Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2019	Apr. 30, 2019
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Trading Symbol	AEE
Entity Current Reporting Status	Yes
Entity Registrant Name	AMEREN CORP
Entity Central Index Key	0001002910
Current Fiscal Year End Date	--12-31
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		245,597,156
Union Electric Company
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Entity Current Reporting Status	Yes
Entity Registrant Name	UNION ELECTRIC CO
Entity Central Index Key	0000100826
Current Fiscal Year End Date	--12-31
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		102,123,834
Ameren Illinois Company
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Entity Current Reporting Status	Yes
Entity Registrant Name	AMEREN ILLINOIS CO
Entity Central Index Key	0000018654
Current Fiscal Year End Date	--12-31
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Filer Category	Non-accelerated Filer
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
	Mar. 31, 2019	Mar. 31, 2018
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,556	$ 1,585
Operating Expenses:
Fuel	160	188
Utilities Operating Expense, Purchased Power	156	163
Utilities Operating Expense, Gas and Petroleum Purchased	161	171
Other operations and maintenance	417	431
Depreciation and amortization	248	234
Taxes other than income taxes	126	125
Total operating expenses	1,268	1,312
Operating Income	288	273
Other Income (Expense), Net	29	23
Interest Charges	97	101
Income Before Income Taxes	220	195
Income Taxes	27	42
Net income	193	153
Less: Net Income Attributable to Noncontrolling Interests	2	2
Net Income (Loss) Available to Common Stockholders, Basic	191	151
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	1	1
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	194	154
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	2	2
Comprehensive Income	$ 192	$ 152
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic and Diluted	$ 0.78	$ 0.62
Average Common Shares Outstanding - Basic (in shares)	244.9	242.9
Weighted Average Number of Shares Outstanding, Diluted	246.4	244.4
Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,182	$ 1,223
Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	374	362
Union Electric Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	758	792
Operating Expenses:
Fuel	160	188
Utilities Operating Expense, Purchased Power	51	42
Utilities Operating Expense, Gas and Petroleum Purchased	27	24
Other operations and maintenance	224	232
Depreciation and amortization	140	136
Taxes other than income taxes	77	80
Total operating expenses	679	702
Operating Income	79	90
Other Income (Expense), Net	12	13
Interest Charges	47	51
Income Before Income Taxes	44	52
Income Taxes	4	13
Net income	40	39
Preferred Stock Dividends	1	1
Net Income (Loss) Attributable to Parent	39	38
Union Electric Company \| Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	704	741
Union Electric Company \| Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	54	51
Ameren Illinois Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	762	760
Operating Expenses:
Utilities Operating Expense, Purchased Power	105	124
Utilities Operating Expense, Gas and Petroleum Purchased	134	147
Other operations and maintenance	191	199
Depreciation and amortization	101	90
Taxes other than income taxes	45	41
Total operating expenses	576	601
Operating Income	186	159
Other Income (Expense), Net	11	6
Interest Charges	37	37
Income Before Income Taxes	160	128
Income Taxes	39	32
Net income	121	96
Preferred Stock Dividends	1	1
Net Income (Loss) Attributable to Parent	120	95
Ameren Illinois Company \| Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	442	449
Ameren Illinois Company \| Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 320	$ 311

Consolidated Statement of Income (Loss) and Comprehensive Income (Loss) (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Pension and other postretirement benefit plan activity, tax expense (benefit)	$ 0	$ 0

