Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2019	Jul. 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	Q2
Amendment Flag	false
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 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		245,803,323
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	Q2
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	Q2
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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,379	$ 1,563	$ 2,935	$ 3,148
Operating Expenses:
Fuel	102	186	262	374
Purchased Power	136	142	292	305
Natural gas purchased for resale	44	51	205	222
Other operations and maintenance	450	439	867	870
Depreciation and amortization	249	238	497	472
Taxes other than income taxes	118	122	244	247
Total operating expenses	1,099	1,178	2,367	2,490
Operating Income	280	385	568	658
Other Income (Expense), Net	36	29	65	52
Interest Charges	97	100	194	201
Income Before Income Taxes	219	314	439	509
Income Taxes	39	74	66	116
Net income	180	240	373	393
Less: Net Income Attributable to Noncontrolling Interests	1	1	3	3
Net Income (Loss) Available to Common Stockholders, Basic	179	239	370	390
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	180	238	374	392
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	1	1	3	3
Comprehensive Income	$ 179	$ 237	$ 371	$ 389
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic	$ 0.73	$ 0.98	$ 1.51	$ 1.60
Earnings Per Share, Diluted	$ 0.72	$ 0.97	$ 1.50	$ 1.59
Average Common Shares Outstanding - Basic (in shares)	245.6	243.7	245.3	243.3
Weighted Average Number of Shares Outstanding, Diluted (in shares)	247.2	245.8	246.8	245.1
Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,218	$ 1,396	$ 2,400	$ 2,619
Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	161	167	535	529
Union Electric Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	798	955	1,556	1,747
Operating Expenses:
Fuel	102	186	262	374
Purchased Power	60	40	111	82
Natural gas purchased for resale	8	8	35	32
Other operations and maintenance	254	241	478	473
Depreciation and amortization	139	138	279	274
Taxes other than income taxes	83	84	160	164
Total operating expenses	646	697	1,325	1,399
Operating Income	152	258	231	348
Other Income (Expense), Net	16	16	28	29
Interest Charges	45	51	92	102
Income Before Income Taxes	123	223	167	275
Income Taxes	15	54	19	67
Net income	108	169	148	208
Preferred Stock Dividends	1	1	2	2
Net Income (Loss) Attributable to Parent	107	168	146	206
Union Electric Company \| Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	773	930	1,477	1,671
Union Electric Company \| Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	25	25	79	76
Ameren Illinois Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	547	578	1,309	1,338
Operating Expenses:
Purchased Power	78	105	183	229
Natural gas purchased for resale	36	43	170	190
Other operations and maintenance	196	196	387	395
Depreciation and amortization	101	94	202	184
Taxes other than income taxes	32	35	77	76
Total operating expenses	443	473	1,019	1,074
Operating Income	104	105	290	264
Other Income (Expense), Net	15	13	26	19
Interest Charges	36	37	73	74
Income Before Income Taxes	83	81	243	209
Income Taxes	20	18	59	50
Net income	63	63	184	159
Preferred Stock Dividends	1	1	2	2
Net Income (Loss) Attributable to Parent	62	62	182	157
Ameren Illinois Company \| Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	411	436	853	885
Ameren Illinois Company \| Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 136	$ 142	$ 456	$ 453

Consolidated Statement of Income (Loss) and Comprehensive Income (Loss) (Parenthetical) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 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	Jun. 30, 2019	Dec. 31, 2018
Current Assets:
Cash and cash equivalents	$ 6	$ 16
Accounts receivable - trade (less allowance for doubtful accounts)	461	463
Unbilled revenue	326	295
Miscellaneous accounts receivable	91	79
Inventories	433	483
Current regulatory assets	107	134
Other current assets	90	63
Total current assets	1,514	1,533
Property, Plant, and Equipment, Net	23,479	22,810
Investments and Other Assets:
Nuclear decommissioning trust fund	783	684
Goodwill	411	411
Regulatory Assets, Noncurrent	1,175	1,127
Other assets	741	650
Total investments and other assets	3,110	2,872
TOTAL ASSETS	28,103	27,215
Current Liabilities:
Current maturities of long-term debt	336	580
Short-term Debt	999	597
Accounts and wages payable	593	817
Current regulatory liabilities	156	149
Other current liabilities	668	544
Total current liabilities	2,752	2,687
Long-term Debt, Net	8,222	7,859
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	2,758	2,666
Regulatory Liability, Noncurrent	4,768	4,637
Asset retirement obligations	667	627
Pension and other postretirement benefits	539	558
Other deferred credits and liabilities	464	408
Total deferred credits and other liabilities	9,196	8,896
Commitments and Contingencies
Stockholders' Equity:
Common Stock	2	2
Other paid-in capital	5,649	5,627
Retained earnings	2,161	2,024
Accumulated other comprehensive loss	(21)	(22)
Stockholder's equity	7,791	7,631
Noncontrolling Interest	142	142
Total equity	7,933	7,773
TOTAL LIABILITIES AND EQUITY	28,103	27,215
Union Electric Company
Current Assets:
Cash and cash equivalents	0	0
Accounts receivable - trade (less allowance for doubtful accounts)	221	223
Accounts receivable - affiliates	21	14
Unbilled revenue	220	155
Miscellaneous accounts receivable	52	42
Inventories	336	358
Other current assets	52	40
Total current assets	902	832
Property, Plant, and Equipment, Net	12,315	12,103
Investments and Other Assets:
Nuclear decommissioning trust fund	783	684
Regulatory Assets, Noncurrent	343	366
Other assets	355	306
Total investments and other assets	1,481	1,356
TOTAL ASSETS	14,698	14,291
Current Liabilities:
Current maturities of long-term debt	336	580
Short-term Debt	205	55
Accounts and wages payable	240	428
Accounts payable - affiliates	76	69
Taxes accrued	110	27
Current regulatory liabilities	65	68
Other current liabilities	210	175
Total current liabilities	1,242	1,402
Long-term Debt, Net	3,780	3,418
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,596	1,576
Regulatory Liability, Noncurrent	2,884	2,799
Asset retirement obligations	663	623
Pension and other postretirement benefits	214	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	1,781	1,735
Stockholder's equity	4,275	4,229
TOTAL LIABILITIES AND EQUITY	14,698	14,291
Ameren Illinois Company
Current Assets:
Cash and cash equivalents	0	0
Accounts receivable - trade (less allowance for doubtful accounts)	225	224
Accounts receivable - affiliates	37	21
Unbilled revenue	106	140
Miscellaneous accounts receivable	34	40
Inventories	97	125
Current regulatory assets	89	110
Other current assets	24	16
Total current assets	612	676
Property, Plant, and Equipment, Net	9,585	9,198
Investments and Other Assets:
Goodwill	411	411
Regulatory Assets, Noncurrent	823	759
Other assets	293	275
Total investments and other assets	1,527	1,445
TOTAL ASSETS	11,724	11,319
Current Liabilities:
Short-term Debt	199	72
Accounts and wages payable	272	302
Accounts payable - affiliates	55	58
Customer deposits	71	76
Current environmental remediation	44	42
Current regulatory liabilities	72	62
Other current liabilities	184	184
Total current liabilities	897	796
Long-term Debt, Net	3,296	3,296
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,175	1,119
Regulatory Liability, Noncurrent	1,786	1,741
Pension and other postretirement benefits	274	280
Environmental remediation	102	109
Other deferred credits and liabilities	238	204
Total deferred credits and other liabilities	3,575	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	1,721	1,539
Stockholder's equity	3,956	3,774
TOTAL LIABILITIES AND EQUITY	$ 11,724	$ 11,319

Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Millions	Jun. 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	245,800,000	244,500,000
Union Electric Company
Accounts receivable - trade allowance for doubtful accounts	$ 7	$ 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	$ 12	$ 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	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash Flows From Operating Activities:
Net income	$ 373	$ 393
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	494	463
Amortization of nuclear fuel	33	48
Amortization of debt issuance costs and premium/discounts	9	11
Deferred income taxes and investment tax credits, net	54	81
Allowance for equity funds used during construction	(13)	(14)
Stock-based compensation costs	10	10
Other	(5)	11
Changes in assets and liabilities:
Receivables	(46)	(170)
Inventories	50	46
Accounts and wages payable	(199)	(209)
Taxes accrued	77	105
Regulatory assets and liabilities	4	83
Assets, other	(11)	8
Liabilities, other	63	(50)
Pension and other postretirement benefits	(14)	4
Net cash provided by operating activities	879	820
Cash Flows From Investing Activities:
Capital expenditures	(1,125)	(1,112)
Nuclear fuel expenditures	(25)	(16)
Purchases of securities - nuclear decommissioning trust fund	(96)	(129)
Sales and maturities of securities - nuclear decommissioning trust fund	95	122
Purchase of bonds	(97)	0
Proceeds from sale of remarketed bonds	97	0
Other	(3)	6
Net cash used in investing activities	(1,154)	(1,129)
Cash Flows From Financing Activities:
Dividends on common stock	(233)	(223)
Dividends paid to noncontrolling interest holders	(3)	(3)
Short-term debt, net	401	21
Repayments of Long-term Debt	(329)	(323)
Proceeds from Issuance of Long-term Debt	450	853
Issuance of common stock	37	40
Employee payroll taxes related to stock-based compensation	(29)	(19)
Payment of Financing and Stock Issuance Costs	(4)	(9)
Net cash provided by financing activities	290	337
Net change in cash, cash equivalents, and restricted cash	15	28
Cash, cash equivalents, and restricted cash at beginning of year	107	68
Cash, cash equivalents, and restricted cash at end of period	122	96
Noncash financing activity - Issuance of common stock for stock-based compensation	54	35
Union Electric Company
Cash Flows From Operating Activities:
Net income	148	208
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	277	265
Amortization of nuclear fuel	33	48
Amortization of debt issuance costs and premium/discounts	2	3
Deferred income taxes and investment tax credits, net	(10)	(24)
Allowance for equity funds used during construction	(8)	(11)
Other	5	10
Changes in assets and liabilities:
Receivables	(83)	(205)
Inventories	22	8
Accounts and wages payable	(158)	(160)
Taxes accrued	109	152
Regulatory assets and liabilities	7	106
Assets, other	(9)	(2)
Liabilities, other	28	11
Pension and other postretirement benefits	(2)	3
Net cash provided by operating activities	361	412
Cash Flows From Investing Activities:
Capital expenditures	(495)	(454)
Nuclear fuel expenditures	(25)	(16)
Purchases of securities - nuclear decommissioning trust fund	(96)	(129)
Sales and maturities of securities - nuclear decommissioning trust fund	95	122
Purchase of bonds	(97)	0
Proceeds from sale of remarketed bonds	97	0
Money pool advances, net	0	(66)
Net cash used in investing activities	(521)	(543)
Cash Flows From Financing Activities:
Dividends on common stock	(100)	(50)
Dividends on preferred stock	(2)	(2)
Short-term debt, net	150	(39)
Repayments of Long-term Debt	(329)	(179)
Proceeds from Issuance of Long-term Debt	450	423
Payment of Financing and Stock Issuance Costs	(4)	(4)
Net cash provided by financing activities	165	149
Net change in cash, cash equivalents, and restricted cash	5	18
Cash, cash equivalents, and restricted cash at beginning of year	8	7
Cash, cash equivalents, and restricted cash at end of period	13	25
Ameren Illinois Company
Cash Flows From Operating Activities:
Net income	184	159
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	201	184
Amortization of debt issuance costs and premium/discounts	6	7
Deferred income taxes and investment tax credits, net	43	13
Allowance for equity funds used during construction	(5)	(3)
Other	0	(3)
Changes in assets and liabilities:
Receivables	35	23
Inventories	28	38
Accounts and wages payable	(38)	(35)
Taxes accrued	(22)	(23)
Regulatory assets and liabilities	1	(20)
Assets, other	1	4
Liabilities, other	24	(58)
Pension and other postretirement benefits	(11)	(2)
Net cash provided by operating activities	452	287
Cash Flows From Investing Activities:
Capital expenditures	(556)	(602)
Other	(2)	3
Net cash used in investing activities	(558)	(599)
Cash Flows From Financing Activities:
Dividends on preferred stock	(2)	(2)
Short-term debt, net	127	(62)
Money pool borrowings, net	0	31
Repayments of Long-term Debt	0	(144)
Proceeds from Issuance of Long-term Debt	0	430
Payment of Financing and Stock Issuance Costs	0	(5)
Capital contribution from parent	0	80
Net cash provided by financing activities	125	328
Net change in cash, cash equivalents, and restricted cash	19	16
Cash, cash equivalents, and restricted cash at beginning of year	80	41
Cash, cash equivalents, and restricted cash at end of period	$ 99	$ 57

Consolidated Statement of Stockholders' Equity Consolidated Statement of Stockholders' Equity - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Other Paid-in Capital	Retained Earnings	Deferred Retirement Benefit Costs	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interest	Total Ameren Corporation Stockholders' Equity	Union Electric Company	Union Electric Company Common Stock	Union Electric Company Other Paid-in Capital	Union Electric Company Preferred Stock Not Subject To Mandatory Redemption	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 Not Subject To Mandatory Redemption	Ameren Illinois Company Retained Earnings
Beginning of year at Dec. 31, 2017			$ 5,540	$ 1,660	$ (18)		$ 142						$ 1,632			$ 2,013		$ 1,235
Stock Issued During Period, Value, New Issues			40
Stock-based compensation activity			(4)
Capital contribution from parent														$ 80		80
Net income	$ 393								$ 208				208	159				159
Net Income (Loss) Available to Common Stockholders, Basic	390			390
Common stock dividends				(223)									(50)
Preferred stock dividends													(2)					(2)
Stockholders' Equity, Other				0
Change in deferred retirement benefit costs	1				(1)
Net income attributable to noncontrolling interest holder	$ 3						3
Dividends paid to noncontrolling interest holders							(3)
Beginning of year (shares) at Dec. 31, 2017	242.6
Stock Issued During Period, Shares, New Issues	0.7
Stock Issued During Period, Shares, Other	0.7
End of year (shares) at Jun. 30, 2018	244.0
End of year at Jun. 30, 2018	$ 7,528	$ 2	5,576	1,827	(19)	$ (19)	142			$ 511	$ 1,858	$ 80	1,788		$ 0	2,093	$ 62	1,392
Dividends per Common Share (in dollars per share)	$ 0.9150
Stockholders' equity, end of year at Jun. 30, 2018								$ 7,386	4,237					3,547
Beginning of year at Mar. 31, 2018			5,546	1,699	(17)		142						1,620			2,033		1,330
Stock Issued During Period, Value, New Issues			23
Stock-based compensation activity			7
Capital contribution from parent																60
Net income	$ 240								169				169	63				63
Net Income (Loss) Available to Common Stockholders, Basic	239			239
Common stock dividends				(112)									0
Preferred stock dividends													(1)					(1)
Stockholders' Equity, Other				1
Change in deferred retirement benefit costs	2				(2)
Net income attributable to noncontrolling interest holder	$ 1						1
Dividends paid to noncontrolling interest holders							(1)
Beginning of year (shares) at Mar. 31, 2018	243.6
Stock Issued During Period, Shares, New Issues	0.4
Stock Issued During Period, Shares, Other	0.0
End of year (shares) at Jun. 30, 2018	244.0
End of year at Jun. 30, 2018	$ 7,528	2	5,576	1,827	(19)	(19)	142			511	1,858	80	1,788		0	2,093	62	1,392
Dividends per Common Share (in dollars per share)	$ 0.4575
Stockholders' equity, end of year at Jun. 30, 2018								7,386	4,237					3,547
Beginning of year at Dec. 31, 2018	$ 7,773		5,627	2,024	(22)		142						1,735			2,173		1,539
Stock Issued During Period, Value, New Issues			37
Stock-based compensation activity			(15)
Capital contribution from parent														0		0
Net income	373								$ 148				148	$ 184				184
Net Income (Loss) Available to Common Stockholders, Basic	370			370
Common stock dividends				(233)									(100)
Preferred stock dividends													(2)					(2)
Stockholders' Equity, Other				0
Change in deferred retirement benefit costs	(1)				1
Net income attributable to noncontrolling interest holder	$ 3						3
Dividends paid to noncontrolling interest holders							(3)
Beginning of year (shares) at Dec. 31, 2018	244.5								102.1					25.5
Stock Issued During Period, Shares, New Issues	0.5
Stock Issued During Period, Shares, Other	0.8
End of year (shares) at Jun. 30, 2019	245.8								102.1					25.5
End of year at Jun. 30, 2019	$ 7,933	2	5,649	2,161	(21)	(21)	142			511	1,903	80	1,781		0	2,173	62	1,721
Dividends per Common Share (in dollars per share)	$ 0.9500
Stockholders' equity, end of year at Jun. 30, 2019	$ 7,791							7,791	$ 4,275					$ 3,956
Beginning of year at Mar. 31, 2019			5,625	2,099	(21)		142						1,774			2,173		1,659
Stock Issued During Period, Value, New Issues			18
Stock-based compensation activity			6
Capital contribution from parent																0
Net income	180								$ 108				108	$ 63				63
Net Income (Loss) Available to Common Stockholders, Basic	179			179
Common stock dividends				(117)									(100)
Preferred stock dividends													(1)					(1)
Stockholders' Equity, Other				0
Change in deferred retirement benefit costs	0				0
Net income attributable to noncontrolling interest holder	$ 1						1
Dividends paid to noncontrolling interest holders							(1)
Beginning of year (shares) at Mar. 31, 2019	245.6
Stock Issued During Period, Shares, New Issues	0.2
Stock Issued During Period, Shares, Other	0.0
End of year (shares) at Jun. 30, 2019	245.8								102.1					25.5
End of year at Jun. 30, 2019	$ 7,933	$ 2	$ 5,649	$ 2,161	$ (21)	$ (21)	$ 142			$ 511	$ 1,903	$ 80	$ 1,781		$ 0	$ 2,173	$ 62	$ 1,721
Dividends per Common Share (in dollars per share)	$ 0.4750
Stockholders' equity, end of year at Jun. 30, 2019	$ 7,791							$ 7,791	$ 4,275					$ 3,956

