedule of Affiliate Receivables and Payables) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Union Electric Company
Related Party Transaction [Line Items]
Accounts payable - affiliates	$ 126	$ 69
Accounts receivable - affiliates	20	14
Union Electric Company \| Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts payable - affiliates	58	16
Union Electric Company \| Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts receivable - affiliates	0	0
Ameren Illinois Company
Related Party Transaction [Line Items]
Accounts payable - affiliates	67	58
Accounts receivable - affiliates	38	21
Ameren Illinois Company \| Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts payable - affiliates	24	7
Ameren Illinois Company \| Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts receivable - affiliates	$ 0	$ 6

Commitments And Contingencies (Other Obligations) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($) MWh
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	$ 291
2020	630
2021	438
2022	235
2023	140
Thereafter	188
Total unrecorded unconditional purchase obligation	1,922
Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	142
2020	226
2021	195
2022	137
2023	46
Thereafter	0
Total unrecorded unconditional purchase obligation	746
Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	51
2020	172
2021	105
2022	49
2023	25
Thereafter	48
Total unrecorded unconditional purchase obligation	450
Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	21
2020	43
2021	59
2022	12
2023	42
Thereafter	29
Total unrecorded unconditional purchase obligation	206
Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	56
2020	146
2021	50
2022	12
2023	2
Thereafter	0
Total unrecorded unconditional purchase obligation	$ 266
Amount of Megawatts \| MWh	102
Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	$ 1
2020	3
2021	3
2022	3
2023	3
Thereafter	26
Total unrecorded unconditional purchase obligation	39
Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	20
2020	40
2021	26
2022	22
2023	22
Thereafter	85
Total unrecorded unconditional purchase obligation	215
Renewable Energy Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Total unrecorded unconditional purchase obligation	11
Zero Emission Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Total unrecorded unconditional purchase obligation	44
Union Electric Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	190
2020	340
2021	302
2022	185
2023	123
Thereafter	120
Total unrecorded unconditional purchase obligation	1,260
Union Electric Company \| Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	142
2020	226
2021	195
2022	137
2023	46
Thereafter	0
Total unrecorded unconditional purchase obligation	746
Union Electric Company \| Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	10
2020	39
2021	23
2022	11
2023	10
Thereafter	17
Total unrecorded unconditional purchase obligation	110
Union Electric Company \| Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	21
2020	43
2021	59
2022	12
2023	42
Thereafter	29
Total unrecorded unconditional purchase obligation	206
Union Electric Company \| Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	0
2020	0
2021	0
2022	0
2023	0
Thereafter	0
Total unrecorded unconditional purchase obligation	$ 0
Amount of Megawatts \| MWh	102
Union Electric Company \| Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	$ 1
2020	3
2021	3
2022	3
2023	3
Thereafter	26
Total unrecorded unconditional purchase obligation	39
Union Electric Company \| Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	16
2020	29
2021	22
2022	22
2023	22
Thereafter	48
Total unrecorded unconditional purchase obligation	159
Ameren Illinois Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	99
2020	280
2021	132
2022	50
2023	17
Thereafter	31
Total unrecorded unconditional purchase obligation	609
Ameren Illinois Company \| Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	0
2020	0
2021	0
2022	0
2023	0
Thereafter	0
Total unrecorded unconditional purchase obligation	0
Ameren Illinois Company \| Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	41
2020	133
2021	82
2022	38
2023	15
Thereafter	31
Total unrecorded unconditional purchase obligation	340
Ameren Illinois Company \| Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	0
2020	0
2021	0
2022	0
2023	0
Thereafter	0
Total unrecorded unconditional purchase obligation	0
Ameren Illinois Company \| Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	56
2020	146
2021	50
2022	12
2023	2
Thereafter	0
Total unrecorded unconditional purchase obligation	266
Ameren Illinois Company \| Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	0
2020	0
2021	0
2022	0
2023	0
Thereafter	0
Total unrecorded unconditional purchase obligation	0
Ameren Illinois Company \| Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	2
2020	1
2021	0
2022	0
2023	0
Thereafter	0
Total unrecorded unconditional purchase obligation	3
Ameren Illinois Company \| Renewable Energy Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Total unrecorded unconditional purchase obligation	11
Ameren Illinois Company \| Zero Emission Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Total unrecorded unconditional purchase obligation	$ 44