Consolidated Balance Sheet - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Current Assets:
Cash and cash equivalents	$ 8	$ 16
Accounts receivable - trade (less allowance for doubtful accounts)	517	463
Unbilled revenue	238	295
Miscellaneous accounts and notes receivable	74	79
Inventories	402	483
Current regulatory assets	113	134
Other current assets	70	63
Total current assets	1,422	1,533
Property, Plant, and Equipment, Net	23,044	22,810
Investments and Other Assets:
Nuclear decommissioning trust fund	754	684
Goodwill	411	411
Regulatory Assets, Noncurrent	1,134	1,127
Other assets	744	650
Total investments and other assets	3,043	2,872
TOTAL ASSETS	27,509	27,215
Current Liabilities:
Current maturities of long-term debt	336	580
Short-term Debt	799	597
Accounts and wages payable	522	817
Taxes accrued	81	53
Interest accrued	81	93
Customer deposits	111	116
Current regulatory liabilities	161	149
Other current liabilities	301	282
Total current liabilities	2,392	2,687
Long-term Debt, Net	8,221	7,859
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	2,666	2,623
Accumulated Deferred Investment Tax Credit	42	43
Regulatory Liability, Noncurrent	4,731	4,637
Asset retirement obligations	630	627
Pension and other postretirement benefits	558	558
Other deferred credits and liabilities	422	408
Total deferred credits and other liabilities	9,049	8,896
Commitments and Contingencies
Stockholders' Equity:
Common Stock	2	2
Other paid-in capital	5,625	5,627
Retained earnings	2,099	2,024
Accumulated other comprehensive loss	(21)	(22)
Stockholder's equity	7,705	7,631
Noncontrolling Interest	142	142
Total equity	7,847	7,773
TOTAL LIABILITIES AND EQUITY	27,509	27,215
Union Electric Company
Current Assets:
Cash and cash equivalents	0	0
Accounts receivable - trade (less allowance for doubtful accounts)	189	223
Accounts receivable - affiliates	20	14
Unbilled revenue	128	155
Miscellaneous accounts and notes receivable	40	42
Inventories	329	358
Current regulatory assets	13	14
Other current assets	32	26
Total current assets	751	832
Property, Plant, and Equipment, Net	12,159	12,103
Investments and Other Assets:
Nuclear decommissioning trust fund	754	684
Regulatory Assets, Noncurrent	351	366
Other assets	356	306
Total investments and other assets	1,461	1,356
TOTAL ASSETS	14,371	14,291
Current Liabilities:
Current maturities of long-term debt	336	580
Short-term Debt	55	55
Accounts and wages payable	224	428
Accounts payable - affiliates	69	69
Taxes accrued	62	27
Interest accrued	47	52
Current regulatory liabilities	64	68
Other current liabilities	128	123
Total current liabilities	985	1,402
Long-term Debt, Net	3,780	3,418
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,536	1,534
Accumulated Deferred Investment Tax Credit	41	42
Regulatory Liability, Noncurrent	2,868	2,799
Asset retirement obligations	626	623
Pension and other postretirement benefits	227	228
Other deferred credits and liabilities	40	16
Total deferred credits and other liabilities	5,338	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	1,774	1,735
Stockholder's equity	4,268	4,229
TOTAL LIABILITIES AND EQUITY	14,371	14,291
Ameren Illinois Company
Current Assets:
Cash and cash equivalents	0	0
Accounts receivable - trade (less allowance for doubtful accounts)	315	224
Accounts receivable - affiliates	18	21
Unbilled revenue	110	140
Miscellaneous accounts and notes receivable	34	40
Inventories	73	125
Current regulatory assets	92	110
Other current assets	15	16
Total current assets	657	676
Property, Plant, and Equipment, Net	9,347	9,198
Investments and Other Assets:
Goodwill	411	411
Regulatory Assets, Noncurrent	776	759
Other assets	303	275
Total investments and other assets	1,490	1,445
TOTAL ASSETS	11,494	11,319
Current Liabilities:
Short-term Debt	126	72
Accounts and wages payable	242	302
Accounts payable - affiliates	63	58
Taxes accrued	19	23
Interest accrued	27	31
Customer deposits	73	76
Current environmental remediation	45	42
Current regulatory liabilities	79	62
Other current liabilities	129	130
Total current liabilities	803	796
Long-term Debt, Net	3,296	3,296
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,151	1,119
Regulatory Liability, Noncurrent	1,765	1,741
Pension and other postretirement benefits	281	280
Environmental remediation	101	109
Other deferred credits and liabilities	203	204
Total deferred credits and other liabilities	3,501	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	1,659	1,539
Stockholder's equity	3,894	3,774
TOTAL LIABILITIES AND EQUITY	$ 11,494	$ 11,319

Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Millions	Mar. 31, 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	245,600,000	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	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash Flows From Operating Activities:
Net income	$ 193	$ 153
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	245	230
Amortization of nuclear fuel	23	24
Amortization of debt issuance costs and premium/discounts	5	5
Deferred income taxes and investment tax credits, net	32	26
Allowance for equity funds used during construction	(6)	(5)
Stock-based compensation costs	6	6
Other	(8)	2
Changes in assets and liabilities:
Receivables	4	(26)
Inventories	81	68
Accounts and wages payable	(213)	(249)
Taxes accrued	28	49
Regulatory assets and liabilities	26	20
Assets, other	(14)	1
Liabilities, other	(11)	(57)
Pension and other postretirement benefits	(4)	11
Net cash provided by operating activities	387	258
Cash Flows From Investing Activities:
Capital expenditures	(544)	(579)
Nuclear fuel expenditures	(21)	(12)
Purchases of securities - nuclear decommissioning trust fund	(39)	(38)
Sales and maturities of securities - nuclear decommissioning trust fund	36	34
Other	1	(2)
Net cash used in investing activities	(567)	(597)
Cash Flows From Financing Activities:
Dividends on common stock	(116)	(111)
Dividends paid to noncontrolling interest holders	(2)	(2)
Short-term debt, net	202	475
Repayments of Long-term Debt	(329)	0
Proceeds from Issuance of Long-term Debt	450	0
Issuance of common stock	19	17
Employee payroll taxes related to stock-based compensation	(29)	(19)
Payment of Financing and Stock Issuance Costs	(4)	0
Net cash provided by financing activities	191	360
Net change in cash, cash equivalents, and restricted cash	11	21
Cash, cash equivalents, and restricted cash at beginning of year	107	68
Cash, cash equivalents, and restricted cash at end of period	118	89
Noncash financing activity - Issuance of common stock for stock-based compensation	54	35
Union Electric Company
Cash Flows From Operating Activities:
Net income	40	39
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	138	132
Amortization of nuclear fuel	23	24
Amortization of debt issuance costs and premium/discounts	2	2
Deferred income taxes and investment tax credits, net	(1)	(1)
Allowance for equity funds used during construction	(4)	(4)
Other	2	4
Changes in assets and liabilities:
Receivables	56	4
Inventories	29	5
Accounts and wages payable	(167)	(192)
Taxes accrued	44	36
Regulatory assets and liabilities	11	38
Assets, other	(16)	(5)
Liabilities, other	(4)	(10)
Pension and other postretirement benefits	(1)	4
Net cash provided by operating activities	152	76
Cash Flows From Investing Activities:
Capital expenditures	(240)	(249)
Nuclear fuel expenditures	(21)	(12)
Purchases of securities - nuclear decommissioning trust fund	(39)	(38)
Sales and maturities of securities - nuclear decommissioning trust fund	36	34
Net cash used in investing activities	(264)	(265)
Cash Flows From Financing Activities:
Dividends on common stock	0	(50)
Dividends on preferred stock	(1)	(1)
Short-term debt, net	0	243
Repayments of Long-term Debt	(329)	0
Proceeds from Issuance of Long-term Debt	450	0
Payment of Financing and Stock Issuance Costs	(4)	0
Net cash provided by financing activities	116	192
Net change in cash, cash equivalents, and restricted cash	4	3
Cash, cash equivalents, and restricted cash at beginning of year	8	7
Cash, cash equivalents, and restricted cash at end of period	12	10
Ameren Illinois Company
Cash Flows From Operating Activities:
Net income	121	96
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	100	90
Amortization of debt issuance costs and premium/discounts	3	3
Deferred income taxes and investment tax credits, net	24	9
Allowance for equity funds used during construction	(2)	(1)
Other	(1)	(2)
Changes in assets and liabilities:
Receivables	(56)	(34)
Inventories	52	63
Accounts and wages payable	(34)	(52)
Taxes accrued	12	1
Regulatory assets and liabilities	18	(16)
Assets, other	3	2
Liabilities, other	(12)	(36)
Pension and other postretirement benefits	(3)	6
Net cash provided by operating activities	227	130
Cash Flows From Investing Activities:
Capital expenditures	(267)	(300)
Net cash used in investing activities	(267)	(300)
Cash Flows From Financing Activities:
Dividends on preferred stock	(1)	(1)
Short-term debt, net	54	162
Capital contribution from parent	0	20
Net cash provided by financing activities	53	181
Net change in cash, cash equivalents, and restricted cash	13	11
Cash, cash equivalents, and restricted cash at beginning of year	80	41
Cash, cash equivalents, and restricted cash at end of period	$ 93	$ 52