Summary Of Significant Accounting Policies

6 Months Ended

Jun. 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 placed the Spoon River project in service in February 2018, and 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. 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 June 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 June 30, 2019, the maximum exposure to loss related to these variable interest entities was approximately $20 million, which primarily represents legal costs incurred and the portion of interconnection study costs that may be incurred by Ameren and Ameren Missouri. 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 June 30, 2019, and December 31, 2018, Ameren had unconsolidated variable interests as a limited partner in various equity method investments, totaling $23 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 June 30, 2019, the maximum exposure to loss related to these variable interests is limited to the investment in these partnerships of $23 million plus associated outstanding funding commitments of $14 million. In July 2019, Ameren made $25 million in additional funding commitments.

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 June 30, 2019, the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $262 million (December 31, 2018 – $244 million) and

$125 million (December 31, 2018 – $122 million), respectively, while total borrowings against the policies were $118 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

6 Months Ended

Jun. 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 of which approximately $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 in late 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 2018, Ameren Missouri entered into a build-transfer agreement to acquire, after construction, an up-to 400-megawatt wind generation facility. In the second quarter of 2019, Ameren Missouri entered into a build-transfer agreement to acquire, after construction, an up-to 300-megawatt wind generation facility. The two build-transfer agreements collectively represent approximately $1.2 billion of capital expenditures, expected in 2020, and would support Ameren Missouri’s compliance with the Missouri renewable energy standard. Both acquisitions are subject to certain conditions, including the issuance of a certificate of convenience and necessity by the MoPSC and obtaining FERC

approval for the 300-megawatt facility, entering into an RTO transmission interconnection agreement at an acceptable cost for each facility, and other customary contract terms and conditions. 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	Enel Green Power North America, Inc.
Location	Northeastern Missouri	Northwestern Missouri
Status of certificate of convenience and necessity from the MoPSC	Approved October 2018	Requested in May 2019^(a)
Status of final interconnection costs	Received in July 2019	Received in July 2019
Status of RTO transmission interconnection agreement	Expected by the fall of 2019	Expected by the fall of 2019
Status of FERC approval	Received December 2018	To be requested in the third quarter of 2019
Expected completion date	By the end of 2020	By the end of 2020


(a)	In July 2019, Ameren Missouri, the MoPSC staff, and certain intervenors filed a nonunanimous stipulation and agreement with the MoPSC regarding the requested certificate of convenience and necessity.

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 agreement due to unacceptable interconnection costs, which made the project uneconomic and not in the best interest of Ameren Missouri’s customers.

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. Ameren Missouri expects a decision by the end of 2019. 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. RESRAM regulatory assets earn carrying costs at short-term interest rates.

MEEIA

As a result of MoPSC orders issued in September 2017, October 2018, and January 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.

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 July 2019, Ameren Missouri, the MoPSC staff, and certain intervenors filed a nonunanimous stipulation and agreement with the MoPSC to decrease Ameren Missouri’s annual revenues for natural gas delivery service by $1 million. The remaining intervenors to the regulatory rate review did not object to the agreement. The requested 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 agreement allows for the use of ISRS, which will be calculated using an ROE of 9.725%. The agreement 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. A decision by the MoPSC is expected in August 2019, with new rates expected to be effective in September 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 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. It also 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. In June 2019, the ICC staff submitted its calculation of the revenue requirement included in Ameren Illinois’ update filing, recommending an amount comparable to 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 $44 million reflected in 2020 rates, which represents an increase of $10 million from 2019 rates. 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, ATXI 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 June 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 an 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 in June 2019, and reply comments are due by late August 2019. The Notice of Inquiry addressing the FERC’s return on common equity policy, among other things, broadens the ability to comment on the new methodology beyond electric utilities that are participants in the complaint cases, and the transmission incentives Notice of Inquiry is open for industry 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 June 30, 2019, Ameren and Ameren Illinois had recorded current regulatory liabilities of $45 million and $26 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

6 Months Ended

Jun. 30, 2019

Debt Disclosure [Abstract]

SHORT-TERM DEBT AND LIQUIDITY

NOTE 3 – 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 six months ended June 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 June 30, 2019, was $1.1 billion. The Ameren Companies were in compliance with the covenants in their Credit Agreements as of June 30, 2019. As of June 30, 2019, the ratios of consolidated indebtedness to consolidated total capitalization, calculated in accordance with the provisions of the Credit Agreements, were 54%, 49%, 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 June 30, 2019, and December 31, 2018:



	June 30, 2019		December 31, 2018
Ameren (parent)	$	595	$	470
Ameren Missouri	205		55
Ameren Illinois	199		72
Ameren consolidated	$	999	$	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 six months ended June 30, 2019 and 2018:



	Ameren (parent)			Ameren Missouri			Ameren Illinois			Ameren Consolidated
2019
Average daily commercial paper outstanding at par value	$	542		$	174		$	106		$	822
Weighted-average interest rate	2.80		%	2.79		%	2.72		%	2.79		%
Peak commercial paper during period at par value^(a)	$	636		$	549		$	202		$	1,113
Peak interest rate	3.10		%	2.97		%	2.90		%	3.10		%
2018
Average daily commercial paper outstanding at par value	$	397		$	123		$	174		$	693
Weighted-average interest rate	2.14		%	1.94		%	2.20		%	2.12		%
Peak commercial paper during period at par value^(a)	$	506		$	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.

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 six months ended June 30, 2019, was 2.75% and 2.81%, respectively (2018 - 2.17% and 2.04%, 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 six months ended June 30, 2019 and 2018.

Long-Term Debt And Equity Financings

6 Months Ended

Jun. 30, 2019

Long-Term Debt And Equity Financings [Abstract]

LONG-TERM DEBT AND EQUITY FINANCINGS

Ameren

For the three and six months ended June 30, 2019, Ameren issued a total of 0.2 million and 0.5 million shares of common stock under its DRPlus and 401(k) plan, and received proceeds of $18 million and $37 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.

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 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.

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 June 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 June 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

6 Months Ended

Jun. 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 six months ended June 30, 2019 and 2018:



	Three Months						Six Months
	2019			2018			2019			2018
Ameren:
Allowance for equity funds used during construction	$	8		$	9		$	13		$	14
Interest income on industrial development revenue bonds	6			7			13			13
Other interest income	3			2			4			4
Non-service cost components of net periodic benefit income^(a)	22			19			44			35
Miscellaneous income	1			2			4			3
Donations	(1		)	(6		)	(7		)	(11		)
Miscellaneous expense	(3		)	(4		)	(6		)	(6		)



	Three Months						Six Months
	2019			2018			2019			2018
Total Other Income, Net	$	36		$	29		$	65		$	52
Ameren Missouri:
Allowance for equity funds used during construction	$	5		$	7		$	8		$	11
Interest income on industrial development revenue bonds	6			7			13			13
Other interest income	—			1			—			1
Non-service cost components of net periodic benefit income^(a)	4			4			9			9
Miscellaneous income	2			—			2			1
Donations	—			(2		)	(2		)	(3		)
Miscellaneous expense	(1		)	(1		)	(2		)	(3		)
Total Other Income, Net	$	16		$	16		$	28		$	29
Ameren Illinois:
Allowance for equity funds used during construction	$	3		$	2		$	5		$	3
Interest income	2			1			4			3
Non-service cost components of net periodic benefit income	12			10			24			17
Miscellaneous income	1			2			2			2
Donations	(1		)	(1		)	(5		)	(5		)
Miscellaneous expense	(2		)	(1		)	(4		)	(1		)
Total Other Income, Net	$	15		$	13		$	26		$	19

(a)

For the three and six months ended June 30, 2019, the non-service cost components of net periodic benefit income were partially offset by a $8 million and $15 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 - $4 million and $8 million, respectively).

Derivative Financial Instruments

6 Months Ended

Jun. 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.

The following table presents open gross commodity contract volumes by commodity type for derivative assets and liabilities as of June 30, 2019, and December 31, 2018. As of June 30, 2019, these contracts extended through October 2022, October 2023, 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)	67	—	67	66	—	66
Natural gas (in mmbtu)	17	146	163	19	154	173
Power (in megawatthours)	4	8	12	1	8	9


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

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. See Note 7 – Fair Value Measurements for discussion of our methods of assessing the fair value of derivative instruments. 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.

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



			June 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		4		—		4			5		—		5
Natural gas	Other current assets		—		2		2			—		1		1
	Other assets		—		2		2			—		2		2
Power	Other current assets		13		—		13			4		—		4
	Other assets		5		—		5			—		—		—
	Total assets	$	26	$	4	$	30		$	12	$	3	$	15
Fuel oils	Other current liabilities	$	4	$	—	$	4		$	4	$	—	$	4
	Other deferred credits and liabilities		5		—		5			9		—		9
Natural gas	Other current liabilities		3		13		16			4		8		12
	Other deferred credits and liabilities		—		4		4			1		6		7
Power	Other current liabilities		3		16		19			4		14		18
	Other deferred credits and liabilities		—		175		175			—		169		169
	Total liabilities	$	15	$	208	$	223		$	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 June 30, 2019, and December 31, 2018.

Concentrations of 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. We calculate maximum exposures based on the gross fair value of financial instruments, including NPNS and other accrual contracts. These exposures are calculated on a gross basis, which include affiliate exposure not eliminated at the consolidated Ameren level. As of June 30, 2019, if counterparty groups were to fail completely to perform on contracts, the Ameren Companies’ maximum exposure would have been immaterial with or without consideration of the application of master netting arrangements or similar agreements and collateral held.

Derivative Instruments with Credit Risk-related Contingent Features

Our commodity contracts 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 following table presents, as of June 30, 2019, the aggregate fair value of all 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. The additional collateral required is the net liability position allowed under the master netting arrangements or similar agreements, assuming (1) the credit risk-related contingent features underlying these arrangements were triggered on June 30, 2019, and (2) those counterparties with rights to do so requested collateral.



	Aggregate Fair Value of Derivative Liabilities^(a)		Cash Collateral Posted		Potential Aggregate Amount of Additional Collateral Required^(b)
Ameren Missouri	$	72	$	4	$	63
Ameren Illinois	35		—		28
Ameren	$	107	$	4	$	91


(a)	Before consideration of master netting arrangements or similar agreements and including NPNS and other accrual contract exposures.

(b)

As collateral requirements with certain counterparties are based on master netting arrangements or similar agreements, the aggregate amount of additional collateral required to be posted is determined after consideration of the effects of such arrangements.

Fair Value Measurements

6 Months Ended

Jun. 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. Authoritative accounting guidance provides a fair value hierarchy that prioritizes the inputs used to measure fair value. On a quarterly basis, all financial assets and liabilities carried at fair value are classified and disclosed in one of three hierarchy levels. Financial assets and liabilities are classified in their entirety according to the lowest level of input that is significant to the fair value measurement. See Note 8 – Fair Value Measurements under Part II, Item 8, of the Form 10-K for information related to hierarchy levels.