Commitments And Contingencies (Environmental Matters) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($) waste_stream scrubber center site	Dec. 31, 2018 USD ($)
Loss Contingencies [Line Items]
Percentage of Rate Base Related to Carbon Dioxide Energy Centers		16.00%
Asset Retirement Obligation	$ 694	$ 650
Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	300
Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 400
Union Electric Company
Loss Contingencies [Line Items]
Percentage of Rate Base Related to Carbon Dioxide Energy Centers		32.00%
Number of Energy Center Scrubbers \| scrubber	2
Postponement of EPA's 2015 Rule	two years
Waste Streams with Postponed Compliance Dates \| waste_stream	2
Number of Energy Centers Constructing Wastewater Treatment Facilities \| center	3
Asset Retirement Obligation	$ 690	$ 646
Union Electric Company \| Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	300
Union Electric Company \| Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	400
Ameren Illinois Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	4	$ 4
Coal Combustion Residuals Estimate \| Union Electric Company \| Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	150
Coal Combustion Residuals Estimate \| Union Electric Company \| Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	200
Manufactured Gas Plant
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 145
Manufactured Gas Plant \| Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites \| site	44
Accrual for environmental loss contingencies	$ 139
Manufactured Gas Plant \| Ameren Illinois Company \| Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	139
Manufactured Gas Plant \| Ameren Illinois Company \| Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	209
Rush Island Energy Center [Member] \| Union Electric Company
Loss Contingencies [Line Items]
Estimated capital costs to comply with preliminary court order	1,000
Rush Island Energy Center [Member] \| Union Electric Company \| Minimum
Loss Contingencies [Line Items]
Estimated operations and maintenance costs to comply with preliminary court order	30
Rush Island Energy Center [Member] \| Union Electric Company \| Maximum
Loss Contingencies [Line Items]
Estimated operations and maintenance costs to comply with preliminary court order	50
New CCR Rules Estimate
Loss Contingencies [Line Items]
Asset Retirement Obligation	162
New CCR Rules Estimate \| Union Electric Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 160

Callaway Energy Center (Insurance Disclosure) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($)
Nuclear Waste Matters [Line Items]
Inflationary Adjustment Prescribed By Most Recent Price-Anderson Act Renewal, In Years	five years
Number Of Weeks Of Coverage After The First Twelve Weeks Of An Outage	1
Number Of Additional Weeks After Initial Indemnity Coverage For Power Outage	1.365
Public Liability And Nuclear Worker Liability - American Nuclear Insurers
Nuclear Waste Matters [Line Items]
Maximum Coverages	$ 450.0
Maximum Assessments for Single Incidents	0.0
Public Liability And Nuclear Worker Liability - Pool Participation
Nuclear Waste Matters [Line Items]
Maximum Coverages	13,486.0
Maximum Assessments for Single Incidents	138.0
Threshold For Which A Retrospective Assessment For A Covered Loss Is Necessary	450.0
Annual Payment In The Event Of An Incident At Any Licensed Commercial Reactor	21.0
Public Liability
Nuclear Waste Matters [Line Items]
Maximum Coverages	13,936.0
Maximum Assessments for Single Incidents	138.0
Property Damage - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Maximum Coverages	3,200.0
Maximum Assessments for Single Incidents	28.0
Replacement Power - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Maximum Coverages	490.0
Maximum Assessments for Single Incidents	7.0
Amount Of Weekly Indemnity Coverage Commencing Twelve Weeks After Power Outage	4.5
Amount Of Additional Weekly Indemnity Coverage Commencing After Initial Indemnity Coverage	3.6
Amount Of Weekly Indemnity Coverage Thereafter Not Exceeding Policy Limit	490.0
Sub-Limit Of Amount Of Weekly Indemnity Coverage Thereafter Not Exceeding Policy Limit For Non-Nuclear Events	328.0
Radiation Event
Nuclear Waste Matters [Line Items]
Maximum Coverages	2,700.0
Aggregate Nuclear Power Industry Insurance Policy Limit For Losses From Terrorist Attacks Within Twelve Month Period	3,200.0
Non-Radiation Event
Nuclear Waste Matters [Line Items]
Maximum Coverages	2,300.0
Aggregate Nuclear Power Industry Insurance Policy Limit For Losses From Terrorist Attacks Within Twelve Month Period	1,800.0
Property Damage European Mutual Association for Nuclear Insurance
Nuclear Waste Matters [Line Items]
Maximum Coverages	$ 490.0

Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	$ (23)	$ (17)	$ (67)	$ (52)
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	22	25	66	75
Interest cost	46	42	139	126
Expected return on plan assets	(69)	(68)	(207)	(206)
Prior service cost (benefit)	0	0	0	0
Actuarial loss	6	17	19	51
Total non-service cost components	(17)	(9)	(49)	(29)
Net periodic benefit cost	5	16	17	46
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	4	6	13	16
Interest cost	11	10	32	30
Expected return on plan assets	(19)	(20)	(57)	(58)
Prior service cost (benefit)	(1)	(1)	(4)	(3)
Actuarial loss	(4)	(2)	(11)	(5)
Total non-service cost components	(13)	(13)	(40)	(36)
Net periodic benefit cost	(9)	(7)	(27)	(20)
Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(4)	(4)	(13)	(13)
Union Electric Company \| Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	1	6	3	17
Union Electric Company \| Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(1)	(1)	(4)	(1)
Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(12)	(8)	(36)	(25)
Ameren Illinois Company \| Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	5	11	15	30
Ameren Illinois Company \| Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ (8)	$ (6)	$ (23)	$ (19)

Retirement Benefits (Summary of Benefit Plan Costs Incurred) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 5	$ 16	$ 17	$ 46
Pension Plan \| Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	1	6	3	17
Pension Plan \| Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	5	11	15	30
Pension Plan \| Other Affiliated Entities and Intercompany Eliminations [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(1)	(1)	(1)	(1)
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(9)	(7)	(27)	(20)
Other Postretirement Benefit Plan, Defined Benefit \| Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(1)	(1)	(4)	(1)
Other Postretirement Benefit Plan, Defined Benefit \| Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(8)	(6)	(23)	(19)
Other Postretirement Benefit Plan, Defined Benefit \| Other Affiliated Entities and Intercompany Eliminations [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 0	$ 0	$ 0	$ 0

Income Taxes Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Taxes [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	21.00%	21.00%	21.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent	(7.00%)	(6.00%)	(7.00%)	(3.00%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent	(1.00%)	(1.00%)	(1.00%)	(1.00%)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	6.00%	6.00%	6.00%	6.00%
Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Percent	1.00%	0.00%	(1.00%)	0.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	0.00%	3.00%	0.00%	1.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent			0.00%	(1.00%)
Effective Income Tax Rate Reconciliation, Percent	20.00%	23.00%	18.00%	23.00%
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability				$ 13
Union Electric Company
Income Taxes [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	21.00%	21.00%	21.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent	(11.00%)	(7.00%)	(12.00%)	(4.00%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent	(1.00%)	0.00%	(1.00%)	0.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	5.00%	4.00%	5.00%	4.00%
Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Percent	0.00%	0.00%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	0.00%	0.00%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent			0.00%	0.00%
Effective Income Tax Rate Reconciliation, Percent	14.00%	18.00%	13.00%	21.00%
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability				$ 4
Ameren Illinois Company
Income Taxes [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	21.00%	21.00%	21.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent	(3.00%)	(3.00%)	(4.00%)	(4.00%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent	0.00%	(1.00%)
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent	0.00%	0.00%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	6.00%	5.00%	7.00%	7.00%
Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Percent	0.00%	0.00%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	0.00%	4.00%	0.00%	1.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent			0.00%	0.00%
Effective Income Tax Rate Reconciliation, Percent	24.00%	26.00%	24.00%	25.00%
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability				$ 4

Supplemental Information (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Supplemental Information [Abstract]
Payables for purchased receivables	$ 37	$ 33
Deferred Compensation Liability, Classified, Noncurrent	$ 78	$ 80

Supplemental Information (Cash and Cash Equivalents) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	$ 20	$ 16
Restricted Cash and Cash Equivalents, Current	14	13
Restricted Cash and Cash Equivalents, Noncurrent	111	74
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	10	4
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	155	107	$ 92	$ 68
Union Electric Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	0	0
Restricted Cash and Cash Equivalents, Current	4	4
Restricted Cash and Cash Equivalents, Noncurrent	0	0
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	10	4
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	14	8	7	7
Ameren Illinois Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	0	0
Restricted Cash and Cash Equivalents, Current	6	6
Restricted Cash and Cash Equivalents, Noncurrent	111	74
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 117	$ 80	$ 71	$ 41