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			17
Stock-based compensation activity			(11)
Capital contribution from parent														$ 20		20
Net income	$ 153								$ 39				39	96				96
Net Income (Loss) Available to Common Stockholders, Basic	151			151
Common stock dividends				(111)									(50)
Preferred stock dividends													(1)					(1)
Stockholders' Equity, Other				1
Change in deferred retirement benefit costs	(1)				1
Net income attributable to noncontrolling interest holder	$ 2						2
Dividends paid to noncontrolling interest holders							(2)
Beginning of year (shares) at Dec. 31, 2017	242.6
Stock Issued During Period, Shares, New Issues	0.3
Stock Issued During Period, Shares, Other	0.7
End of year (shares) at Mar. 31, 2018	243.6
End of year at Mar. 31, 2018	$ 7,372	$ 2	5,546	1,699	(17)	$ (17)	142			$ 511	$ 1,858	$ 80	1,620		$ 0	2,033	$ 62	1,330
Dividends per Common Share (in dollars per share)	$ 0.4575
Stockholders' equity, end of year at Mar. 31, 2018								$ 7,230	4,069					3,425
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			19
Stock-based compensation activity			(21)
Capital contribution from parent														0		0
Net income	193								$ 40				40	$ 121				121
Net Income (Loss) Available to Common Stockholders, Basic	191			191
Common stock dividends				(116)									0
Preferred stock dividends													(1)					(1)
Stockholders' Equity, Other				0
Change in deferred retirement benefit costs	(1)				1
Net income attributable to noncontrolling interest holder	$ 2						2
Dividends paid to noncontrolling interest holders							(2)
Beginning of year (shares) at Dec. 31, 2018	244.5								102.1					25.5
Stock Issued During Period, Shares, New Issues	0.3
Stock Issued During Period, Shares, Other	0.8
End of year (shares) at Mar. 31, 2019	245.6								102.1					25.5
End of year at Mar. 31, 2019	$ 7,847	$ 2	$ 5,625	$ 2,099	$ (21)	$ (21)	$ 142			$ 511	$ 1,903	$ 80	$ 1,774		$ 0	$ 2,173	$ 62	$ 1,659
Dividends per Common Share (in dollars per share)	$ 0.4750
Stockholders' equity, end of year at Mar. 31, 2019	$ 7,705							$ 7,705	$ 4,268					$ 3,894

Summary Of Significant Accounting Policies

3 Months Ended

Mar. 31, 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 two additional MISO-approved electric transmission projects, the Illinois Rivers and Mark Twain 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 March 31, 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 March 31, 2019, the maximum exposure to loss related to these variable interest entities was approximately $16 million, which represents 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 March 31, 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 March 31, 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 $15 million.