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 six months ended June 30, 2019 or 2018. At June 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 June 30, 2019, and December 31, 2018:



		June 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	$	2	$	—	$	6	$	8		$	1	$	—	$	7	$	8
	Natural gas	—		1		3		4			—		2		1		3
	Power	—		—		18		18			—		1		3		4
	Total derivative assets – commodity contracts	$	2	$	1	$	27	$	30		$	1	$	3	$	11	$	15
	Nuclear decommissioning trust fund:
	Equity securities:
	U.S. large capitalization	$	511	$	—	$	—	$	511		$	427	$	—	$	—	$	427
	Debt securities:
	U.S. Treasury and agency securities	—		146		—		146			—		148		—		148
	Corporate bonds	—		79		—		79			—		72		—		72
	Other	—		38		—		38			—		32		—		32
	Total nuclear decommissioning trust fund	$	511	$	263	$	—	$	774	^(b)	$	427	$	252	$	—	$	679	^(b)
	Total Ameren	$	513	$	264	$	27	$	804		$	428	$	255	$	11	$	694
Ameren Missouri
	Derivative assets – commodity contracts^(a):
	Fuel oils	$	2	$	—	$	6	$	8		$	1	$	—	$	7	$	8
	Power	—		—		18		18			—		1		3		4
	Total derivative assets – commodity contracts	$	2	$	—	$	24	$	26		$	1	$	1	$	10	$	12
	Nuclear decommissioning trust fund:
	Equity securities:
	U.S. large capitalization	$	511	$	—	$	—	$	511		$	427	$	—	$	—	$	427
	Debt securities:
	U.S. Treasury and agency securities	—		146		—		146			—		148		—		148
	Corporate bonds	—		79		—		79			—		72		—		72
	Other	—		38		—		38			—		32		—		32
	Total nuclear decommissioning trust fund	$	511	$	263	$	—	$	774	^(b)	$	427	$	252	$	—	$	679	^(b)
	Total Ameren Missouri	$	513	$	263	$	24	$	800		$	428	$	253	$	10	$	691
Ameren Illinois
	Derivative assets – commodity contracts^(a):
	Natural gas	$	—	$	1	$	3	$	4		$	—	$	2	$	1	$	3
Liabilities:
Ameren
	Derivative liabilities – commodity contracts^(a):
	Fuel oils	$	2	$	—	$	7	$	9		$	2	$	—	$	11	$	13
	Natural gas	2		15		3		20			—		15		4		19
	Power	—		—		194		194			—		1		186		187
	Total Ameren	$	4	$	15	$	204	$	223		$	2	$	16	$	201	$	219
Ameren Missouri
	Derivative liabilities – commodity contracts^(a):
	Fuel oils	$	2	$	—	$	7	$	9		$	2	$	—	$	11	$	13
	Natural gas	—		3		—		3			—		5		—		5
	Power	—		—		3		3			—		1		3		4
	Total Ameren Missouri	$	2	$	3	$	10	$	15		$	2	$	6	$	14	$	22
Ameren Illinois
	Derivative liabilities – commodity contracts^(a):
	Natural gas	$	2	$	12	$	3	$	17		$	—	$	10	$	4	$	14
	Power	—		—		191		191			—		—		183		183
	Total Ameren Illinois	$	2	$	12	$	194	$	208		$	—	$	10	$	187	$	197


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


(b)	Balance excludes $9 million and $5 million of cash and cash equivalents, receivables, payables, and accrued income, net, for June 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 six months ended June 30, 2019 and 2018:



	2019									2018
	Ameren Missouri			Ameren Illinois			Ameren			Ameren Missouri			Ameren Illinois			Ameren
For the three months ended June 30
Beginning balance at April 1	$	—		$	(184	)	$	(184	)	$	4		$	(191	)	$	(187	)
Realized and unrealized gains/(losses) included in regulatory assets/liabilities	16			(11		)	5			(1		)	(2		)	(3		)
Purchases	—			—			—			4			—			4
Settlements	(1		)	4			3			(2		)	3			1
Ending balance at June 30	$	15		$	(191	)	$	(176	)	$	5		$	(190	)	$	(185	)
Change in unrealized gains/(losses) related to assets/liabilities held at June 30	$	16		$	(11	)	$	5		$	—		$	(3	)	$	(3	)
For the six months ended June 30
Beginning balance at January 1	$	—		$	(183	)	$	(183	)	$	7		$	(195	)	$	(188	)
Realized and unrealized gains/(losses) included in regulatory assets/liabilities	16			(15		)	1			(3		)	(1		)	(4		)
Purchases	—			—			—			4			—			4
Settlements	(1		)	7			6			(3		)	6			3
Ending balance at June 30	15			(191		)	(176		)	5			(190		)	(185		)
Change in unrealized gains/(losses) related to assets/liabilities held at June 30	$	16		$	(15	)	$	1		$	(1	)	$	(2	)	$	(3	)

For the three and six months ended June 30, 2019 and 2018, there were no material transfers between Level 1 and Level 2, Level 1 and Level 3, or Level 2 and Level 3 related to derivative commodity contracts.

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 June 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)	$	18	$	(194)	Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps ($/MWh)^(c)	22 – 32	25
							Nodal basis ($/MWh)^(c)	(9) – 0	(2)
						Fundamental energy production model	Estimated future natural gas prices ($/mmbtu)^(c)	3 – 4	3
2018	Power^(b)	$	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 2022. Valuations beyond 2022 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.

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 June 30, 2019, and December 31, 2018:



	June 30, 2019
	Carrying Amount			Fair Value
	Carrying Amount			Level 1		Level 2		Level 3			Total
Ameren:
Cash, cash equivalents, and restricted cash	$	122		$	122	$	—	$	—		$	122
Investments in held-to-maturity debt securities^(a)	270			—		270		—			270
Short-term debt	999			—		999		—			999
Long-term debt (including current portion)^(a)	8,558		^(b)	—		8,925		466		^(c)	9,391
Ameren Missouri:
Cash, cash equivalents, and restricted cash	$	13		$	13	$	—	$	—		$	13
Investments in held-to-maturity debt securities^(a)	270			—		270		—			270
Short-term debt	205			—		205		—			205
Long-term debt (including current portion)^(a)	4,116		^(b)	—		4,568		—			4,568
Ameren Illinois:
Cash, cash equivalents, and restricted cash	$	99		$	99	$	—	$	—		$	99
Short-term debt	199			—		199		—			199
Long-term debt (including current portion)	3,296		^(b)	—		3,642		—			3,642
	December 31, 2018
Ameren:
Cash, cash equivalents, and restricted cash	$	107		$	107	$	—	$	—		$	107
Investments in held-to-maturity debt securities^(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 held-to-maturity debt securities^(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 June 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 $62 million, $26 million, and $31 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of June 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

6 Months Ended

Jun. 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 2019, Ameren Illinois conducted a procurement event, administered by the IPA, to purchase energy products. Ameren Missouri was among the winning suppliers in this event. 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.

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



				Three Months				Six Months
Agreement	Income Statement Line Item			Ameren Missouri		Ameren Illinois		Ameren Missouri		Ameren Illinois
Ameren Missouri power supply	Operating Revenues	2019	$	2	$	(a)	$	2	$	(a)
agreements with Ameren Illinois		2018		3		(a)		6		(a)
Ameren Missouri and Ameren Illinois	Operating Revenues	2019	$	6	$	(b)	$	13	$	1
rent and facility services		2018		6		1		11		2
Ameren Missouri and Ameren Illinois	Operating Revenues	2019	$	(b)	$	1	$	(b)	$	1
miscellaneous support services		2018		(b)		(b)		(b)		(b)
Total Operating Revenues		2019	$	8	$	1	$	15	$	2
		2018		9		1		17		2
Ameren Illinois power supply	Purchased Power	2019	$	(a)	$	2	$	(a)	$	2
agreements with Ameren Missouri		2018		(a)		3		(a)		6
Ameren Illinois transmission	Purchased Power	2019	$	(a)	$	(b)	$	(a)	$	(b)
services with ATXI		2018		(a)		1		(a)		1
Total Purchased Power		2019	$	(a)	$	2	$	(a)	$	2
		2018		(a)		4		(a)		7
Ameren Missouri and Ameren Illinois	Other Operations and Maintenance	2019	$	1	$	2	$	1	$	3
rent and facility services		2018		(b)		1		1		3
Ameren Services support services	Other Operations and Maintenance	2019	$	32	$	31	$	64	$	61
agreement		2018		32		30		65		60
Total Other Operations and		2019	$	33	$	33	$	65	$	64
Maintenance		2018		32		31		66		63
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

6 Months Ended

Jun. 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 June 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 June 30, 2019.



	Coal		Natural Gas^(a)		Nuclear Fuel		Purchased Power^(b)(c)			Methane Gas		Other		Total
Ameren:
2019	$	224	$	113	$	23	$	127	^(d)	$	2	$	27	$	516
2020	207		160		43		114		^(d)	3		33		560
2021	183		96		60		29			3		22		393
2022	125		38		11		5			3		17		199
2023	46		20		41		—			3		18		128
Thereafter	—		36		28		—			26		53		143
Total	$	785	$	463	$	206	$	275		$	40	$	170	$	1,939
Ameren Missouri:
2019	$	224	$	28	$	23	$	—		$	2	$	20	$	297
2020	207		34		43		—			3		21		308
2021	183		16		60		—			3		17		279
2022	125		5		11		—			3		17		161
2023	46		5		41		—			3		18		113
Thereafter	—		13		28		—			26		39		106
Total	$	785	$	101	$	206	$	—		$	40	$	132	$	1,264
Ameren Illinois:
2019	$	—	$	85	$	—	$	127	^(d)	$	—	$	2	$	214
2020	—		126		—		114		^(d)	—		3		243
2021	—		80		—		29			—		—		109
2022	—		33		—		5			—		—		38
2023	—		15		—		—			—		—		15
Thereafter	—		23		—		—			—		—		23
Total	$	—	$	362	$	—	$	275		$	—	$	5	$	642


(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 2034 with various renewable energy suppliers due to the contingent nature of the payment amounts, with the exception of expected payments of $5 million in 2019.


(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 $60 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. 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 emission source reductions 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 generating units. The EPA has identified certain efficiency measures as the best system of emission reduction for coal-fired generating units. The Affordable Clean Energy Rule will go 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. The complaint, as amended in October 2013, alleged 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. The litigation has been divided into liability and remedy phases. In January 2017, the district court issued a liability ruling against Ameren Missouri. A trial on the scope of appropriate remedy began in April 2019. The EPA is seeking broad relief, including installation of pollution control equipment and other mitigation relief. The EPA previously withdrew all claims for penalties and fines. A final order and judgment could be issued by the trial court as early as the fall of 2019. Once the final judgment is entered, Ameren Missouri will seek a stay of that order while it appeals the liability ruling to the United States Court of Appeals for the Eighth Circuit.

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. We are unable to predict the ultimate resolution of this matter or the costs that might be incurred.

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 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 by 2020.

CCR Management

In 2015, the EPA issued the CCR rule, which established regulations regarding the management and disposal of CCR from coal-fired energy centers. 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 and indicated that additional revisions to the CCR rule are likely. Ameren and Ameren Missouri have AROs of $162 million recorded on their respective balance sheets as of June 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, waste water 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 June 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 its 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 June 30, 2019, Ameren Illinois estimated the remaining obligation related to these former MGP sites at $145 million to $212 million. Ameren and Ameren Illinois recorded a liability of $145 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

6 Months Ended

Jun. 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 June 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

6 Months Ended

Jun. 30, 2019

Defined Benefit Plan [Abstract]

RETIREMENT BENEFITS

NOTE 11 – 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 six months ended June 30, 2019 and 2018:



	Pension Benefits												Postretirement Benefits
	Three Months						Six Months						Three Months						Six Months
	2019			2018			2019			2018			2019			2018			2019			2018
Service cost^(a)	$	22		$	25		$	44		$	50		$	5		$	5		$	9		$	10
Non-service cost components:
Interest cost	46			42			93			84			10			9			21			20
Expected return on plan assets	(69		)	(69		)	(138		)	(138		)	(19		)	(19		)	(38		)	(38		)
Amortization of:
Prior service benefit	—			—			—			—			(2		)	(1		)	(3		)	(2		)
Actuarial loss (gain)	7			18			13			34			(3		)	(3		)	(7		)	(3		)
Total non-service cost components^(b)	$	(16	)	$	(9	)	$	(32	)	$	(20	)	$	(14	)	$	(14	)	$	(27	)	$	(23	)
Net periodic benefit cost (income)	$	6		$	16		$	12		$	30		$	(9	)	$	(9	)	$	(18	)	$	(13	)


(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 six months ended June 30, 2019 and 2018:



	Pension Benefits								Postretirement Benefits
	Three Months				Six Months				Three Months						Six Months
	2019		2018		2019		2018		2019			2018			2019			2018
Ameren Missouri^(a)	$	1	$	6	$	2	$	11	$	(1	)	$	—		$	(3	)	$	—
Ameren Illinois	5		10		10		19		(8		)	(9		)	(15		)	(13		)
Ameren^(a)	$	6	$	16	$	12	$	30	$	(9	)	$	(9	)	$	(18	)	$	(13	)

(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

6 Months Ended

Jun. 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 six months ended June 30, 2019 and 2018:



	Ameren			Ameren Missouri			Ameren Illinois
Three Months	2019	2018		2019	2018		2019	2018
Federal statutory corporate income tax rate:	21%	21%		21%	21%		21%	21%
Increases (decreases) from:
Amortization of excess deferred taxes	(9)	(1)	^(a)	(12)	—	^(a)	(4)	(5)
Depreciation differences	—	—		—	—		(1)	(1)
Amortization of deferred investment tax credit	—	—		(1)	(1)		—	—
State tax	6	5		4	4		7	8
Tax credits	—	(1)		—	—		—	—
Effective income tax rate	18%	24%		12%	24%		23%	23%
Six Months
Federal statutory corporate income tax rate:	21%	21%		21%	21%		21%	21%
Increases (decreases) from:
Amortization of excess deferred taxes	(8)	(2)	^(a)	(12)	—	^(a)	(4)	(4)
Depreciation differences	—	—		1	—		—	—
Amortization of deferred investment tax credit	—	(1)		(1)	(1)		—	—
State tax	6	6		4	4		7	7
Stock-based compensation	(3)	(1)		—	—		—	—
Other	(1)	—		(1)	—		—	—
Effective income tax rate	15%	23%		12%	24%		24%	24%

(a)

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

Supplemental Information

6 Months Ended

Jun. 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 June 30, 2019, and December 31, 2018:



	June 30, 2019						December 31, 2018
	Ameren		Ameren Missouri		Ameren Illinois		Ameren		Ameren Missouri		Ameren Illinois
Cash and cash equivalents	$	6	$	—	$	—	$	16	$	—	$	—
Restricted cash included in “Other current assets”	6		—		2		13		4		6
Restricted cash included in “Other assets”	97		—		97		74		—		74
Restricted cash included in “Nuclear decommissioning trust fund”	13		13		—		4		4		—
Total cash, cash equivalents, and restricted cash	$	122	$	13	$	99	$	107	$	8	$	80

At June 30, 2019, restricted cash included in “Other current assets” primarily represents participant funds from Ameren (parent)’s DRPlus. At December 31, 2018, 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 June 30, 2019, and December 31, 2018, “Other current liabilities” on Ameren’s and Ameren Illinois’ balance sheets included payables for purchased receivables of $34 million and $33 million, respectively.

For the three and six months ended June 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 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 six months ended June 30, 2019 and 2018.

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



	Ameren			Ameren Missouri
Other assets	$	35		$	32
Other current liabilities	7			6
Other deferred credits and liabilities	28			26
Weighted average remaining operating lease term	6 years			6 years
Weighted average discount rate^(a)	3.6		%	3.6		%


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

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



	Ameren		Ameren Missouri
2019	$	4	$	4
2020	7		7
2021	7		6
2022	6		5
2023	5		5
Thereafter	10		9
Total lease payments	39		36
Less imputed interest	4		4
Total^(a)	$	35	$	32

(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 June 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 investing activity excluded from the statements of cash flows for the six months ended June 30, 2019 and 2018:



	June 30, 2019						June 30, 2018
	Ameren		Ameren Missouri		Ameren Illinois		Ameren		Ameren Missouri		Ameren Illinois
Accrued capital expenditures	$	263	$	101	$	143	$	233	$	80	$	147
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	90		90		—		1		1		—

Asset Retirement Obligations

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



	Ameren Missouri				Ameren Illinois^(a)			Ameren
Balance at December 31, 2018	$	646		^(a)	$	4	^(b)	$	650		^(a)
Liabilities settled	(7		)		—			(7		)
Accretion	14			^(c)	—			14			^(c)
Change in estimates	33			^(d)	—			33			^(d)
Balance at June 30, 2019	$	686		^(a)	$	4	^(b)	$	690		^(a)


(a)	Balance included $23 million in “Other current liabilities” on the balance sheet as of both December 31, 2018, and June 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 six months ended June 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	294,871		67.42		^(b)	128,415		65.46
Forfeitures	(14,564	)	64.13			(4,465	)	62.62
Vested and undistributed^(c)	(122,451	)	62.19			(27,301	)	61.87
Vested and distributed	(176,923	)	44.13			—		—
Nonvested at June 30, 2019^(d)	663,744		$	63.52		251,902		$	60.80


(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 vested 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 390,459 performance share units and 53,858 restricted stock units that were vested and undistributed.

For the six months ended June 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 June 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 June 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 six months ended June 30, 2019 and 2018:



	Three Months				Six Months
	2019		2018		2019		2018
Ameren Missouri	$	38	$	46	$	69	$	80
Ameren Illinois	25		28		64		63
Ameren	$	63	$	74	$	133	$	143

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 period. Earnings per diluted share reflects the dilution that would occur if certain stock-based performance share units and restricted stock units were assumed to be settled. The number of performance share units and restricted stock units assumed settled was 1.6 million and 1.5 million in the three and six months ended June 30, 2019, respectively, and 2.1 million and 1.8 million, respectively, in the year-ago periods. There were no potentially dilutive securities excluded from the earnings per diluted share calculations for the three and six months ended June 30, 2019 and 2018.