Supplemental Information (Leases - Supplemental Balance Sheet Information) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	$ 38
Operating Lease, Right-of-Use Asset	38
Operating Lease, Liability, Current	8
Operating Lease, Liability, Noncurrent	$ 30
Operating Lease, Weighted Average Remaining Lease Term	6 years
Operating Lease, Weighted Average Discount Rate, Percent	3.50%
Union Electric Company
Lessee, Lease, Description [Line Items]
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	$ 36
Lessee, Operating Lease, Renewal Term	5 years
Operating Lease, Right-of-Use Asset	$ 36
Operating Lease, Liability, Current	7
Operating Lease, Liability, Noncurrent	$ 29
Operating Lease, Weighted Average Remaining Lease Term	6 years
Operating Lease, Weighted Average Discount Rate, Percent	3.40%

Supplemental Information (Leases - Maturities of Operating Lease Liabilities) (Details) $ in Millions	Sep. 30, 2019 USD ($)
Lessee, Lease, Description [Line Items]
2019	$ 2
2020	8
2021	8
2022	7
2023	6
Thereafter	11
Total lease payments	42
Less imputed interest	4
Total	38
Union Electric Company
Lessee, Lease, Description [Line Items]
2019	2
2020	8
2021	7
2022	6
2023	6
Thereafter	10
Total lease payments	39
Less imputed interest	3
Total	$ 36

Supplemental Information (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Supplemental Cash Flow Information [Line Items]
Exchange of bond investments for extinguishment of senior unsecured notes	$ 17	$ 0
Accrued Capital Expenditures	273	240
Net Realized and Unrealized Gain (Loss) - nuclear decommissioning trust fund	100	33
Noncash financing activity – Issuance of common stock for stock-based compensation	54	35
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Exchange of bond investments for extinguishment of senior unsecured notes	0	0
Accrued Capital Expenditures	138	94
Net Realized and Unrealized Gain (Loss) - nuclear decommissioning trust fund	100	33
Noncash financing activity – Issuance of common stock for stock-based compensation	0	0
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Exchange of bond investments for extinguishment of senior unsecured notes	17	0
Accrued Capital Expenditures	128	133
Net Realized and Unrealized Gain (Loss) - nuclear decommissioning trust fund	0	0
Noncash financing activity – Issuance of common stock for stock-based compensation	$ 0	$ 0

Supplemental Information (Schedule of Asset Retirement Obligations) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	$ 694	$ 650
Liabilities settled	(10)
Accretion	21
Asset Retirement Obligation, Revision of Estimate	33
Other current liabilities	522	491
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	690	646
Liabilities settled	(10)
Accretion	21
Asset Retirement Obligation, Revision of Estimate	33
Other current liabilities	240	243
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	4	4
Liabilities settled	0
Accretion	0
Asset Retirement Obligation, Revision of Estimate	0
Other current liabilities	176	184
Asset Retirement Obligation Balance [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Other current liabilities	23	23
Asset Retirement Obligation Balance [Member] \| Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Other current liabilities	$ 23	$ 23