Company-owned Life Insurance

Ameren and Ameren Illinois have company-owned life insurance, which is recorded at the net cash surrender value. The net cash surrender value is the amount that can be realized under the insurance policies at the balance sheet date. As of March 31, 2019, the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $258 million (December 31, 2018 – $244 million) and $123 million (December 31, 2018 – $122 million), respectively, while total borrowings against the policies were $113 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	3 Months Ended
Rate And Regulatory Matters	Mar. 31, 2019
Public Utilities, General Disclosures [Abstract]
RATE AND REGULATORY MATTERS	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 Wind Generation Facilities and RESRAM In May 2019, Ameren Missouri entered into a build-transfer agreement with a subsidiary of Enel Green Power North America, Inc. to acquire, after construction, an up-to 300-megawatt wind generation facility to be located in northwestern Missouri. Ameren Missouri expects to file for a certificate of convenience and necessity with the MoPSC in May 2019. Final RTO interconnection costs are expected to be determined in June 2019, and a related RTO transmission interconnection agreement is expected in the fall of 2019. In 2018, Ameren Missouri entered into two build-transfer agreements to acquire, after construction, an up-to 400-megawatt wind generation facility and an up-to 157-megawatt wind generation facility. In October 2018, the MoPSC issued an order approving a unanimous stipulation and agreement regarding a requested certificate of convenience and necessity for the up-to 400-megawatt facility. In March 2019, the MoPSC issued an order approving a nonunanimous stipulation and agreement regarding a requested certificate of convenience and necessity for the up-to 157-megawatt facility. Final MISO interconnection costs for the up-to 400-megawatt facility are expected to be determined in June 2019, and a related transmission interconnection agreement with the MISO is expected in the fall of 2019. Final MISO interconnection costs for the up-to 157-megawatt facility are expected to be determined in the fall of 2019, and a related transmission interconnection agreement with the MISO is expected in early 2020. All three facilities are expected to be completed by the end of 2020, which would support Ameren Missouri’s compliance with the Missouri renewable energy standard. Each acquisition is subject to certain conditions, including the issuance of a certificate of convenience and necessity by the MoPSC for the up-to 300-megawatt facility, obtaining FERC approval for the up-to 157-megawatt facility and the up-to 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 three build-transfer agreements collectively represent approximately $1.4 billion of capital expenditures expected in 2020. 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 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 gains of $20 million and $5 million during the three months ended March 31, 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 by approximately $4 million, prior to the interim rate reduction discussed below. The natural gas delivery service rate increase request is based on a 10.30% return on equity, a capital structure composed of 51.84% common equity, a rate base of $259 million, and a test year ended June 30, 2018, with certain pro-forma adjustments through the anticipated true-up date of May 31, 2019. In December 2018, the MoPSC issued an order approving a stipulation and agreement for an interim rate reduction of $2 million to reflect cost of service updates including the reduction in the federal corporate income tax rate and the amortization of excess deferred taxes as a result of the TCJA. As a result of the interim rate reduction, Ameren Missouri's requested annual revenues reflect a $6 million increase from interim rates. The interim rate reduction became effective January 2, 2019, and will continue until new rates are approved by the MoPSC in this regulatory rate review. In April 2019, the MoPSC staff recommended a $1 million increase to interim rates, based on a 9.5% return on equity and a 50% ceiling on the common equity ratio. 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. An ICC decision in this proceeding is expected by December 2019. As of March 31, 2019, Ameren Illinois had recorded a regulatory asset of $41 million to reflect the difference between Ameren Illinois’ estimate of its 2019 revenue requirement and the revenue requirement reflected in customer rates, including interest. ATXI’s Illinois Rivers Project In August 2017, the Illinois Circuit Court for Edgar County dismissed several of ATXI’s condemnation cases related to one line segment 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 December 2018, the Illinois Supreme Court issued an order to stay its October 2018 ruling. 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 intends to seek to reinstate 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 $38 million was invested as of March 31, 2019. The other eight line segments of the Illinois Rivers project have already been completed and placed in-service and are not affected by these proceedings. 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 and 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, with comments due in June 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 March 31, 2019, Ameren and Ameren Illinois had recorded current regulatory liabilities of $44 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

3 Months Ended

Mar. 31, 2019

Debt Disclosure [Abstract]

SHORT-TERM DEBT AND LIQUIDITY

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

The Missouri Credit Agreement and the Illinois Credit Agreement were not utilized for direct borrowings during the three months ended March 31, 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 March 31, 2019, was $1.3 billion. The Ameren Companies were in compliance with the covenants in their Credit Agreements as of March 31, 2019. As of March 31, 2019, the ratios of consolidated indebtedness to consolidated total capitalization, calculated in accordance with the provisions of the Credit Agreements, were 54%, 48%, and 47% for Ameren, Ameren Missouri, and Ameren Illinois, respectively.

Commercial Paper

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



	March 31, 2019		December 31, 2018
Ameren (parent)	$	618	$	470
Ameren Missouri	55		55
Ameren Illinois	126		72
Ameren consolidated	$	799	$	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 three months ended March 31, 2019 and 2018:



	Ameren (parent)			Ameren Missouri			Ameren Illinois			Ameren Consolidated
2019
Average daily commercial paper outstanding at par value	$	480		$	246		$	89		$	815
Weighted-average interest rate	2.87		%	2.84		%	2.76		%	2.85		%
Peak commercial paper during period at par value^(a)	$	618		$	549		$	130		$	1,113
Peak interest rate	3.10		%	2.97		%	2.90		%	3.10		%
2018
Average daily commercial paper outstanding at par value	$	378		$	213		$	137		$	728
Weighted-average interest rate	1.90		%	1.88		%	1.96		%	1.90		%
Peak commercial paper during period at par value^(a)	$	454		$	282		$	238		$	960
Peak interest rate	2.35		%	2.40		%	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 months ended March 31, 2019 and 2018, was 2.87% and 1.90%, 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 months ended March 31, 2019 and 2018.