Segment Information

6 Months Ended

Jun. 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 certain factors, which 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 six months ended June 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	$	790	$	358	$	136	$	95		$	—		$	—		$	1,379
Intersegment revenues	8		1		—		14			—			(23		)	—
Net income attributable to Ameren common shareholders	107		37		1		42		^(a)	(8		)	—			179
Capital expenditures	255		127		77		127			—			(5		)	581
Three Months 2018:
External revenues	$	946	$	386	$	142	$	89		$	—		$	—		$	1,563
Intersegment revenues	9		1		—		14			—			(24		)	—
Net income attributable to Ameren common shareholders	168		33		7		36		^(a)	(5		)	—			239
Capital expenditures	205		132		66		130			(2		)	2			533
Six Months 2019:
External revenues	$	1,541	$	744	$	456	$	194		$	—		$	—		$	2,935
Intersegment revenues	15		2		—		29			—			(46		)	—
Net income attributable to Ameren common shareholders	146		73		58		86		^(a)	7			—			370
Capital expenditures	495		251		128		248			10			(7		)	1,125
Six Months 2018:
External revenues	$	1,730	$	785	$	453	$	180		$	—		$	—		$	3,148
Intersegment revenues	17		2		—		27			—			(46		)	—
Net income attributable to Ameren common shareholders	206		66		49		73		^(a)	(4		)	—			390
Capital expenditures	454		254		126		275			5			(2		)	1,112


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

Ameren Illinois



	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Illinois Transmission		Intersegment Eliminations			Ameren Illinois
Three Months 2019:
External revenues	$	359	$	136	$	52	$	—		$	547
Intersegment revenues	—		—		14		(14		)	—
Net income available to common shareholder	37		1		24		—			62
Capital expenditures	127		77		85		—			289
Three Months 2018:
External revenues	$	387	$	142	$	49	$	—		$	578
Intersegment revenues	—		—		13		(13		)	—
Net income available to common shareholder	33		7		22		—			62
Capital expenditures	132		66		104		—			302
Six Months 2019:
External revenues	$	746	$	456	$	107	$	—		$	1,309
Intersegment revenues	—		—		29		(29		)	—
Net income available to common shareholder	73		58		51		—			182
Capital expenditures	251		128		177		—			556
Six Months 2018:
External revenues	$	787	$	453	$	98	$	—		$	1,338
Intersegment revenues	—		—		26		(26		)	—
Net income available to common shareholder	66		49		42		—			157
Capital expenditures	254		126		222		—			602

The following tables present disaggregated revenues by segment at Ameren and Ameren Illinois for the three and six months ended June 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	$	333		$	199	$	—	$	—	$	—		$	532
Commercial	310			124		—		—		—			434
Industrial	77			33		—		—		—			110
Other	53			3		—		109		(23		)	142
Total electric revenues	$	773		$	359	$	—	$	109	$	(23	)	$	1,218
Residential	$	10		$	—	$	88	$	—	$	—		$	98
Commercial	4			—		23		—		—			27
Industrial	—			—		3		—		—			3
Other	11			—		22		—		—			33
Total gas revenues	$	25		$	—	$	136	$	—	$	—		$	161
Total revenues^(b)	$	798		$	359	$	136	$	109	$	(23	)	$	1,379
Three Months 2018:
Residential	$	432		$	221	$	—	$	—	$	—		$	653
Commercial	364			126		—		—		—			490
Industrial	87			33		—		—		—			120
Other	47		^(a)	7		—		103		(24		)	133		^(a)
Total electric revenues	$	930		$	387	$	—	$	103	$	(24	)	$	1,396
Residential	$	13		$	—	$	97	$	—	$	—		$	110
Commercial	6			—		26		—		—			32
Industrial	—			—		5		—		—			5
Other	6			—		14		—		—			20
Total gas revenues	$	25		$	—	$	142	$	—	$	—		$	167
Total revenues^(b)	$	955		$	387	$	142	$	103	$	(24	)	$	1,563



	Ameren Missouri				Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Transmission		Intersegment Eliminations			Ameren
Six Months 2019:
Residential	$	645			$	416	$	—	$	—	$	—		$	1,061
Commercial	549				247		—		—		—			796
Industrial	132				67		—		—		—			199
Other	151				16		—		223		(46		)	344
Total electric revenues	$	1,477			$	746	$	—	$	223	$	(46	)	$	2,400
Residential	$	48			$	—	$	334	$	—	$	—		$	382
Commercial	20				—		88		—		—			108
Industrial	2				—		7		—		—			9
Other	9				—		27		—		—			36
Total gas revenues	$	79			$	—	$	456	$	—	$	—		$	535
Total revenues^(b)	$	1,556			$	746	$	456	$	223	$	(46	)	$	2,935
Six Months 2018:
Residential	$	764			$	440	$	—	$	—	$	—		$	1,204
Commercial	616				250		—		—		—			866
Industrial	148				68		—		—		—			216
Other	143			^(a)	29		—		207		(46		)	333		^(a)
Total electric revenues	$	1,671			$	787	$	—	$	207	$	(46	)	$	2,619
Residential	$	54			$	—	$	340	$	—	$	—		$	394
Commercial	22				—		93		—		—			115
Industrial	2				—		11		—		—			13
Other	(2		)		—		9		—		—			7
Total gas revenues	$	76			$	—	$	453	$	—	$	—		$	529
Total revenues^(b)	$	1,747			$	787	$	453	$	207	$	(46	)	$	3,148


(a)	Includes $37 million and $47 million for the three and six months ended June 30, 2018, respectively, for the reduction to revenue for the excess amounts collected in rates related to the TCJA from January 1, 2018, through June 30, 2018. See Note 2 – Rate and Regulatory Matters under Part II, Item 8, of the Form 10-K 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 three and six months ended June 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	$	—		$	12	$	4		$	(8	)	$	8
Other revenues not from contracts with customers	4			1		—			—			5
Three Months 2018:
Revenues from alternative revenue programs	$	(5	)	$	15	$	(5	)	$	(5	)	$	—
Other revenues not from contracts with customers	5			3		—			—			8
Six Months 2019:
Revenues from alternative revenue programs	$	15		$	34	$	1		$	(13	)	$	37
Other revenues not from contracts with customers	9			4		1			—			14
Six Months 2018:
Revenues from alternative revenue programs	$	(9	)	$	46	$	(8	)	$	(9	)	$	20
Other revenues not from contracts with customers	19			13		1			—			33

Ameren Illinois



	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Illinois Transmission		Intersegment Eliminations			Ameren Illinois
Three Months 2019:
Residential	$	199	$	88	$	—	$	—		$	287
Commercial	124		23		—		—			147
Industrial	33		3		—		—			36
Other	3		22		66		(14		)	77
Total revenues^(a)	$	359	$	136	$	66	$	(14	)	$	547
Three Months 2018:
Residential	$	221	$	97	$	—	$	—		$	318
Commercial	126		26		—		—			152
Industrial	33		5		—		—			38
Other	7		14		62		(13		)	70
Total revenues^(a)	$	387	$	142	$	62	$	(13	)	$	578
Six Months 2019:
Residential	$	416	$	334	$	—	$	—		$	750
Commercial	247		88		—		—			335
Industrial	67		7		—		—			74
Other	16		27		136		(29		)	150
Total revenues^(a)	$	746	$	456	$	136	$	(29	)	$	1,309
Six Months 2018:
Residential	$	440	$	340	$	—	$	—		$	780
Commercial	250		93		—		—			343
Industrial	68		11		—		—			79
Other	29		9		124		(26		)	136
Total revenues^(a)	$	787	$	453	$	124	$	(26	)	$	1,338


(a)	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 six months ended June 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	$	12	$	4		$	(9	)	$	7
Other revenues not from contracts with customers	1		—			—			1
Three Months 2018:
Revenues from alternative revenue programs	$	15	$	(5	)	$	(5	)	$	5
Other revenues not from contracts with customers	3		—			—			3
Six Months 2019:
Revenues from alternative revenue programs	$	34	$	1		$	(14	)	$	21
Other revenues not from contracts with customers	4		1			—			5
Six Months 2018:
Revenues from alternative revenue programs	$	46	$	(8	)	$	(9	)	$	29
Other revenues not from contracts with customers	13		1			—			14

Summary Of Significant Accounting Policies (Policies)

6 Months Ended

Jun. 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 placed the Spoon River project in service in February 2018, and is developing the MISO-approved Illinois Rivers and Mark Twain electric transmission projects.

Consolidation

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. 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 June 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 June 30, 2019, the maximum exposure to loss related to these variable interest entities was approximately $20 million, which primarily represents legal costs incurred and the portion of interconnection study costs that may be incurred by Ameren and Ameren Missouri. 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 June 30, 2019, and December 31, 2018, Ameren had unconsolidated variable interests as a limited partner in various equity method investments, totaling $23 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 June 30, 2019, the maximum exposure to loss related to these variable interests is limited to the investment in these partnerships of $23 million plus associated outstanding funding commitments of $14 million. In July 2019, Ameren made $25 million in additional funding commitments.

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 June 30, 2019, the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $262 million (December 31, 2018 – $244 million) and

$125 million (December 31, 2018 – $122 million), respectively, while total borrowings against the policies were $118 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 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 June 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 June 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 Rate And Regulatory Matters (Tables)

6 Months Ended

Jun. 30, 2019

Public Utilities, General Disclosures [Abstract]

Schedule of Wind Generation Facilities [Table Text Block]

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	Enel Green Power North America, Inc.
Location	Northeastern Missouri	Northwestern Missouri
Status of certificate of convenience and necessity from the MoPSC	Approved October 2018	Requested in May 2019^(a)
Status of final interconnection costs	Received in July 2019	Received in July 2019
Status of RTO transmission interconnection agreement	Expected by the fall of 2019	Expected by the fall of 2019
Status of FERC approval	Received December 2018	To be requested in the third quarter of 2019
Expected completion date	By the end of 2020	By the end of 2020

(a)

In July 2019, Ameren Missouri, the MoPSC staff, and certain intervenors filed a nonunanimous stipulation and agreement with the MoPSC regarding the requested certificate of convenience and necessity.

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Tables)

6 Months Ended

Jun. 30, 2019

Debt Disclosure [Abstract]

Schedule of Short-term Debt

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



	June 30, 2019		December 31, 2018
Ameren (parent)	$	595	$	470
Ameren Missouri	205		55
Ameren Illinois	199		72
Ameren consolidated	$	999	$	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 six months ended June 30, 2019 and 2018:



	Ameren (parent)			Ameren Missouri			Ameren Illinois			Ameren Consolidated
2019
Average daily commercial paper outstanding at par value	$	542		$	174		$	106		$	822
Weighted-average interest rate	2.80		%	2.79		%	2.72		%	2.79		%
Peak commercial paper during period at par value^(a)	$	636		$	549		$	202		$	1,113
Peak interest rate	3.10		%	2.97		%	2.90		%	3.10		%
2018
Average daily commercial paper outstanding at par value	$	397		$	123		$	174		$	693
Weighted-average interest rate	2.14		%	1.94		%	2.20		%	2.12		%
Peak commercial paper during period at par value^(a)	$	506		$	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.

Long-Term Debt And Equity Financings Long-Term Debt and Equity Financings (Tables)

6 Months Ended

Jun. 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
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-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
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-Paragraph 1
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-Topic 835
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 8: http://fasb.org/us-gaap/role/ref/legacyRef
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-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

Reference 9: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
-Topic 835
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-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)

6 Months Ended

Jun. 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 six months ended June 30, 2019 and 2018:



	Three Months						Six Months
	2019			2018			2019			2018
Ameren:
Allowance for equity funds used during construction	$	8		$	9		$	13		$	14
Interest income on industrial development revenue bonds	6			7			13			13
Other interest income	3			2			4			4
Non-service cost components of net periodic benefit income^(a)	22			19			44			35
Miscellaneous income	1			2			4			3
Donations	(1		)	(6		)	(7		)	(11		)
Miscellaneous expense	(3		)	(4		)	(6		)	(6		)



	Three Months						Six Months
	2019			2018			2019			2018
Total Other Income, Net	$	36		$	29		$	65		$	52
Ameren Missouri:
Allowance for equity funds used during construction	$	5		$	7		$	8		$	11
Interest income on industrial development revenue bonds	6			7			13			13
Other interest income	—			1			—			1
Non-service cost components of net periodic benefit income^(a)	4			4			9			9
Miscellaneous income	2			—			2			1
Donations	—			(2		)	(2		)	(3		)
Miscellaneous expense	(1		)	(1		)	(2		)	(3		)
Total Other Income, Net	$	16		$	16		$	28		$	29
Ameren Illinois:
Allowance for equity funds used during construction	$	3		$	2		$	5		$	3
Interest income	2			1			4			3
Non-service cost components of net periodic benefit income	12			10			24			17
Miscellaneous income	1			2			2			2
Donations	(1		)	(1		)	(5		)	(5		)
Miscellaneous expense	(2		)	(1		)	(4		)	(1		)
Total Other Income, Net	$	15		$	13		$	26		$	19

(a)

For the three and six months ended June 30, 2019, the non-service cost components of net periodic benefit income were partially offset by a $8 million and $15 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 - $4 million and $8 million, respectively).

Derivative Financial Instruments (Tables)

6 Months Ended

Jun. 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 June 30, 2019, and December 31, 2018. As of June 30, 2019, these contracts extended through October 2022, October 2023, 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)	67	—	67	66	—	66
Natural gas (in mmbtu)	17	146	163	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 June 30, 2019, and December 31, 2018:



			June 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		4		—		4			5		—		5
Natural gas	Other current assets		—		2		2			—		1		1
	Other assets		—		2		2			—		2		2
Power	Other current assets		13		—		13			4		—		4
	Other assets		5		—		5			—		—		—
	Total assets	$	26	$	4	$	30		$	12	$	3	$	15
Fuel oils	Other current liabilities	$	4	$	—	$	4		$	4	$	—	$	4
	Other deferred credits and liabilities		5		—		5			9		—		9
Natural gas	Other current liabilities		3		13		16			4		8		12
	Other deferred credits and liabilities		—		4		4			1		6		7
Power	Other current liabilities		3		16		19			4		14		18
	Other deferred credits and liabilities		—		175		175			—		169		169
	Total liabilities	$	15	$	208	$	223		$	22	$	197	$	219

Derivative Instruments With Credit Risk-Related Contingent Features

The following table presents, as of June 30, 2019, the aggregate fair value of all 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. The additional collateral required is the net liability position allowed under the master netting arrangements or similar agreements, assuming (1) the credit risk-related contingent features underlying these arrangements were triggered on June 30, 2019, and (2) those counterparties with rights to do so requested collateral.



	Aggregate Fair Value of Derivative Liabilities^(a)		Cash Collateral Posted		Potential Aggregate Amount of Additional Collateral Required^(b)
Ameren Missouri	$	72	$	4	$	63
Ameren Illinois	35		—		28
Ameren	$	107	$	4	$	91


(a)	Before consideration of master netting arrangements or similar agreements and including NPNS and other accrual contract exposures.

(b)

As collateral requirements with certain counterparties are based on master netting arrangements or similar agreements, the aggregate amount of additional collateral required to be posted is determined after consideration of the effects of such arrangements.