Supplemental Information (Summary of Nonvested Shares Related To Long-Term Incentive Plan) (Details) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Weighted-average Fair Value per Unit at Grant Date
Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Amount	$ 14	$ 6
Performance Shares
Share Units
Nonvested at the beginning of the period (in shares)	682,811
Granted (in shares)	297,728
Forfeitures (in shares)	(33,195)
Undistributed vested units (in shares)	(180,823)
Vested and distributed (in shares)	(176,923)
Nonvested at the end of the period (in shares)	589,598
Fully Vested Undistributed to Retirement-eligible employees (in shares)	448,831		619,783
Weighted-average Fair Value per Unit at Grant Date
Weighted-average Fair Value per Unit, Nonvested as of Beginning of year (in dollars per share)	$ 56.58
Weighted-average Fair Value per Unit, Granted (in dollars per share)	67.42
Weighted-average Fair Value per Unit, Forfeitures (in dollars per share)	64.34
Weighted-average Fair Value per Unit, Undistributed Vested Units (in dollars per share)	62.24
Weighted-average Fair Value per Unit, Vested (in dollars per share)	44.13
Weighted-average Fair Value per Unit, Nonvested as of Current period end (in dollars per share)	$ 63.62
Performance period	three year
Closing common share price (in dollars per share)			$ 65.23
Expected Volatility Rate	17.00%
Three-year risk-free rate	2.46%
Volatility rate, minimum	15.00%
Volatility rate, maximum	25.00%
Restricted Stock Units (RSUs)
Share Units
Nonvested at the beginning of the period (in shares)	155,253
Granted (in shares)	128,883
Forfeitures (in shares)	(11,028)
Undistributed vested units (in shares)	(40,616)
Vested and distributed (in shares)	(2,403)
Nonvested at the end of the period (in shares)	230,089
Fully Vested Undistributed to Retirement-eligible employees (in shares)	67,173		26,557
Weighted-average Fair Value per Unit at Grant Date
Weighted-average Fair Value per Unit, Nonvested as of Beginning of year (in dollars per share)	$ 57.38
Weighted-average Fair Value per Unit, Granted (in dollars per share)	65.49
Weighted-average Fair Value per Unit, Forfeitures (in dollars per share)	62.74
Weighted-average Fair Value per Unit, Undistributed Vested Units (in dollars per share)	61.91
Weighted-average Fair Value per Unit, Vested (in dollars per share)	54.30
Weighted-average Fair Value per Unit, Nonvested as of Current period end (in dollars per share)	$ 60.90

Supplemental Information (Schedule Of Excise Taxes) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Accounting Policies [Line Items]
Excise tax expense	$ 76	$ 78	$ 209	$ 222
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	49	52	118	133
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 27	$ 26	$ 91	$ 89

Supplemental Information (Earnings Per Share) (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share Reconciliation [Abstract]
Average Common Shares Outstanding - Basic (in shares)	245,900,000	244,100,000	245,500,000	243,600,000
Assumed settlement of performance share units and restricted stock units (in shares)	1,400,000	2,200,000	1,400,000	1,900,000
Dilutive effect of forward sale agreement (in shares)	200,000	0	100,000	0
Weighted Average Number of Shares Outstanding, Diluted (in shares)	247,500,000	246,300,000	247,000,000.0	245,500,000
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount (in shares)	0	0	0	0

Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) segment	Sep. 30, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments \| segment			4
External revenues	$ 1,640	$ 1,724	$ 4,575	$ 4,872
Intersegment revenues	19	0	19	0
Net income (loss) attributable to common shareholders	364	357	734	747
Capital expenditures	636	577	$ 1,761	1,689
Union Electric Company
Segment Reporting Information [Line Items]
Number of Reportable Segments \| segment			1
Ameren Illinois Electric Distribution \| Ameren Missouri Software Licensing with Ameren Illinois
Segment Reporting Information [Line Items]
Intersegment revenues	14		$ 14
Ameren Illinois Natural Gas \| Ameren Missouri Software Licensing with Ameren Illinois
Segment Reporting Information [Line Items]
Intersegment revenues	5		$ 5
Ameren Illinois Company
Segment Reporting Information [Line Items]
Number of Reportable Segments \| segment			3
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	564	564	$ 1,873	1,902
Intersegment revenues	0	0	0	0
Net income (loss) attributable to common shareholders	65	63	247	220
Capital expenditures	344	345	900	947
Ameren Illinois Company \| Ameren Missouri Software Licensing with Ameren Illinois
Segment Reporting Information [Line Items]
Intersegment revenues	19		19
Operating Segments \| Union Electric Company
Segment Reporting Information [Line Items]
External revenues	1,050	1,118	2,591	2,848
Intersegment revenues	9	11	24	28
Net income (loss) attributable to common shareholders	300	294	446	500
Capital expenditures	256	210	751	664
Operating Segments \| Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	374	392	1,118	1,177
Intersegment revenues	15	0	17	2
Net income (loss) attributable to common shareholders	32	35	105	101
Capital expenditures	139	135	390	389
Operating Segments \| Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
External revenues	107	116	563	569
Intersegment revenues	5	0	5	0
Net income (loss) attributable to common shareholders	(1)	0	57	49
Capital expenditures	113	111	241	237
Operating Segments \| Ameren Transmission
Segment Reporting Information [Line Items]
External revenues	109	98	303	278
Intersegment revenues	19	15	48	42
Net income (loss) attributable to common shareholders	53	48	139	121
Capital expenditures	129	124	377	399
Operating Segments \| Other
Segment Reporting Information [Line Items]
External revenues	0	0	0	0
Intersegment revenues	0	0	0	0
Net income (loss) attributable to common shareholders	(20)	(20)	(13)	(24)
Capital expenditures	(7)	1	3	6
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	389	392	1,135	1,179
Intersegment revenues	0	0	0	0
Net income (loss) attributable to common shareholders	32	35	105	101
Capital expenditures	139	135	390	389
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
External revenues	112	116	568	569
Intersegment revenues	0	0	0	0
Net income (loss) attributable to common shareholders	(1)	0	57	49
Capital expenditures	113	111	241	237
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Transmission
Segment Reporting Information [Line Items]
External revenues	63	56	170	154
Intersegment revenues	18	15	47	41
Net income (loss) attributable to common shareholders	34	28	85	70
Capital expenditures	92	99	269	321
Intersegment Eliminations
Segment Reporting Information [Line Items]
External revenues	0	0	0	0
Intersegment revenues	(29)	(26)	(75)	(72)
Net income (loss) attributable to common shareholders	0	0	0	0
Capital expenditures	6	(4)	(1)	(6)
Intersegment Eliminations \| Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	0	0	0	0
Intersegment revenues	(18)	(15)	(47)	(41)
Net income (loss) attributable to common shareholders	0	0	0	0
Capital expenditures	$ 0	$ 0	$ 0	$ 0