Long-Term Debt And Equity Financings	3 Months Ended
Long-Term Debt And Equity Financings	Mar. 31, 2019
Long-Term Debt And Equity Financings [Abstract]
LONG-TERM DEBT AND EQUITY FINANCINGS	LONG-TERM DEBT AND EQUITY FINANCINGS Ameren For the three months ended March 31, 2019, Ameren issued a total of 0.3 million shares of common stock under its DRPlus and 401(k) plan, and received proceeds of $19 million. 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. 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 March 31, 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 March 31, 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

3 Months Ended

Mar. 31, 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 months ended March 31, 2019 and 2018:



	Three Months
	2019	2018
Ameren:



	Three Months
	2019			2018
Allowance for equity funds used during construction	$	6		$	5
Interest income on industrial development revenue bonds	6			6
Other interest income	2			2
Non-service cost components of net periodic benefit income^(a)	22			16
Miscellaneous income	2			1
Donations	(6		)	(5		)
Miscellaneous expense	(3		)	(2		)
Total Other Income, Net	$	29		$	23
Ameren Missouri:
Allowance for equity funds used during construction	$	4		$	4
Interest income on industrial development revenue bonds	6			6
Non-service cost components of net periodic benefit income^(a)	4			5
Miscellaneous income	1			1
Donations	(2		)	(1		)
Miscellaneous expense	(1		)	(2		)
Total Other Income, Net	$	12		$	13
Ameren Illinois:
Allowance for equity funds used during construction	$	2		$	1
Interest income	2			2
Non-service cost components of net periodic benefit income	12			7
Miscellaneous income	1			—
Donations	(4		)	(4		)
Miscellaneous expense	(2		)	—
Total Other Income, Net	$	11		$	6


(a)	For the three months ended March 31, 2019 and 2018, the non-service cost components of net periodic benefit income were partially offset by a $7 million and $4 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.

Derivative Financial Instruments

3 Months Ended

Mar. 31, 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 March 31, 2019, and December 31, 2018. As of March 31, 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)	65	—	65	66	—	66
Natural gas (in mmbtu)	18	156	174	19	154	173
Power (in megawatthours)	2	8	10	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 March 31, 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 March 31, 2019, and December 31, 2018:



			March 31, 2019					December 31, 2018
	Balance Sheet Location		Ameren Missouri		Ameren Illinois		Ameren			Ameren Missouri		Ameren Illinois		Ameren
Fuel oils	Other current assets	$	5	$	—	$	5		$	3	$	—	$	3
	Other assets		4		—		4			5		—		5
Natural gas	Other current assets		—		3		3			—		1		1
	Other assets		—		3		3			—		2		2
Power	Other current assets		1		—		1			4		—		4
	Other assets		1		—		1			—		—		—
	Total assets	$	11	$	6	$	17		$	12	$	3	$	15
Fuel oils	Other current liabilities	$	3	$	—	$	3		$	4	$	—	$	4
	Other deferred credits and liabilities		4		—		4			9		—		9
Natural gas	Other current liabilities		2		6		8			4		8		12
	Other deferred credits and liabilities		1		4		5			1		6		7
Power	Other current liabilities		2		14		16			4		14		18
	Other deferred credits and liabilities		—		170		170			—		169		169
	Total liabilities	$	12	$	194	$	206		$	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 March 31, 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 March 31, 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 March 31, 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 March 31, 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	$	64	$	4	$	58
Ameren Illinois	39		—		29
Ameren	$	103	$	4	$	87


(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

3 Months Ended

Mar. 31, 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 months ended March 31, 2019 or 2018. At March 31, 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 March 31, 2019, and December 31, 2018:



		March 31, 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	$	—	$	7	$	9		$	1	$	—	$	7	$	8
	Natural gas	—		3		3		6			—		2		1		3
	Power	—		—		2		2			—		1		3		4
	Total derivative assets – commodity contracts	$	2	$	3	$	12	$	17		$	1	$	3	$	11	$	15
	Nuclear decommissioning trust fund:
	Equity securities:
	U.S. large capitalization	$	488	$	—	$	—	$	488		$	427	$	—	$	—	$	427
	Debt securities:
	U.S. Treasury and agency securities	—		151		—		151			—		148		—		148
	Corporate bonds	—		76		—		76			—		72		—		72
	Other	—		33		—		33			—		32		—		32
	Total nuclear decommissioning trust fund	$	488	$	260	$	—	$	748	^(b)	$	427	$	252	$	—	$	679	^(b)
	Total Ameren	$	490	$	263	$	12	$	765		$	428	$	255	$	11	$	694
Ameren Missouri
	Derivative assets – commodity contracts^(a):
	Fuel oils	$	2	$	—	$	7	$	9		$	1	$	—	$	7	$	8
	Power	—		—		2		2			—		1		3		4
	Total derivative assets – commodity contracts	$	2	$	—	$	9	$	11		$	1	$	1	$	10	$	12
	Nuclear decommissioning trust fund:
	Equity securities:
	U.S. large capitalization	$	488	$	—	$	—	$	488		$	427	$	—	$	—	$	427
	Debt securities:
	U.S. Treasury and agency securities	—		151		—		151			—		148		—		148
	Corporate bonds	—		76		—		76			—		72		—		72
	Other	—		33		—		33			—		32		—		32
	Total nuclear decommissioning trust fund	$	488	$	260	$	—	$	748	^(b)	$	427	$	252	$	—	$	679	^(b)
	Total Ameren Missouri	$	490	$	260	$	9	$	759		$	428	$	253	$	10	$	691
Ameren Illinois
	Derivative assets – commodity contracts^(a):
	Natural gas	$	—	$	3	$	3	$	6		$	—	$	2	$	1	$	3
Liabilities:
Ameren
	Derivative liabilities – commodity contracts^(a):
	Fuel oils	$	1	$	—	$	6	$	7		$	2	$	—	$	11	$	13
	Natural gas	—		10		3		13			—		15		4		19
	Power	—		—		186		186			—		1		186		187
	Total Ameren	$	1	$	10	$	195	$	206		$	2	$	16	$	201	$	219
Ameren Missouri
	Derivative liabilities – commodity contracts^(a):
	Fuel oils	$	1	$	—	$	6	$	7		$	2	$	—	$	11	$	13
	Natural gas	—		3		—		3			—		5		—		5
	Power	—		—		2		2			—		1		3		4
	Total Ameren Missouri	$	1	$	3	$	8	$	12		$	2	$	6	$	14	$	22
Ameren Illinois
	Derivative liabilities – commodity contracts^(a):
	Natural gas	$	—	$	7	$	3	$	10		$	—	$	10	$	4	$	14
	Power	—		—		184		184			—		—		183		183
	Total Ameren Illinois	$	—	$	7	$	187	$	194		$	—	$	10	$	187	$	197