Fair Value Measurements (Tables)

6 Months Ended

Jun. 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 June 30, 2019, and December 31, 2018:



		June 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	$	2	$	—	$	6	$	8		$	1	$	—	$	7	$	8
	Natural gas	—		1		3		4			—		2		1		3
	Power	—		—		18		18			—		1		3		4
	Total derivative assets – commodity contracts	$	2	$	1	$	27	$	30		$	1	$	3	$	11	$	15
	Nuclear decommissioning trust fund:
	Equity securities:
	U.S. large capitalization	$	511	$	—	$	—	$	511		$	427	$	—	$	—	$	427
	Debt securities:
	U.S. Treasury and agency securities	—		146		—		146			—		148		—		148
	Corporate bonds	—		79		—		79			—		72		—		72
	Other	—		38		—		38			—		32		—		32
	Total nuclear decommissioning trust fund	$	511	$	263	$	—	$	774	^(b)	$	427	$	252	$	—	$	679	^(b)
	Total Ameren	$	513	$	264	$	27	$	804		$	428	$	255	$	11	$	694
Ameren Missouri
	Derivative assets – commodity contracts^(a):
	Fuel oils	$	2	$	—	$	6	$	8		$	1	$	—	$	7	$	8
	Power	—		—		18		18			—		1		3		4
	Total derivative assets – commodity contracts	$	2	$	—	$	24	$	26		$	1	$	1	$	10	$	12
	Nuclear decommissioning trust fund:
	Equity securities:
	U.S. large capitalization	$	511	$	—	$	—	$	511		$	427	$	—	$	—	$	427
	Debt securities:
	U.S. Treasury and agency securities	—		146		—		146			—		148		—		148
	Corporate bonds	—		79		—		79			—		72		—		72
	Other	—		38		—		38			—		32		—		32
	Total nuclear decommissioning trust fund	$	511	$	263	$	—	$	774	^(b)	$	427	$	252	$	—	$	679	^(b)
	Total Ameren Missouri	$	513	$	263	$	24	$	800		$	428	$	253	$	10	$	691
Ameren Illinois
	Derivative assets – commodity contracts^(a):
	Natural gas	$	—	$	1	$	3	$	4		$	—	$	2	$	1	$	3
Liabilities:
Ameren
	Derivative liabilities – commodity contracts^(a):
	Fuel oils	$	2	$	—	$	7	$	9		$	2	$	—	$	11	$	13
	Natural gas	2		15		3		20			—		15		4		19
	Power	—		—		194		194			—		1		186		187
	Total Ameren	$	4	$	15	$	204	$	223		$	2	$	16	$	201	$	219
Ameren Missouri
	Derivative liabilities – commodity contracts^(a):
	Fuel oils	$	2	$	—	$	7	$	9		$	2	$	—	$	11	$	13
	Natural gas	—		3		—		3			—		5		—		5
	Power	—		—		3		3			—		1		3		4
	Total Ameren Missouri	$	2	$	3	$	10	$	15		$	2	$	6	$	14	$	22
Ameren Illinois
	Derivative liabilities – commodity contracts^(a):
	Natural gas	$	2	$	12	$	3	$	17		$	—	$	10	$	4	$	14
	Power	—		—		191		191			—		—		183		183
	Total Ameren Illinois	$	2	$	12	$	194	$	208		$	—	$	10	$	187	$	197


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


(b)	Balance excludes $9 million and $5 million of cash and cash equivalents, receivables, payables, and accrued income, net, for June 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 six months ended June 30, 2019 and 2018:



	2019									2018
	Ameren Missouri			Ameren Illinois			Ameren			Ameren Missouri			Ameren Illinois			Ameren
For the three months ended June 30
Beginning balance at April 1	$	—		$	(184	)	$	(184	)	$	4		$	(191	)	$	(187	)
Realized and unrealized gains/(losses) included in regulatory assets/liabilities	16			(11		)	5			(1		)	(2		)	(3		)
Purchases	—			—			—			4			—			4
Settlements	(1		)	4			3			(2		)	3			1
Ending balance at June 30	$	15		$	(191	)	$	(176	)	$	5		$	(190	)	$	(185	)
Change in unrealized gains/(losses) related to assets/liabilities held at June 30	$	16		$	(11	)	$	5		$	—		$	(3	)	$	(3	)
For the six months ended June 30
Beginning balance at January 1	$	—		$	(183	)	$	(183	)	$	7		$	(195	)	$	(188	)
Realized and unrealized gains/(losses) included in regulatory assets/liabilities	16			(15		)	1			(3		)	(1		)	(4		)
Purchases	—			—			—			4			—			4
Settlements	(1		)	7			6			(3		)	6			3
Ending balance at June 30	15			(191		)	(176		)	5			(190		)	(185		)
Change in unrealized gains/(losses) related to assets/liabilities held at June 30	$	16		$	(15	)	$	1		$	(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 June 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)	$	18	$	(194)	Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps ($/MWh)^(c)	22 – 32	25
							Nodal basis ($/MWh)^(c)	(9) – 0	(2)
						Fundamental energy production model	Estimated future natural gas prices ($/mmbtu)^(c)	3 – 4	3
2018	Power^(b)	$	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 2022. Valuations beyond 2022 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.

Schedule of Financial Assets and Liabilities

ower) fair value measurement.

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 June 30, 2019, and December 31, 2018:



	June 30, 2019
	Carrying Amount			Fair Value
	Carrying Amount			Level 1		Level 2		Level 3			Total
Ameren:
Cash, cash equivalents, and restricted cash	$	122		$	122	$	—	$	—		$	122
Investments in held-to-maturity debt securities^(a)	270			—		270		—			270
Short-term debt	999			—		999		—			999
Long-term debt (including current portion)^(a)	8,558		^(b)	—		8,925		466		^(c)	9,391
Ameren Missouri:
Cash, cash equivalents, and restricted cash	$	13		$	13	$	—	$	—		$	13
Investments in held-to-maturity debt securities^(a)	270			—		270		—			270
Short-term debt	205			—		205		—			205
Long-term debt (including current portion)^(a)	4,116		^(b)	—		4,568		—			4,568
Ameren Illinois:
Cash, cash equivalents, and restricted cash	$	99		$	99	$	—	$	—		$	99
Short-term debt	199			—		199		—			199
Long-term debt (including current portion)	3,296		^(b)	—		3,642		—			3,642
	December 31, 2018
Ameren:
Cash, cash equivalents, and restricted cash	$	107		$	107	$	—	$	—		$	107
Investments in held-to-maturity debt securities^(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 held-to-maturity debt securities^(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 June 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 $62 million, $26 million, and $31 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of June 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

Related Party Transactions (Tables)

6 Months Ended

Jun. 30, 2019

Schedule of Related Party Transactions

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



				Three Months				Six Months
Agreement	Income Statement Line Item			Ameren Missouri		Ameren Illinois		Ameren Missouri		Ameren Illinois
Ameren Missouri power supply	Operating Revenues	2019	$	2	$	(a)	$	2	$	(a)
agreements with Ameren Illinois		2018		3		(a)		6		(a)
Ameren Missouri and Ameren Illinois	Operating Revenues	2019	$	6	$	(b)	$	13	$	1
rent and facility services		2018		6		1		11		2
Ameren Missouri and Ameren Illinois	Operating Revenues	2019	$	(b)	$	1	$	(b)	$	1
miscellaneous support services		2018		(b)		(b)		(b)		(b)
Total Operating Revenues		2019	$	8	$	1	$	15	$	2
		2018		9		1		17		2
Ameren Illinois power supply	Purchased Power	2019	$	(a)	$	2	$	(a)	$	2
agreements with Ameren Missouri		2018		(a)		3		(a)		6
Ameren Illinois transmission	Purchased Power	2019	$	(a)	$	(b)	$	(a)	$	(b)
services with ATXI		2018		(a)		1		(a)		1
Total Purchased Power		2019	$	(a)	$	2	$	(a)	$	2
		2018		(a)		4		(a)		7
Ameren Missouri and Ameren Illinois	Other Operations and Maintenance	2019	$	1	$	2	$	1	$	3
rent and facility services		2018		(b)		1		1		3
Ameren Services support services	Other Operations and Maintenance	2019	$	32	$	31	$	64	$	61
agreement		2018		32		30		65		60
Total Other Operations and		2019	$	33	$	33	$	65	$	64
Maintenance		2018		32		31		66		63
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)

6 Months Ended

Jun. 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 June 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 June 30, 2019.



	Coal		Natural Gas^(a)		Nuclear Fuel		Purchased Power^(b)(c)			Methane Gas		Other		Total
Ameren:
2019	$	224	$	113	$	23	$	127	^(d)	$	2	$	27	$	516
2020	207		160		43		114		^(d)	3		33		560
2021	183		96		60		29			3		22		393
2022	125		38		11		5			3		17		199
2023	46		20		41		—			3		18		128
Thereafter	—		36		28		—			26		53		143
Total	$	785	$	463	$	206	$	275		$	40	$	170	$	1,939
Ameren Missouri:
2019	$	224	$	28	$	23	$	—		$	2	$	20	$	297
2020	207		34		43		—			3		21		308
2021	183		16		60		—			3		17		279
2022	125		5		11		—			3		17		161
2023	46		5		41		—			3		18		113
Thereafter	—		13		28		—			26		39		106
Total	$	785	$	101	$	206	$	—		$	40	$	132	$	1,264
Ameren Illinois:
2019	$	—	$	85	$	—	$	127	^(d)	$	—	$	2	$	214
2020	—		126		—		114		^(d)	—		3		243
2021	—		80		—		29			—		—		109
2022	—		33		—		5			—		—		38
2023	—		15		—		—			—		—		15
Thereafter	—		23		—		—			—		—		23
Total	$	—	$	362	$	—	$	275		$	—	$	5	$	642


(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 2034 with various renewable energy suppliers due to the contingent nature of the payment amounts, with the exception of expected payments of $5 million in 2019.


(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 $60 million of zero emission credits through May 2020.

Callaway Energy Center (Tables)

6 Months Ended

Jun. 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 June 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)

6 Months Ended

Jun. 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 six months ended June 30, 2019 and 2018:



	Pension Benefits												Postretirement Benefits
	Three Months						Six Months						Three Months						Six Months
	2019			2018			2019			2018			2019			2018			2019			2018
Service cost^(a)	$	22		$	25		$	44		$	50		$	5		$	5		$	9		$	10
Non-service cost components:
Interest cost	46			42			93			84			10			9			21			20
Expected return on plan assets	(69		)	(69		)	(138		)	(138		)	(19		)	(19		)	(38		)	(38		)
Amortization of:
Prior service benefit	—			—			—			—			(2		)	(1		)	(3		)	(2		)
Actuarial loss (gain)	7			18			13			34			(3		)	(3		)	(7		)	(3		)
Total non-service cost components^(b)	$	(16	)	$	(9	)	$	(32	)	$	(20	)	$	(14	)	$	(14	)	$	(27	)	$	(23	)
Net periodic benefit cost (income)	$	6		$	16		$	12		$	30		$	(9	)	$	(9	)	$	(18	)	$	(13	)


(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 six months ended June 30, 2019 and 2018:



	Pension Benefits								Postretirement Benefits
	Three Months				Six Months				Three Months						Six Months
	2019		2018		2019		2018		2019			2018			2019			2018
Ameren Missouri^(a)	$	1	$	6	$	2	$	11	$	(1	)	$	—		$	(3	)	$	—
Ameren Illinois	5		10		10		19		(8		)	(9		)	(15		)	(13		)
Ameren^(a)	$	6	$	16	$	12	$	30	$	(9	)	$	(9	)	$	(18	)	$	(13	)

(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)

6 Months Ended

Jun. 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 six months ended June 30, 2019 and 2018:



	Ameren			Ameren Missouri			Ameren Illinois
Three Months	2019	2018		2019	2018		2019	2018
Federal statutory corporate income tax rate:	21%	21%		21%	21%		21%	21%
Increases (decreases) from:
Amortization of excess deferred taxes	(9)	(1)	^(a)	(12)	—	^(a)	(4)	(5)
Depreciation differences	—	—		—	—		(1)	(1)
Amortization of deferred investment tax credit	—	—		(1)	(1)		—	—
State tax	6	5		4	4		7	8
Tax credits	—	(1)		—	—		—	—
Effective income tax rate	18%	24%		12%	24%		23%	23%
Six Months
Federal statutory corporate income tax rate:	21%	21%		21%	21%		21%	21%
Increases (decreases) from:
Amortization of excess deferred taxes	(8)	(2)	^(a)	(12)	—	^(a)	(4)	(4)
Depreciation differences	—	—		1	—		—	—
Amortization of deferred investment tax credit	—	(1)		(1)	(1)		—	—
State tax	6	6		4	4		7	7
Stock-based compensation	(3)	(1)		—	—		—	—
Other	(1)	—		(1)	—		—	—
Effective income tax rate	15%	23%		12%	24%		24%	24%

Supplemental Information (Tables)

6 Months Ended

Jun. 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 June 30, 2019, and December 31, 2018:



	June 30, 2019						December 31, 2018
	Ameren		Ameren Missouri		Ameren Illinois		Ameren		Ameren Missouri		Ameren Illinois
Cash and cash equivalents	$	6	$	—	$	—	$	16	$	—	$	—
Restricted cash included in “Other current assets”	6		—		2		13		4		6
Restricted cash included in “Other assets”	97		—		97		74		—		74
Restricted cash included in “Nuclear decommissioning trust fund”	13		13		—		4		4		—
Total cash, cash equivalents, and restricted cash	$	122	$	13	$	99	$	107	$	8	$	80

Lease Supplemental Information

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



	Ameren			Ameren Missouri
Other assets	$	35		$	32
Other current liabilities	7			6
Other deferred credits and liabilities	28			26
Weighted average remaining operating lease term	6 years			6 years
Weighted average discount rate^(a)	3.6		%	3.6		%

(a)

As most of our lease agreements do not provide an implicit rate, we use our incremental borrowing rate based on the information available at commencement date in determining the present value of lease payments. We use the 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 June 30, 2019:



	Ameren		Ameren Missouri
2019	$	4	$	4
2020	7		7
2021	7		6
2022	6		5
2023	5		5
Thereafter	10		9
Total lease payments	39		36
Less imputed interest	4		4
Total^(a)	$	35	$	32

(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 June 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 investing activity excluded from the statements of cash flows for the six months ended June 30, 2019 and 2018:



	June 30, 2019						June 30, 2018
	Ameren		Ameren Missouri		Ameren Illinois		Ameren		Ameren Missouri		Ameren Illinois
Accrued capital expenditures	$	263	$	101	$	143	$	233	$	80	$	147
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	90		90		—		1		1		—

Asset Retirement Obligation Disclosure

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



	Ameren Missouri				Ameren Illinois^(a)			Ameren
Balance at December 31, 2018	$	646		^(a)	$	4	^(b)	$	650		^(a)
Liabilities settled	(7		)		—			(7		)
Accretion	14			^(c)	—			14			^(c)
Change in estimates	33			^(d)	—			33			^(d)
Balance at June 30, 2019	$	686		^(a)	$	4	^(b)	$	690		^(a)


(a)	Balance included $23 million in “Other current liabilities” on the balance sheet as of both December 31, 2018, and June 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 six months ended June 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	294,871		67.42		^(b)	128,415		65.46
Forfeitures	(14,564	)	64.13			(4,465	)	62.62
Vested and undistributed^(c)	(122,451	)	62.19			(27,301	)	61.87
Vested and distributed	(176,923	)	44.13			—		—
Nonvested at June 30, 2019^(d)	663,744		$	63.52		251,902		$	60.80