Segment Information (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 1,659	$ 1,724	$ 4,594	$ 4,872
Intersegment revenues	19	0	19	0
TCJA Revenue Reduction		13		60
Revenues from alternative revenue programs	(130)	(111)	(93)	(91)
Other revenues not from contracts with customers	7	5	21	38
Ameren Illinois Electric Distribution \| Ameren Missouri Software Licensing with Ameren Illinois
Disaggregation of Revenue [Line Items]
Intersegment revenues	14		14
Ameren Illinois Natural Gas \| Ameren Missouri Software Licensing with Ameren Illinois
Disaggregation of Revenue [Line Items]
Intersegment revenues	5		5
Electricity
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,528	1,590	3,928	4,209
Electricity \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	713	731	1,774	1,935
Electricity \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	517	548	1,313	1,414
Electricity \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	121	129	320	345
Electricity \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	177	182	521	515
Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	131	134	666	663
Natural Gas \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	73	76	455	470
Natural Gas \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	21	23	129	138
Natural Gas \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	3	2	12	15
Natural Gas \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	34	33	70	40
Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	564	564	1,873	1,902
Intersegment revenues	0	0	0	0
Ameren Illinois Company \| Ameren Missouri Software Licensing with Ameren Illinois
Disaggregation of Revenue [Line Items]
Intersegment revenues	19		19
Ameren Illinois Company \| Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	564	564	1,873	1,902
Revenues from alternative revenue programs	(156)	(110)	(135)	(81)
Other revenues not from contracts with customers	2	2	7	16
Ameren Illinois Company \| Ameren Illinois Company \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	289	291	1,039	1,071
Ameren Illinois Company \| Ameren Illinois Company \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	140	151	475	494
Ameren Illinois Company \| Ameren Illinois Company \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	29	29	103	108
Ameren Illinois Company \| Ameren Illinois Company \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	106	93	256	229
Ameren Illinois Company \| Electricity
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	452	448	1,305	1,333
Ameren Illinois Company \| Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	112	116	568	569
Operating Segments \| Union Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,059	1,129	2,615	2,876
Intersegment revenues	9	11	24	28
Revenues from alternative revenue programs	26	1	41	(8)
Other revenues not from contracts with customers	5	3	14	22
Operating Segments \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	389	392	1,135	1,179
Intersegment revenues	15	0	17	2
Revenues from alternative revenue programs	(145)	(98)	(111)	(52)
Other revenues not from contracts with customers	1	1	5	14
Operating Segments \| Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	112	116	568	569
Intersegment revenues	5	0	5	0
Revenues from alternative revenue programs	1	(2)	2	(10)
Other revenues not from contracts with customers	1	1	2	2
Operating Segments \| Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	128	113	351	320
Intersegment revenues	19	15	48	42
Revenues from alternative revenue programs	(12)	(12)	(25)	(21)
Other revenues not from contracts with customers	0	0	0	0
Operating Segments \| Electricity \| Union Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,040	1,111	2,517	2,782
Operating Segments \| Electricity \| Union Electric Company \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	489	508	1,134	1,272
Operating Segments \| Electricity \| Union Electric Company \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	394	417	943	1,033
Operating Segments \| Electricity \| Union Electric Company \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	94	101	226	249
Operating Segments \| Electricity \| Union Electric Company \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	63	85	214	228
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	389	392	1,135	1,179
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	224	223	640	663
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	123	131	370	381
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	27	28	94	96
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	15	10	31	39
Operating Segments \| Electricity \| Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Electricity \| Ameren Illinois Natural Gas \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Electricity \| Ameren Illinois Natural Gas \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Electricity \| Ameren Illinois Natural Gas \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Electricity \| Ameren Illinois Natural Gas \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Electricity \| Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	128	113	351	320
Operating Segments \| Electricity \| Ameren Transmission \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Electricity \| Ameren Transmission \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Electricity \| Ameren Transmission \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Electricity \| Ameren Transmission \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	128	113	351	320