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


(b)	Balance excludes $6 million and $5 million of cash and cash equivalents, receivables, payables, and accrued income, net, for March 31, 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 months ended March 31, 2019 and 2018:



	2019								2018
	Ameren Missouri		Ameren Illinois			Ameren			Ameren Missouri			Ameren Illinois			Ameren
Beginning balance at January 1	$	—	$	(183	)	$	(183	)	$	7		$	(195	)	$	(188	)
Realized and unrealized losses included in regulatory assets/liabilities	—		(4		)	(4		)	(2		)	1			(1		)
Settlements	—		3			3			(1		)	3			2
Ending balance at March 31	$	—	$	(184	)	$	(184	)	$	4		$	(191	)	$	(187	)
Change in unrealized losses related to assets/liabilities held at March 31	$	—	$	(4	)	$	(4	)	$	(1	)	$	1		$	—

For the three months ended March 31, 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 March 31, 2019, and December 31, 2018:



			Fair Value^(a)						Weighted Average
	Commodity		Assets		Liabilities	Valuation Technique(s)	Unobservable Input	Range	Weighted Average
2019	Power^(b)	$	2	$	(186)	Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps ($/MWh)^(c)	24 – 38	28
							Nodal basis ($/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 March 31, 2019, and December 31, 2018:



	March 31, 2019
	Carrying Amount			Fair Value
	Carrying Amount			Level 1		Level 2		Level 3			Total
Ameren:
Cash, cash equivalents, and restricted cash	$	118		$	118	$	—	$	—		$	118
Investments in held-to-maturity debt securities^(a)	270			—		270		—			270
Short-term debt	799			—		799		—			799
Long-term debt (including current portion)^(a)	8,557		^(b)	—		8,623		447		^(c)	9,070
Ameren Missouri:
Cash, cash equivalents, and restricted cash	$	12		$	12	$	—	$	—		$	12
Investments in held-to-maturity debt securities^(a)	270			—		270		—			270
Short-term debt	55			—		55		—			55
Long-term debt (including current portion)^(a)	4,116		^(b)	—		4,404		—			4,404
Ameren Illinois:
Cash, cash equivalents, and restricted cash	$	93		$	93	$	—	$	—		$	93
Short-term debt	126			—		126		—			126
Long-term debt (including current portion)	3,296		^(b)	—		3,515		—			3,515
	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 March 31, 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)

Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $62 million, $25 million, and $32 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of March 31, 2019. Included 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

3 Months Ended

Mar. 31, 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 months ended March 31, 2019 and 2018:



				Three Months
Agreement	Income Statement Line Item			Ameren Missouri		Ameren Illinois
Ameren Missouri power supply	Operating Revenues	2019	$	(b)	$	(a)
agreements with Ameren Illinois		2018		3		(a)
Ameren Missouri and Ameren Illinois	Operating Revenues	2019		7		1
rent and facility services		2018		5		1
Ameren Missouri and Ameren Illinois	Operating Revenues	2019		(b)		(b)
miscellaneous support services		2018		(b)		(b)
Total Operating Revenues		2019	$	7	$	1
		2018		8		1
Ameren Illinois power supply	Purchased Power	2019	$	(a)	$	(b)
agreements with Ameren Missouri		2018		(a)		3
Ameren Illinois transmission	Purchased Power	2019		(a)		(b)
services with ATXI		2018		(a)		(b)
Total Purchased Power		2019	$	(a)	$	(b)
		2018		(a)		3
Ameren Missouri and Ameren Illinois	Other Operations and Maintenance	2019	$	(b)	$	1
rent and facility services		2018		(b)		2
Ameren Services support services	Other Operations and Maintenance	2019		32		30
agreement		2018		33		30
Total Other Operations and Maintenance		2019	$	32	$	31
		2018		33		32
Money pool borrowings (advances)	Interest Charges/ Other Income, Net	2019	$	—	$	—
		2018		(b)		(b)


(a)	Not applicable.


(b)	Amount less than $1 million.

Commitments And Contingencies

3 Months Ended

Mar. 31, 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 March 31, 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 March 31, 2019.