(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 vested 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 390,459 performance share units and 53,858 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 six months ended June 30, 2019 and 2018:



	Three Months				Six Months
	2019		2018		2019		2018
Ameren Missouri	$	38	$	46	$	69	$	80
Ameren Illinois	25		28		64		63
Ameren	$	63	$	74	$	133	$	143

Segment Information (Tables)

6 Months Ended

Jun. 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 six months ended June 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	$	790	$	358	$	136	$	95		$	—		$	—		$	1,379
Intersegment revenues	8		1		—		14			—			(23		)	—
Net income attributable to Ameren common shareholders	107		37		1		42		^(a)	(8		)	—			179
Capital expenditures	255		127		77		127			—			(5		)	581
Three Months 2018:
External revenues	$	946	$	386	$	142	$	89		$	—		$	—		$	1,563
Intersegment revenues	9		1		—		14			—			(24		)	—
Net income attributable to Ameren common shareholders	168		33		7		36		^(a)	(5		)	—			239
Capital expenditures	205		132		66		130			(2		)	2			533
Six Months 2019:
External revenues	$	1,541	$	744	$	456	$	194		$	—		$	—		$	2,935
Intersegment revenues	15		2		—		29			—			(46		)	—
Net income attributable to Ameren common shareholders	146		73		58		86		^(a)	7			—			370
Capital expenditures	495		251		128		248			10			(7		)	1,125
Six Months 2018:
External revenues	$	1,730	$	785	$	453	$	180		$	—		$	—		$	3,148
Intersegment revenues	17		2		—		27			—			(46		)	—
Net income attributable to Ameren common shareholders	206		66		49		73		^(a)	(4		)	—			390
Capital expenditures	454		254		126		275			5			(2		)	1,112


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

Ameren Illinois



	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Illinois Transmission		Intersegment Eliminations			Ameren Illinois
Three Months 2019:
External revenues	$	359	$	136	$	52	$	—		$	547
Intersegment revenues	—		—		14		(14		)	—
Net income available to common shareholder	37		1		24		—			62
Capital expenditures	127		77		85		—			289
Three Months 2018:
External revenues	$	387	$	142	$	49	$	—		$	578
Intersegment revenues	—		—		13		(13		)	—
Net income available to common shareholder	33		7		22		—			62
Capital expenditures	132		66		104		—			302
Six Months 2019:
External revenues	$	746	$	456	$	107	$	—		$	1,309
Intersegment revenues	—		—		29		(29		)	—
Net income available to common shareholder	73		58		51		—			182
Capital expenditures	251		128		177		—			556
Six Months 2018:
External revenues	$	787	$	453	$	98	$	—		$	1,338
Intersegment revenues	—		—		26		(26		)	—
Net income available to common shareholder	66		49		42		—			157
Capital expenditures	254		126		222		—			602

Disaggregation of Revenue

The following tables present disaggregated revenues by segment at Ameren and Ameren Illinois for the three and six months ended June 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	$	333		$	199	$	—	$	—	$	—		$	532
Commercial	310			124		—		—		—			434
Industrial	77			33		—		—		—			110
Other	53			3		—		109		(23		)	142
Total electric revenues	$	773		$	359	$	—	$	109	$	(23	)	$	1,218
Residential	$	10		$	—	$	88	$	—	$	—		$	98
Commercial	4			—		23		—		—			27
Industrial	—			—		3		—		—			3
Other	11			—		22		—		—			33
Total gas revenues	$	25		$	—	$	136	$	—	$	—		$	161
Total revenues^(b)	$	798		$	359	$	136	$	109	$	(23	)	$	1,379
Three Months 2018:
Residential	$	432		$	221	$	—	$	—	$	—		$	653
Commercial	364			126		—		—		—			490
Industrial	87			33		—		—		—			120
Other	47		^(a)	7		—		103		(24		)	133		^(a)
Total electric revenues	$	930		$	387	$	—	$	103	$	(24	)	$	1,396
Residential	$	13		$	—	$	97	$	—	$	—		$	110
Commercial	6			—		26		—		—			32
Industrial	—			—		5		—		—			5
Other	6			—		14		—		—			20
Total gas revenues	$	25		$	—	$	142	$	—	$	—		$	167
Total revenues^(b)	$	955		$	387	$	142	$	103	$	(24	)	$	1,563



	Ameren Missouri				Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Transmission		Intersegment Eliminations			Ameren
Six Months 2019:
Residential	$	645			$	416	$	—	$	—	$	—		$	1,061
Commercial	549				247		—		—		—			796
Industrial	132				67		—		—		—			199
Other	151				16		—		223		(46		)	344
Total electric revenues	$	1,477			$	746	$	—	$	223	$	(46	)	$	2,400
Residential	$	48			$	—	$	334	$	—	$	—		$	382
Commercial	20				—		88		—		—			108
Industrial	2				—		7		—		—			9
Other	9				—		27		—		—			36
Total gas revenues	$	79			$	—	$	456	$	—	$	—		$	535
Total revenues^(b)	$	1,556			$	746	$	456	$	223	$	(46	)	$	2,935
Six Months 2018:
Residential	$	764			$	440	$	—	$	—	$	—		$	1,204
Commercial	616				250		—		—		—			866
Industrial	148				68		—		—		—			216
Other	143			^(a)	29		—		207		(46		)	333		^(a)
Total electric revenues	$	1,671			$	787	$	—	$	207	$	(46	)	$	2,619
Residential	$	54			$	—	$	340	$	—	$	—		$	394
Commercial	22				—		93		—		—			115
Industrial	2				—		11		—		—			13
Other	(2		)		—		9		—		—			7
Total gas revenues	$	76			$	—	$	453	$	—	$	—		$	529
Total revenues^(b)	$	1,747			$	787	$	453	$	207	$	(46	)	$	3,148


(a)	Includes $37 million and $47 million for the three and six months ended June 30, 2018, respectively, for the reduction to revenue for the excess amounts collected in rates related to the TCJA from January 1, 2018, through June 30, 2018. See Note 2 – Rate and Regulatory Matters under Part II, Item 8, of the Form 10-K 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 three and six months ended June 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	$	—		$	12	$	4		$	(8	)	$	8
Other revenues not from contracts with customers	4			1		—			—			5
Three Months 2018:
Revenues from alternative revenue programs	$	(5	)	$	15	$	(5	)	$	(5	)	$	—
Other revenues not from contracts with customers	5			3		—			—			8
Six Months 2019:
Revenues from alternative revenue programs	$	15		$	34	$	1		$	(13	)	$	37
Other revenues not from contracts with customers	9			4		1			—			14
Six Months 2018:
Revenues from alternative revenue programs	$	(9	)	$	46	$	(8	)	$	(9	)	$	20
Other revenues not from contracts with customers	19			13		1			—			33

Ameren Illinois



	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Illinois Transmission		Intersegment Eliminations			Ameren Illinois
Three Months 2019:
Residential	$	199	$	88	$	—	$	—		$	287
Commercial	124		23		—		—			147
Industrial	33		3		—		—			36
Other	3		22		66		(14		)	77
Total revenues^(a)	$	359	$	136	$	66	$	(14	)	$	547
Three Months 2018:
Residential	$	221	$	97	$	—	$	—		$	318
Commercial	126		26		—		—			152
Industrial	33		5		—		—			38
Other	7		14		62		(13		)	70
Total revenues^(a)	$	387	$	142	$	62	$	(13	)	$	578
Six Months 2019:
Residential	$	416	$	334	$	—	$	—		$	750
Commercial	247		88		—		—			335
Industrial	67		7		—		—			74
Other	16		27		136		(29		)	150
Total revenues^(a)	$	746	$	456	$	136	$	(29	)	$	1,309
Six Months 2018:
Residential	$	440	$	340	$	—	$	—		$	780
Commercial	250		93		—		—			343
Industrial	68		11		—		—			79
Other	29		9		124		(26		)	136
Total revenues^(a)	$	787	$	453	$	124	$	(26	)	$	1,338


(a)	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 six months ended June 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	$	12	$	4		$	(9	)	$	7
Other revenues not from contracts with customers	1		—			—			1
Three Months 2018:
Revenues from alternative revenue programs	$	15	$	(5	)	$	(5	)	$	5
Other revenues not from contracts with customers	3		—			—			3
Six Months 2019:
Revenues from alternative revenue programs	$	34	$	1		$	(14	)	$	21
Other revenues not from contracts with customers	4		1			—			5
Six Months 2018:
Revenues from alternative revenue programs	$	46	$	(8	)	$	(9	)	$	29
Other revenues not from contracts with customers	13		1			—			14

Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Basis Of Presentation And Significant Accounting Policies [Line Items]
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets and Liabilities, Net		$ 23	$ 22
Cash Surrender Value of Life Insurance		262	244
Corporate owned life insurance, borrowings		118	113
Unrecorded Unconditional Purchase Obligation		1,939
Ameren Illinois Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Cash Surrender Value of Life Insurance		125	122
Corporate owned life insurance, borrowings		118	$ 113
Unrecorded Unconditional Purchase Obligation		642
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		20
Partnership Funding Commitment
Basis Of Presentation And Significant Accounting Policies [Line Items]
Unrecorded Unconditional Purchase Obligation		$ 14
Subsequent Event \| Partnership Funding Commitment
Basis Of Presentation And Significant Accounting Policies [Line Items]
Unrecorded Unconditional Purchase Obligation	$ 25

Rate And Regulatory Matters (Narrative-Missouri) (Detail) $ in Millions	3 Months Ended				6 Months Ended
	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($) MWh	Dec. 31, 2018 MWh	Jun. 30, 2018 USD ($) MWh	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)
Rate And Regulatory Matters [Line Items]
Total operating revenues		$ 1,379		$ 1,563	$ 2,935	$ 3,148
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%
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 [Domain] \| 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						$ 5
Subsequent Event \| Pending Rate Case [Member] \| 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
Subsequent Event \| Pending Rate Case [Member] \| 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
Subsequent Event \| Components of Rate Decrease - Net Energy Costs [Member] \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	100
Subsequent Event \| Components of Rate Decrease - Customer Sales Volume [Member] \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	55
Subsequent Event \| Components of Rate Decrease - Other Expenses [Member] \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	20
Subsequent Event \| Components of Rate Decrease - Other Expenses Subject to Cost Recovery Mechanisms [Member] \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	80
Subsequent Event \| Components of Rate Decrease - Depreciation and Amortization [Member] \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	115
Subsequent Event \| Components of Rate Decrease - Higher Depreciation Rates [Member] \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	35
Subsequent Event \| Components of Rate Decrease - PISA Eligible Depreciation [Member] \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	35
Subsequent Event \| Components of Rate Decrease - Return on Rate Base [Member] \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	60
Subsequent Event \| Components of Rate Decrease - PISA Eligible Return on Rate Base [Member] \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 30
Minimum \| Subsequent Event \| Pending Rate Case [Member] \| Natural Gas \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage	9.40%
Maximum \| Subsequent Event \| Pending Rate Case [Member] \| 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) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
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 [Member] \| 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 [Member] \| IEIMA \| Ameren Illinois Company \| Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	7
Pending Rate Case [Member] \| FEJA energy-efficiency rider [Domain] \| 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	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 156	$ 149
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 72	$ 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	$ 45
Midwest Independent Transmission System Operator, Inc \| Pending Ferc Case \| Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 26
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)	3 Months Ended		6 Months Ended
Short-Term Debt And Liquidity (Narrative) (Detail)	Jun. 30, 2019 USD ($)	Jun. 30, 2018	Jun. 30, 2019 USD ($)	Jun. 30, 2018
Credit Agreements
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,100,000,000		$ 1,100,000,000
Actual debt-to-capital ratio	0.54		0.54
Utilities
Short-term Debt [Line Items]
Short-term Debt, Weighted Average Interest Rate, over Time	2.75%	2.17%	2.81%	2.04%
Union Electric Company \| Missouri Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.49		0.49
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	Jun. 30, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Commercial paper outstanding	$ 999	$ 597
Ameren (parent)
Short-term Debt [Line Items]
Commercial paper outstanding	595	470
Union Electric Company
Short-term Debt [Line Items]
Commercial paper outstanding	205	55
Ameren Illinois Company
Short-term Debt [Line Items]
Commercial paper outstanding	$ 199	$ 72

Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 822	$ 693
Weighted-average interest rate	2.79%	2.12%
Short-term Debt, Maximum Amount Outstanding During Period	$ 1,113	$ 1,295
Peak interest rate	3.10%	2.55%
Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 542	$ 397
Weighted-average interest rate	2.80%	2.14%
Short-term Debt, Maximum Amount Outstanding During Period	$ 636	$ 506
Peak interest rate	3.10%	2.45%
Union Electric Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 174	$ 123
Weighted-average interest rate	2.79%	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	$ 106	$ 174
Weighted-average interest rate	2.72%	2.20%
Short-term Debt, Maximum Amount Outstanding During Period	$ 202	$ 442
Peak interest rate	2.90%	2.55%

Long-Term debt and Equity Financings (Narrative) (Details) - USD ($) shares in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Long-Term Debt And Equity Financings [Line Items]
Stock Issued During Period, Shares, New Issues	0.2	0.4	0.5	0.7
Issuance of common stock	$ 18,000,000		$ 37,000,000	$ 40,000,000
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
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

Long-Term Debt and Equity Financings Environmental Bonds (Details) - Union Electric Company - Environmental Improvement And Pollution Control Revenue Bonds [Member]	6 Months Ended
	Jun. 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		6 Months Ended
Other Income, Net (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 8	$ 9	$ 13	$ 14
Interest income on industrial development revenue bonds	6	7	13	13
Interest income	3	2	4	4
Non-service cost components of net periodic benefit income	22	19	44	35
Other income	1	2	4	3
Donations	(1)	(6)	(7)	(11)
Other expense	(3)	(4)	(6)	(6)
Other Income (Expense), Net	36	29	65	52
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	5	7	8	11
Interest income on industrial development revenue bonds	6	7	13	13
Interest income	0	1	0	1
Non-service cost components of net periodic benefit income	4	4	9	9
Other income	2	0	2	1
Donations	0	(2)	(2)	(3)
Other expense	(1)	(1)	(2)	(3)
Other Income (Expense), Net	16	16	28	29
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	8	4	15	8
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	3	2	5	3
Interest income	2	1	4	3
Non-service cost components of net periodic benefit income	12	10	24	17
Other income	1	2	2	2
Donations	(1)	(1)	(5)	(5)
Other expense	(2)	(1)	(4)	(1)
Other Income (Expense), Net	$ 15	$ 13	$ 26	$ 19

Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) gal in Millions, MWh in Millions, MMBTU in Millions	Jun. 30, 2019 MWh gal MMBTU	Dec. 31, 2018 MWh gal MMBTU
Fuel Oils
Derivative [Line Items]
Quantity \| gal	67	66
Natural Gas
Derivative [Line Items]
Quantity \| MMBTU	163	173
Power
Derivative [Line Items]
Quantity \| MWh	12	9
Union Electric Company \| Fuel Oils
Derivative [Line Items]
Quantity \| gal	67	66
Union Electric Company \| Natural Gas
Derivative [Line Items]
Quantity \| MMBTU	17	19
Union Electric Company \| Power
Derivative [Line Items]
Quantity \| MWh	4	1
Ameren Illinois Company \| Fuel Oils
Derivative [Line Items]
Quantity \| gal	0	0
Ameren Illinois Company \| Natural Gas
Derivative [Line Items]
Quantity \| MMBTU	146	154
Ameren Illinois Company \| Power
Derivative [Line Items]
Quantity \| MWh	8	8

Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative assets	$ 30	$ 15
Derivative liabilities	223	219
Fuel Oils \| Other Current Assets
Derivative [Line Items]
Derivative assets	4	3
Fuel Oils \| Other Assets
Derivative [Line Items]
Derivative assets	4	5
Fuel Oils \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	4	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	2	2
Natural Gas \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	16	12
Natural Gas \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	4	7
Power \| Other Current Assets
Derivative [Line Items]
Derivative assets	13	4
Power \| Other Assets
Derivative [Line Items]
Derivative assets	5	0
Power \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	19	18
Power \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	175	169
Union Electric Company
Derivative [Line Items]
Derivative assets	26	12
Derivative liabilities	15	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	4	5
Union Electric Company \| Fuel Oils \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	4	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	0	1
Union Electric Company \| Power \| Other Current Assets
Derivative [Line Items]
Derivative assets	13	4
Union Electric Company \| Power \| Other Assets
Derivative [Line Items]
Derivative assets	5	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	0	0
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	4	3
Derivative liabilities	208	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	2	2
Ameren Illinois Company \| Natural Gas \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	13	8
Ameren Illinois Company \| Natural Gas \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	4	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	16	14
Ameren Illinois Company \| Power \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	$ 175	$ 169

Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Detail) $ in Millions	Jun. 30, 2019 USD ($)
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	$ 107
Cash Collateral Posted	4
Potential Aggregate Amount of Additional Collateral Required	91
Union Electric Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	72
Cash Collateral Posted	4
Potential Aggregate Amount of Additional Collateral Required	63
Ameren Illinois Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	35
Cash Collateral Posted	0
Potential Aggregate Amount of Additional Collateral Required	$ 28

Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	$ 774	$ 679
Assets fair value	804	694
Excluded receivables, payables, and accrued income, net	9	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	511	427
Assets fair value	513	428
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	263	252
Assets fair value	264	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	27	11
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	30	15
Derivative liabilities	223	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	2	1
Derivative liabilities	4	2
Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	3
Derivative liabilities	15	16
Commodity Contract \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	27	11
Derivative liabilities	204	201
Fuel Oils \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	8	8
Derivative liabilities	9	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	2	1
Derivative liabilities	2	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	7	11
Power \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	18	4
Derivative liabilities	194	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	0	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	0	1
Power \| Commodity Contract \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	18	3
Derivative liabilities	194	186
Natural Gas \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	3
Derivative liabilities	20	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	1	2
Derivative liabilities	15	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	3	4
Equity Securities \| U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	511	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	511	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	79	72
Debt Securities \| US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	146	148
Debt Securities \| Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	38	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	79	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	146	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	38	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	774	679
Assets fair value	800	691
Excluded receivables, payables, and accrued income, net	6	4
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	511	427
Assets fair value	513	428
Union Electric Company \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	263	252
Assets fair value	263	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	24	10
Union Electric Company \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	26	12
Derivative liabilities	15	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	2	1
Derivative liabilities	2	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	3	6
Union Electric Company \| Commodity Contract \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	24	10
Derivative liabilities	10	14
Union Electric Company \| Fuel Oils \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	8	8
Derivative liabilities	9	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	2	1
Derivative liabilities	2	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	7	11
Union Electric Company \| Power \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	18	4
Derivative liabilities	3	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	0	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	0	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	18	3
Derivative liabilities	3	3
Union Electric Company \| Natural Gas \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	3	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	0	0
Union Electric Company \| Equity Securities \| U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	511	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	511	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	79	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	146	148
Union Electric Company \| Debt Securities \| Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	38	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	79	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	146	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	38	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	208	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	12	10
Ameren Illinois Company \| Commodity Contract \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	194	187
Ameren Illinois Company \| Power \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	191	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	191	183
Ameren Illinois Company \| Natural Gas \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	3
Derivative liabilities	17	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	1	2
Derivative liabilities	12	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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 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	$ 4	$ 0	$ 4
Change in unrealized gains (losses) related to assets/liabilities held at period end	5	(3)	1	(3)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	(176)	(185)	(176)	(185)	$ (184)	$ (183)	$ (187)	$ (188)
Included in regulatory assets/liabilities	5	(3)	1	(4)
Settlements	3	1	6	3
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	4	0	4
Change in unrealized gains (losses) related to assets/liabilities held at period end	16	0	16	(1)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	15	5	15	5	0	0	4	7
Included in regulatory assets/liabilities	16	(1)	16	(3)
Settlements	(1)	(2)	(1)	(3)
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	(11)	(3)	(15)	(2)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	(191)	(190)	(191)	(190)	$ (184)	$ (183)	$ (191)	$ (195)
Included in regulatory assets/liabilities	(11)	(2)	(15)	(1)
Settlements	$ 4	$ 3	$ 7	$ 6

Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) - Power $ in Millions	Jun. 30, 2019 USD ($) $ / MMBTU $ / MWh	Jun. 30, 2018 USD ($) $ / MMBTU $ / MWh
Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets \| $	$ 18	$ 3
Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities \| $	$ (194)	$ (186)
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		28
Nodal Basis \| Discounted Cash Flow \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	(9)	(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	(2)	(2)
Quoted Price \| Discounted Cash Flow \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	22
Quoted Price \| Discounted Cash Flow \| Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	32
Quoted Price \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	25
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	4	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	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt	$ 999	$ 597
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	122	107	$ 96	$ 68
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt	205	55
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	13	8	25	7
Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt	199	72
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	99	80	$ 57	$ 41
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities and Held-to-maturity Securities	270	270
Short-term Debt	999	597
Long-term debt (including current portion)	8,558	8,439
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	122	107
Debt Issuance Costs, Net	62	58
Carrying Amount \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities and Held-to-maturity Securities	270	270
Short-term Debt	205	55
Long-term debt (including current portion)	4,116	3,998
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	13	8
Debt Issuance Costs, Net	26	22
Carrying Amount \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt	199	72
Long-term debt (including current portion)	3,296	3,296
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	99	80
Debt Issuance Costs, Net	31	31
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	999	597
Investments, Fair Value Disclosure	270	270
Long-term Debt, Fair Value	9,391	8,669
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	122	107
Fair Value \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	122	107
Fair Value \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	999	597
Investments, Fair Value Disclosure	270	270
Long-term Debt, Fair Value	8,925	8,240
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Fair Value \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	466	429
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Fair Value \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	205	55
Investments, Fair Value Disclosure	270	270
Long-term Debt, Fair Value	4,568	4,156
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	13	8
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]
Short-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	13	8
Fair Value \| Union Electric Company \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	205	55
Investments, Fair Value Disclosure	270	270
Long-term Debt, Fair Value	4,568	4,156
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Fair Value \| Union Electric Company \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Fair Value \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	199	72
Long-term Debt, Fair Value	3,642	3,391
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	99	80
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]
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	99	80
Fair Value \| Ameren Illinois Company \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	199	72
Long-term Debt, Fair Value	3,642	3,391
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Fair Value \| Ameren Illinois Company \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 0	$ 0

Related Party Transactions (Schedule of Related Party Transactions) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Related Party Transaction [Line Items]
Operating Revenues	$ 0	$ 0	$ 0	$ 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	2	3	2	6
Ameren Missouri and Ameren Illinois Rent and Facility Services \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	6	6	13	11
Operating Expenses	1	1	1	1
Ameren Missouri and Ameren Illinois Rent and Facility Services \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	2
Operating Expenses	2	1	3	3
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
Total Related Party Operating Revenues \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	8	9	15	17
Total Related Party Operating Revenues \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	1	2	2
Ameren Illinois Power Supply Agreements with Ameren Missouri \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	2	3	2	6
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	4	2	7
Ameren Services Support Services Agreement \| Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	32	32	64	65
Ameren Services Support Services Agreement \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	31	30	61	60
Total Related Party Other Operations and Maintenance \| Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	33	32	65	66
Total Related Party Other Operations and Maintenance \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	33	31	64	63
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) - April 2019 Procurement - Ameren Illinois Company - Ameren Illinois Power Supply Agreements with Ameren Missouri	3 Months Ended
	Jun. 30, 2019 MWh $ / MWh
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

Commitments And Contingencies (Other Obligations) (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($) MWh
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	$ 516
Unrecorded Unconditional Purchase Obligation, Due within Two Years	560
Unrecorded Unconditional Purchase Obligation, Due within Three Years	393
Unrecorded Unconditional Purchase Obligation, Due within Four Years	199
Unrecorded Unconditional Purchase Obligation, Due within Five Years	128
Unrecorded Unconditional Purchase Obligation, Due after Five Years	143
Unrecorded Unconditional Purchase Obligation	1,939
Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	224
Unrecorded Unconditional Purchase Obligation, Due within Two Years	207
Unrecorded Unconditional Purchase Obligation, Due within Three Years	183
Unrecorded Unconditional Purchase Obligation, Due within Four Years	125
Unrecorded Unconditional Purchase Obligation, Due within Five Years	46
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	785
Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	113
Unrecorded Unconditional Purchase Obligation, Due within Two Years	160
Unrecorded Unconditional Purchase Obligation, Due within Three Years	96
Unrecorded Unconditional Purchase Obligation, Due within Four Years	38
Unrecorded Unconditional Purchase Obligation, Due within Five Years	20
Unrecorded Unconditional Purchase Obligation, Due after Five Years	36
Unrecorded Unconditional Purchase Obligation	463
Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	23
Unrecorded Unconditional Purchase Obligation, Due within Two Years	43
Unrecorded Unconditional Purchase Obligation, Due within Three Years	60
Unrecorded Unconditional Purchase Obligation, Due within Four Years	11
Unrecorded Unconditional Purchase Obligation, Due within Five Years	41
Unrecorded Unconditional Purchase Obligation, Due after Five Years	28
Unrecorded Unconditional Purchase Obligation	206
Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	127
Unrecorded Unconditional Purchase Obligation, Due within Two Years	114
Unrecorded Unconditional Purchase Obligation, Due within Three Years	29
Unrecorded Unconditional Purchase Obligation, Due within Four Years	5
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	$ 275
Amount of Megawatts \| MWh	102
Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	$ 2
Unrecorded Unconditional Purchase Obligation, Due within Two Years	3
Unrecorded Unconditional Purchase Obligation, Due within Three Years	3
Unrecorded Unconditional Purchase Obligation, Due within Four Years	3
Unrecorded Unconditional Purchase Obligation, Due within Five Years	3
Unrecorded Unconditional Purchase Obligation, Due after Five Years	26
Unrecorded Unconditional Purchase Obligation	40
Other Purchased Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	27
Unrecorded Unconditional Purchase Obligation, Due within Two Years	33
Unrecorded Unconditional Purchase Obligation, Due within Three Years	22
Unrecorded Unconditional Purchase Obligation, Due within Four Years	17
Unrecorded Unconditional Purchase Obligation, Due within Five Years	18
Unrecorded Unconditional Purchase Obligation, Due after Five Years	53
Unrecorded Unconditional Purchase Obligation	170
Renewable Energy Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation	5
Zero Emission Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation	60
Union Electric Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	297
Unrecorded Unconditional Purchase Obligation, Due within Two Years	308
Unrecorded Unconditional Purchase Obligation, Due within Three Years	279
Unrecorded Unconditional Purchase Obligation, Due within Four Years	161
Unrecorded Unconditional Purchase Obligation, Due within Five Years	113
Unrecorded Unconditional Purchase Obligation, Due after Five Years	106
Unrecorded Unconditional Purchase Obligation	1,264
Union Electric Company \| Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	224
Unrecorded Unconditional Purchase Obligation, Due within Two Years	207
Unrecorded Unconditional Purchase Obligation, Due within Three Years	183
Unrecorded Unconditional Purchase Obligation, Due within Four Years	125
Unrecorded Unconditional Purchase Obligation, Due within Five Years	46
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	785
Union Electric Company \| Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	28
Unrecorded Unconditional Purchase Obligation, Due within Two Years	34
Unrecorded Unconditional Purchase Obligation, Due within Three Years	16
Unrecorded Unconditional Purchase Obligation, Due within Four Years	5
Unrecorded Unconditional Purchase Obligation, Due within Five Years	5
Unrecorded Unconditional Purchase Obligation, Due after Five Years	13
Unrecorded Unconditional Purchase Obligation	101
Union Electric Company \| Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	23
Unrecorded Unconditional Purchase Obligation, Due within Two Years	43
Unrecorded Unconditional Purchase Obligation, Due within Three Years	60
Unrecorded Unconditional Purchase Obligation, Due within Four Years	11
Unrecorded Unconditional Purchase Obligation, Due within Five Years	41
Unrecorded Unconditional Purchase Obligation, Due after Five Years	28
Unrecorded Unconditional Purchase Obligation	206
Union Electric Company \| Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	0
Unrecorded Unconditional Purchase Obligation, Due within Two Years	0
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	$ 0
Amount of Megawatts \| MWh	102
Union Electric Company \| Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	$ 2
Unrecorded Unconditional Purchase Obligation, Due within Two Years	3
Unrecorded Unconditional Purchase Obligation, Due within Three Years	3
Unrecorded Unconditional Purchase Obligation, Due within Four Years	3
Unrecorded Unconditional Purchase Obligation, Due within Five Years	3
Unrecorded Unconditional Purchase Obligation, Due after Five Years	26
Unrecorded Unconditional Purchase Obligation	40
Union Electric Company \| Other Purchased Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	20
Unrecorded Unconditional Purchase Obligation, Due within Two Years	21
Unrecorded Unconditional Purchase Obligation, Due within Three Years	17
Unrecorded Unconditional Purchase Obligation, Due within Four Years	17
Unrecorded Unconditional Purchase Obligation, Due within Five Years	18
Unrecorded Unconditional Purchase Obligation, Due after Five Years	39
Unrecorded Unconditional Purchase Obligation	132
Ameren Illinois Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	214
Unrecorded Unconditional Purchase Obligation, Due within Two Years	243
Unrecorded Unconditional Purchase Obligation, Due within Three Years	109
Unrecorded Unconditional Purchase Obligation, Due within Four Years	38
Unrecorded Unconditional Purchase Obligation, Due within Five Years	15
Unrecorded Unconditional Purchase Obligation, Due after Five Years	23
Unrecorded Unconditional Purchase Obligation	642
Ameren Illinois Company \| Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	0
Unrecorded Unconditional Purchase Obligation, Due within Two Years	0
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	0
Ameren Illinois Company \| Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	85
Unrecorded Unconditional Purchase Obligation, Due within Two Years	126
Unrecorded Unconditional Purchase Obligation, Due within Three Years	80
Unrecorded Unconditional Purchase Obligation, Due within Four Years	33
Unrecorded Unconditional Purchase Obligation, Due within Five Years	15
Unrecorded Unconditional Purchase Obligation, Due after Five Years	23
Unrecorded Unconditional Purchase Obligation	362
Ameren Illinois Company \| Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	0
Unrecorded Unconditional Purchase Obligation, Due within Two Years	0
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	0
Ameren Illinois Company \| Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	127
Unrecorded Unconditional Purchase Obligation, Due within Two Years	114
Unrecorded Unconditional Purchase Obligation, Due within Three Years	29
Unrecorded Unconditional Purchase Obligation, Due within Four Years	5
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	275
Ameren Illinois Company \| Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	0
Unrecorded Unconditional Purchase Obligation, Due within Two Years	0
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	0
Ameren Illinois Company \| Other Purchased Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	2
Unrecorded Unconditional Purchase Obligation, Due within Two Years	3
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	5
Ameren Illinois Company \| Renewable Energy Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation	5
Ameren Illinois Company \| Zero Emission Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation	$ 60