Operating Segments \| Natural Gas \| Union Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	19	18	98	94
Operating Segments \| Natural Gas \| Union Electric Company \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	8	8	56	62
Operating Segments \| Natural Gas \| Union Electric Company \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	4	3	24	25
Operating Segments \| Natural Gas \| Union Electric Company \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1	1	3	3
Operating Segments \| Natural Gas \| Union Electric Company \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	6	6	15	4
Operating Segments \| Natural Gas \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Illinois Electric Distribution \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Illinois Electric Distribution \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Illinois Electric Distribution \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Illinois Electric Distribution \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	112	116	568	569
Operating Segments \| Natural Gas \| Ameren Illinois Natural Gas \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	65	68	399	408
Operating Segments \| Natural Gas \| Ameren Illinois Natural Gas \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	17	20	105	113
Operating Segments \| Natural Gas \| Ameren Illinois Natural Gas \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	2	1	9	12
Operating Segments \| Natural Gas \| Ameren Illinois Natural Gas \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	28	27	55	36
Operating Segments \| Natural Gas \| Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Transmission \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Transmission \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Transmission \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Transmission \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Intersegment revenues	0	0	0	0
Revenues from alternative revenue programs	(145)	(98)	(111)	(52)
Other revenues not from contracts with customers	1	1	5	14
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Intersegment revenues	0	0	0	0
Revenues from alternative revenue programs	1	(2)	2	(10)
Other revenues not from contracts with customers	1	1	2	2
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Intersegment revenues	18	15	47	41
Revenues from alternative revenue programs	(12)	(10)	(26)	(19)
Other revenues not from contracts with customers	0	0	0	0
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	389	392	1,135	1,179
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	224	223	640	663
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	123	131	370	381
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	27	28	94	96
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	15	10	31	39
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	81	71	217	195
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Transmission \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Transmission \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Transmission \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Transmission \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	81	71	217	195
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	112	116	568	569
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Natural Gas \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	65	68	399	408
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Natural Gas \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	17	20	105	113
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Natural Gas \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	2	1	9	12
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Natural Gas \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	28	27	55	36
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(29)	(26)	(75)	(72)
Intersegment revenues	(29)	(26)	(75)	(72)
Intersegment Eliminations \| Electricity
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(29)	(26)	(75)	(72)
Intersegment Eliminations \| Electricity \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Intersegment Eliminations \| Electricity \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Intersegment Eliminations \| Electricity \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Intersegment Eliminations \| Electricity \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(29)	(26)	(75)	(72)
Intersegment Eliminations \| Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Intersegment Eliminations \| Natural Gas \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Intersegment Eliminations \| Natural Gas \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Intersegment Eliminations \| Natural Gas \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Intersegment Eliminations \| Natural Gas \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Intersegment Eliminations \| Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(18)	(15)	(47)	(41)
Intersegment revenues	(18)	(15)	(47)	(41)
Intersegment Eliminations \| Ameren Illinois Company \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Intersegment Eliminations \| Ameren Illinois Company \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Intersegment Eliminations \| Ameren Illinois Company \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Intersegment Eliminations \| Ameren Illinois Company \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ (18)	$ (15)	$ (47)	$ (41)