	Coal		Natural Gas^(a)		Nuclear Fuel		Purchased Power^(b)(c)			Methane Gas		Other		Total
Ameren:
2019	$	279	$	154	$	25	$	88	^(d)	$	2	$	52	$	600
2020	160		158		43		54			3		42		460
2021	121		85		58		10			3		30		307
2022	72		32		13		—			3		26		146
2023	—		8		41		—			3		29		81
Thereafter	—		34		29		—			27		71		161
Total	$	632	$	471	$	209	$	152		$	41	$	250	$	1,755
Ameren Missouri:
2019	$	279	$	33	$	25	$	—		$	2	$	39	$	378
2020	160		34		43		—			3		29		269
2021	121		16		58		—			3		25		223
2022	72		5		13		—			3		26		119
2023	—		5		41		—			3		29		78
Thereafter	—		13		29		—			27		55		124
Total	$	632	$	106	$	209	$	—		$	41	$	203	$	1,191
Ameren Illinois:
2019	$	—	$	121	$	—	$	88	^(d)	$	—	$	5	$	214
2020	—		124		—		54			—		4		182
2021	—		69		—		10			—		—		79
2022	—		27		—		—			—		—		27
2023	—		3		—		—			—		—		3
Thereafter	—		21		—		—			—		—		21
Total	$	—	$	365	$	—	$	152		$	—	$	9	$	526


(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.


(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 $10 million of zero emission credits through May 2019.

In April 2019, Ameren Illinois conducted procurement events, administered by the IPA, to purchase energy products through May 2022. In the April 2019 procurement event, Ameren Illinois contracted to purchase 3,704,000 megawatthours of energy products for $108 million from June 2019 through May 2022. See Note 8 – Related-party Transactions for additional information regarding energy product agreements between Ameren Missouri and Ameren Illinois as a result of the April procurement event.

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, pending various legal challenges. In August 2018, the EPA proposed to repeal and replace the Clean Power Plan with a proposed new rule known as the Affordable Clean Energy Rule, which establishes emission guidelines for states to follow in developing plans to limit CO₂ emissions from power plants. The EPA proposes to use certain efficiency measures as the best system of emission reduction for coal-fired power plants. The EPA is expected to finalize the Affordable Clean Energy rule in the first half of 2019. We cannot predict the outcome of EPA’s rulemaking or the outcome of legal challenges related to such rulemaking.

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 two phases: liability and remedy. In the first phase, in January 2017, the district court issued a liability ruling that the projects violated provisions of the Clean Air Act and Missouri law. In the second phase, the district court will determine the actions required to remedy the violations found in the liability phase. The EPA previously withdrew all claims for penalties and fines. A trial on the scope of the appropriate remedy occurred in April 2019, and a final ruling from the court is expected by the end of 2019. Once the court issues its final order and judgment, Ameren Missouri will seek a stay of that order while it seeks an appeal of 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. The rule will be 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. Ameren Missouri is in the process of constructing wastewater treatment facilities that meet the limitations in these guidelines at three of its energy centers.

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 and anticipates the last of such CCR storage facilities to be closed by 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 $131 million recorded on their respective balance sheets as of March 31, 2019, associated with CCR storage facilities that reflect the regulations issued in 2015. 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 by federal or state governments as a potentially responsible party at several contaminated sites.

As of March 31, 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 March 31, 2019, Ameren Illinois estimated the obligation related to these former MGP sites at $145 million to $215 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

3 Months Ended

Mar. 31, 2019

Nuclear Waste Matters [Abstract]

CALLAWAY ENERGY CENTER

Decommissioning

Electric rates charged to customers provide for the recovery of the Callaway energy center’s decommissioning costs, which include decontamination, dismantling, and site restoration costs, over the expected life of the nuclear energy center. Amounts collected from customers are deposited into the external nuclear decommissioning trust fund to provide for the Callaway energy center’s decommissioning. It is assumed that the Callaway energy center site will be decommissioned through the immediate dismantlement method and removed from service. Ameren and Ameren Missouri have recorded an ARO for the Callaway energy center decommissioning costs at fair value, which represents the present value of estimated future cash outflows. Annual decommissioning costs of $7 million are included in the costs used to establish electric rates for Ameren Missouri’s customers.

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. If the assumed return on trust assets is not earned, Ameren Missouri believes that it is probable that any such earnings deficiency will be recovered in rates.

Insurance

The following table presents insurance coverage at Ameren Missouri’s Callaway energy center as of April 1, 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	13,623		^(a)	138		^(b)
	$	14,073	^(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

3 Months Ended

Mar. 31, 2019

Defined Benefit Plan [Abstract]

RETIREMENT BENEFITS

The following table presents the components of the net periodic benefit cost (income) incurred for Ameren’s pension and postretirement benefit plans for the three months ended March 31, 2019 and 2018:



	Pension Benefits						Postretirement Benefits
	Three Months						Three Months
	2019			2018			2019			2018
Service cost^(a)	$	22		$	25		$	4		$	5
Non-service cost components:
Interest cost	47			42			11			11
Expected return on plan assets	(69		)	(69		)	(19		)	(19		)
Amortization of:
Prior service benefit	—			—			(1		)	(1		)
Actuarial loss (gain)	6			16			(4		)	—
Total non-service cost components^(b)	$	(16	)	$	(11	)	$	(13	)	$	(9	)
Net periodic benefit cost (income)	$	6		$	14		$	(9	)	$	(4	)


(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 months ended March 31, 2019 and 2018:



	Pension Benefits				Postretirement Benefits
	Three Months				Three Months
	2019		2018		2019			2018
Ameren Missouri^(a)	$	1	$	5	$	(2	)	$	—
Ameren Illinois	5		9		(7		)	(4		)
Ameren^(a)	$	6	$	14	$	(9	)	$	(4	)