Commitments And Contingencies (Environmental Matters) (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($) center waste_stream scrubber site	Dec. 31, 2018 USD ($)
Loss Contingencies [Line Items]
Percentage of Rate Base Related to Carbon Dioxide Energy Centers		16.00%
Asset Retirement Obligation	$ 690	$ 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	$ 686	$ 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	$ 145
Manufactured Gas Plant \| Ameren Illinois Company \| Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	145
Manufactured Gas Plant \| Ameren Illinois Company \| Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	212
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	$ 162

Callaway Energy Center (Insurance Disclosure) (Detail)	6 Months Ended
Callaway Energy Center (Insurance Disclosure) (Detail)	Jun. 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	$ 450,000,000
Maximum Assessments for Single Incidents	0
Public Liability And Nuclear Worker Liability - Pool Participation
Nuclear Waste Matters [Line Items]
Maximum Coverages	13,486,000,000
Maximum Assessments for Single Incidents	138,000,000
Threshold for which a retrospective assessment for a covered loss is necessary	450,000,000
Annual payment in the event of an incident at any licensed commercial reactor	21,000,000
Public Liability
Nuclear Waste Matters [Line Items]
Maximum Coverages	13,936,000,000
Maximum Assessments for Single Incidents	138,000,000
Property Damage - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Maximum Coverages	3,200,000,000
Maximum Assessments for Single Incidents	28,000,000
Replacement Power - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Maximum Coverages	490,000,000
Maximum Assessments for Single Incidents	7,000,000
Amount of weekly indemnity coverage commencing twelve weeks after power outage	4,500,000
Amount of additional weekly indemnity coverage commencing after initial indemnity coverage	3,600,000
Amount of weekly indemnity coverage thereafter not exceeding policy limit	490,000,000
Sub-limit of for non-nuclear events	328,000,000
Radiation Event
Nuclear Waste Matters [Line Items]
Maximum Coverages	2,700,000,000
Aggregate nuclear power industry insurance policy limit for losses from terrorist attacks within twelve month period	3,200,000,000
Non-radiation event
Nuclear Waste Matters [Line Items]
Maximum Coverages	2,300,000,000
Aggregate nuclear power industry insurance policy limit for losses from terrorist attacks within twelve month period	1,800,000,000
Property Damage European Mutual Association for Nuclear Insurance
Nuclear Waste Matters [Line Items]
Maximum Coverages	$ 490,000,000

Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	$ (22)	$ (19)	$ (44)	$ (35)
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	22	25	44	50
Interest cost	46	42	93	84
Expected return on plan assets	(69)	(69)	(138)	(138)
Prior service cost (benefit)	0	0	0	0
Actuarial loss	7	18	13	34
Total non-service cost components	(16)	(9)	(32)	(20)
Net periodic benefit cost	6	16	12	30
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	5	5	9	10
Interest cost	10	9	21	20
Expected return on plan assets	(19)	(19)	(38)	(38)
Prior service cost (benefit)	(2)	(1)	(3)	(2)
Actuarial loss	(3)	(3)	(7)	(3)
Total non-service cost components	(14)	(14)	(27)	(23)
Net periodic benefit cost	(9)	(9)	(18)	(13)
Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(4)	(4)	(9)	(9)
Union Electric Company \| Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	1	6	2	11
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)	0	(3)	0
Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(12)	(10)	(24)	(17)
Ameren Illinois Company \| Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	5	10	10	19
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)	$ (9)	$ (15)	$ (13)

Retirement Benefits (Summary of Benefit Plan Costs Incurred) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 6	$ 16	$ 12	$ 30
Pension Plan \| Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	1	6	2	11
Pension Plan \| Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	5	10	10	19
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(9)	(9)	(18)	(13)
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)	0	(3)	0
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)	$ (9)	$ (15)	$ (13)

Income Taxes Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2019 Rate	Jun. 30, 2018 Rate	Jun. 30, 2019 Rate	Jun. 30, 2018 Rate
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	(9.00%)	(1.00%)	(8.00%)	(2.00%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent	0.00%	0.00%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent	0.00%	0.00%	0.00%	(1.00%)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	6.00%	5.00%	6.00%	6.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent			(3.00%)	(1.00%)
Effective Income Tax Rate Reconciliation, Tax Credit, Percent	0.00%	(1.00%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent			(1.00%)	0.00%
Effective Income Tax Rate Reconciliation, Percent	18.00%	24.00%	15.00%	23.00%
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	(12.00%)	0.00%	(12.00%)	0.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent	0.00%	0.00%	1.00%	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	4.00%	4.00%	4.00%	4.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent			0.00%	0.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Percent	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent			(1.00%)	0.00%
Effective Income Tax Rate Reconciliation, Percent	12.00%	24.00%	12.00%	24.00%
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	(4.00%)	(5.00%)	(4.00%)	(4.00%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent	(1.00%)	(1.00%)	0.00%	0.00%
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	7.00%	8.00%	7.00%	7.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent			0.00%	0.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Percent	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent			0.00%	0.00%
Effective Income Tax Rate Reconciliation, Percent	23.00%	23.00%	24.00%	24.00%

Supplemental Information (Narrative) (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Supplemental Information [Abstract]
Payables for purchased receivables	$ 34		$ 34		$ 33
Deferred Compensation Liability, Classified, Noncurrent	$ 78		$ 78		$ 80
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements (in shares)	1.6	2.1	1.5	1.8

Supplemental Information (Cash and Cash Equivalents) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	$ 6	$ 16
Restricted Cash and Cash Equivalents, Current	6	13
Restricted Cash and Cash Equivalents, Noncurrent	97	74
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	13	4
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	122	107	$ 96	$ 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	0	4
Restricted Cash and Cash Equivalents, Noncurrent	0	0
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	13	4
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	13	8	25	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	2	6
Restricted Cash and Cash Equivalents, Noncurrent	97	74
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 99	$ 80	$ 57	$ 41

Supplemental Information (Leases - Supplemental Balance Sheet Information) (Details) $ in Millions	6 Months Ended
	Jun. 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	35
Operating Lease, Liability, Current	7
Operating Lease, Liability, Noncurrent	$ 28
Operating Lease, Weighted Average Remaining Lease Term	6 years
Operating Lease, Weighted Average Discount Rate, Percent	3.60%
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	$ 32
Operating Lease, Liability, Current	6
Operating Lease, Liability, Noncurrent	$ 26
Operating Lease, Weighted Average Remaining Lease Term	6 years
Operating Lease, Weighted Average Discount Rate, Percent	3.60%

Supplemental Information (Leases - Maturities of Operating Lease Liabilities) (Details) $ in Millions	Jun. 30, 2019 USD ($)
Lessee, Lease, Description [Line Items]
2019	$ 4
2020	7
2021	7
2022	6
2023	5
Thereafter	10
Total lease payments	39
Less imputed interest	4
Total	35
Union Electric Company
Lessee, Lease, Description [Line Items]
2019	4
2020	7
2021	6
2022	5
2023	5
Thereafter	9
Total lease payments	36
Less imputed interest	4
Total	$ 32

Supplemental Information (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	$ 263	$ 233
Net Realized and Unrealized Gain (Loss) on Trading Securities	90	1
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	101	80
Net Realized and Unrealized Gain (Loss) on Trading Securities	90	1
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	143	147
Net Realized and Unrealized Gain (Loss) on Trading Securities	$ 0	$ 0

Supplemental Information (Schedule of Asset Retirement Obligations) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	$ 690	$ 650
Liabilities settled	(7)
Accretion	14
Asset Retirement Obligation, Revision of Estimate	33
Other current liabilities	668	544
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	686	646
Liabilities settled	(7)
Accretion	14
Asset Retirement Obligation, Revision of Estimate	33
Other current liabilities	210	175
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	184	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	6 Months Ended
	Jun. 30, 2019	Jun. 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)	294,871
Forfeitures (in shares)	(14,564)
Undistributed vested units (in shares)	(122,451)
Vested and distributed (in shares)	(176,923)
Nonvested at the end of the period (in shares)	663,744
Fully Vested Undistributed to Retirement-eligible employees (in shares)	390,459		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)	64.13
Weighted-average Fair Value per Unit, Undistributed Vested Units (in dollars per share)	62.19
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)	$ 63.52
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)	128,415
Forfeitures (in shares)	(4,465)
Undistributed vested units (in shares)	(27,301)
Vested and distributed (in shares)	0
Nonvested at the end of the period (in shares)	251,902
Fully Vested Undistributed to Retirement-eligible employees (in shares)	53,858		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)	65.46
Weighted-average Fair Value per Unit, Forfeitures (in dollars per share)	62.62
Weighted-average Fair Value per Unit, Undistributed Vested Units (in dollars per share)	61.87
Weighted-average Fair Value per Unit, Vested (in dollars per share)	0
Weighted-average Fair Value per Unit, Nonvested as of Current period end (in dollars per share)	$ 60.80

Supplemental Information (Schedule Of Excise Taxes) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Accounting Policies [Line Items]
Excise tax expense	$ 63	$ 74	$ 133	$ 143
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	38	46	69	80
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 25	$ 28	$ 64	$ 63

Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) segment	Jun. 30, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments \| segment			4
External revenues	$ 1,379	$ 1,563	$ 2,935	$ 3,148
Intersegment revenues	0	0	0	0
Net Income (Loss) Available to Common Stockholders, Basic	179	239	370	390
Capital expenditures	581	533	$ 1,125	1,112
Union Electric Company
Segment Reporting Information [Line Items]
Number of Reportable Segments \| segment			1
Ameren Illinois Company
Segment Reporting Information [Line Items]
Number of Reportable Segments \| segment			3
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	547	578	$ 1,309	1,338
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	62	62	182	157
Capital expenditures	289	302	556	602
Operating Segments \| Union Electric Company
Segment Reporting Information [Line Items]
External revenues	790	946	1,541	1,730
Intersegment revenues	8	9	15	17
Net Income Available to Common Shareholder	107	168	146	206
Capital expenditures	255	205	495	454
Operating Segments \| Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	358	386	744	785
Intersegment revenues	1	1	2	2
Net Income Available to Common Shareholder	37	33	73	66
Capital expenditures	127	132	251	254
Operating Segments \| Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues	136	142	456	453
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	1	7	58	49
Capital expenditures	77	66	128	126
Operating Segments \| Ameren Transmission
Segment Reporting Information [Line Items]
External revenues	95	89	194	180
Intersegment revenues	14	14	29	27
Net Income Available to Common Shareholder	42	36	86	73
Capital expenditures	127	130	248	275
Operating Segments \| Other
Segment Reporting Information [Line Items]
External revenues	0	0	0	0
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	(8)	(5)	7	(4)
Capital expenditures	0	(2)	10	5
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	359	387	746	787
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	37	33	73	66
Capital expenditures	127	132	251	254
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues	136	142	456	453
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	1	7	58	49
Capital expenditures	77	66	128	126
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Transmission
Segment Reporting Information [Line Items]
External revenues	52	49	107	98
Intersegment revenues	14	13	29	26
Net Income Available to Common Shareholder	24	22	51	42
Capital expenditures	85	104	177	222
Intersegment Eliminations
Segment Reporting Information [Line Items]
External revenues	0	0	0	0
Intersegment revenues	(23)	(24)	(46)	(46)
Net Income Available to Common Shareholder	0	0	0	0
Capital expenditures	(5)	2	(7)	(2)
Intersegment Eliminations \| Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	0	0	0	0
Intersegment revenues	(14)	(13)	(29)	(26)
Net Income Available to Common Shareholder	0	0	0	0
Capital expenditures	$ 0	$ 0	$ 0	$ 0

Segment Information (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 1,379	$ 1,563	$ 2,935	$ 3,148
TCJA Revenue Reduction		37		47
Revenues from alternative revenue programs	8	0	37	20
Other revenues not from contracts with customers	5	8	14	33
Electricity
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,218	1,396	2,400	2,619
Electricity \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	532	653	1,061	1,204
Electricity \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	434	490	796	866
Electricity \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	110	120	199	216
Electricity \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	142	133	344	333
Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	161	167	535	529
Natural Gas \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	98	110	382	394
Natural Gas \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	27	32	108	115
Natural Gas \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	3	5	9	13
Natural Gas \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	33	20	36	7
Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	547	578	1,309	1,338
Ameren Illinois Company \| Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	547	578	1,309	1,338
Revenues from alternative revenue programs	7	5	21	29
Other revenues not from contracts with customers	1	3	5	14
Ameren Illinois Company \| Ameren Illinois Company \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	287	318	750	780
Ameren Illinois Company \| Ameren Illinois Company \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	147	152	335	343
Ameren Illinois Company \| Ameren Illinois Company \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	36	38	74	79
Ameren Illinois Company \| Ameren Illinois Company \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	77	70	150	136
Ameren Illinois Company \| Electricity
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	411	436	853	885
Ameren Illinois Company \| Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	136	142	456	453
Operating Segments \| Union Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	798	955	1,556	1,747
Revenues from alternative revenue programs	0	(5)	15	(9)
Other revenues not from contracts with customers	4	5	9	19
Operating Segments \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	359	387	746	787
Revenues from alternative revenue programs	12	15	34	46
Other revenues not from contracts with customers	1	3	4	13
Operating Segments \| Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	136	142	456	453
Revenues from alternative revenue programs	4	(5)	1	(8)
Other revenues not from contracts with customers	0	0	1	1
Operating Segments \| Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	109	103	223	207
Revenues from alternative revenue programs	(8)	(5)	(13)	(9)
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	773	930	1,477	1,671
Operating Segments \| Electricity \| Union Electric Company \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	333	432	645	764
Operating Segments \| Electricity \| Union Electric Company \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	310	364	549	616
Operating Segments \| Electricity \| Union Electric Company \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	77	87	132	148
Operating Segments \| Electricity \| Union Electric Company \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	53	47	151	143
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	359	387	746	787
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	199	221	416	440
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	124	126	247	250
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	33	33	67	68
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	3	7	16	29
Operating Segments \| Electricity \| Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Electricity \| Ameren Illinois Gas \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Electricity \| Ameren Illinois Gas \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Electricity \| Ameren Illinois Gas \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments \| Electricity \| Ameren Illinois 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	109	103	223	207
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	109	103	223	207
Operating Segments \| Natural Gas \| Union Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	25	25	79	76
Operating Segments \| Natural Gas \| Union Electric Company \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	10	13	48	54
Operating Segments \| Natural Gas \| Union Electric Company \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	4	6	20	22
Operating Segments \| Natural Gas \| Union Electric Company \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	2	2
Operating Segments \| Natural Gas \| Union Electric Company \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	11	6	9	(2)
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 Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	136	142	456	453
Operating Segments \| Natural Gas \| Ameren Illinois Gas \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	88	97	334	340
Operating Segments \| Natural Gas \| Ameren Illinois Gas \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	23	26	88	93
Operating Segments \| Natural Gas \| Ameren Illinois Gas \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	3	5	7	11
Operating Segments \| Natural Gas \| Ameren Illinois Gas \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	22	14	27	9
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]
Revenues from alternative revenue programs	12	15	34	46
Other revenues not from contracts with customers	1	3	4	13
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	4	(5)	1	(8)
Other revenues not from contracts with customers	0	0	1	1
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(9)	(5)	(14)	(9)
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	359	387	746	787
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	199	221	416	440
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	124	126	247	250
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	33	33	67	68
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	3	7	16	29
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	66	62	136	124
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	66	62	136	124
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	136	142	456	453
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Gas \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	88	97	334	340
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Gas \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	23	26	88	93
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Gas \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	3	5	7	11
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Gas \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	22	14	27	9
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(23)	(24)	(46)	(46)
Intersegment Eliminations \| Electricity
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(23)	(24)	(46)	(46)
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	(23)	(24)	(46)	(46)
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	(14)	(13)	(29)	(26)
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	$ (14)	$ (13)	$ (29)	$ (26)