(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

3 Months Ended

Mar. 31, 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 months ended March 31, 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	(7)	(2)	^(a)	(12)	—	^(a)	(3)	(3)
Other depreciation differences	—	—		—	1		—	—
Amortization of deferred investment tax credit	(1)	(1)		(1)	(1)		—	—
State tax	6	7		4	4		7	7
Stock-based compensation	(7)	(3)		—	—		—	—
Other	—	—		(3)	—		—	—
Effective income tax rate	12%	22%		9%	25%		25%	25%


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

Supplemental Information

3 Months Ended

Mar. 31, 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 March 31, 2019, and December 31, 2018:



	March 31, 2019						December 31, 2018
	Ameren		Ameren Missouri		Ameren Illinois		Ameren		Ameren Missouri		Ameren Illinois
Cash and cash equivalents	$	8	$	—	$	—	$	16	$	—	$	—
Restricted cash included in “Other current assets”	16		5		6		13		4		6
Restricted cash included in “Other assets”	87		—		87		74		—		74
Restricted cash included in “Nuclear decommissioning trust fund”	7		7		—		4		4		—
Total cash, cash equivalents, and restricted cash	$	118	$	12	$	93	$	107	$	8	$	80

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

Accounts Receivable

“Accounts receivable – trade” on Ameren’s and Ameren Illinois’ balance sheets include certain receivables purchased at a discount from alternative retail electric suppliers that elect to participate in the utility consolidated billing program. At March 31, 2019, and December 31, 2018, “Other current liabilities” on Ameren’s and Ameren Illinois’ balance sheets included payables for purchased receivables of $38 million and $33 million, respectively.

For the three months ended March 31, 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 months ended March 31, 2019 and 2018.

The following table provides supplemental balance sheet information related to operating leases as of March 31, 2019:



	Ameren			Ameren Missouri
Other assets	$	36		$	34
Other current liabilities	7			6
Other deferred credits and liabilities	29			28
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 March 31, 2019:



	Ameren		Ameren Missouri
2019	$	6	$	5
2020	7		7
2021	7		6
2022	6		5
2023	5		5
Thereafter	10		10
Total lease payments	41		38
Less imputed interest	5		4
Total^(a)	$	36	$	34

(a)

The amount of remaining maturities of operating lease liabilities under previous authoritative accounting guidance as of December 31, 2018, is materially consistent to the amount as of March 31, 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 three months ended March 31, 2019 and 2018:



	March 31, 2019						March 31, 2018
	Ameren		Ameren Missouri		Ameren Illinois		Ameren			Ameren Missouri			Ameren Illinois
Accrued capital expenditures	$	208	$	92	$	106	$	202		$	73		$	114
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	64		64		—		(11		)	(11		)	—

Asset Retirement Obligations

The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the three months ended March 31, 2019:



	Ameren Missouri				Ameren Illinois^(a)			Ameren
Balance at December 31, 2018	$	646		^(a)	$	4	^(b)	$	650		^(a)
Liabilities settled	(3		)		—			(3		)
Accretion	6			^(c)	—			6			^(c)
Balance at March 31, 2019	$	649		^(a)	$	4	^(b)	$	653		^(a)


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

Stock-based Compensation

The following table summarizes Ameren's nonvested performance share unit and restricted stock unit activity for the three months ended March 31, 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	288,956		67.42		^(b)	124,925		65.23
Forfeitures	(2,228	)	65.06			(833	)	63.16
Vested and undistributed^(c)	(65,458	)	62.04			(14,047	)	61.77
Vested and distributed	(176,923	)	44.13			—		—
Nonvested at March 31, 2019^(d)	727,158		$	63.40		265,298		$	60.64


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


(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 333,466 vested and undistributed performance share units and 40,604 vested and undistributed restricted stock units.

For the three months ended March 31, 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 March 31, 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 March 31, 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 months ended March 31, 2019 and 2018:



	Three Months
	2019		2018
Ameren Missouri	$	31	$	34
Ameren Illinois	39		35		^(a)
Ameren	$	70	$	69	^(a)


(a)	Amounts have been adjusted from those previously reported to reflect additional excise taxes for the three months ended March 31, 2018.

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 1.5 million for both the three months ended March 31, 2019 and 2018. There were no potentially dilutive securities excluded from the earnings per diluted share calculations for the three months ended March 31, 2019 and 2018.

Segment Information

3 Months Ended

Mar. 31, 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 months ended March 31, 2019 and 2018. Ameren, Ameren Missouri, and Ameren Illinois management review segment capital expenditure information rather than any individual or total asset amount.

Ameren



Three Months	Ameren Missouri		Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Transmission			Other		Intersegment Eliminations			Ameren
2019
External revenues	$	751	$	386	$	320	$	99		$	—	$	—		$	1,556
Intersegment revenues	7		1		—		15			—		(23		)	—
Net income attributable to Ameren common shareholders	39		36		57		44		^(a)	15		—			191
Capital expenditures	240		124		51		121			10		(2		)	544
2018
External revenues	$	784	$	399	$	311	$	91		$	—	$	—		$	1,585
Intersegment revenues	8		1		—		13			—		(22		)	—
Net income attributable to Ameren common shareholders	38		33		42		37		^(a)	1		—			151
Capital expenditures	249		122		60		145			7		(4		)	579


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

Ameren Illinois



Three Months	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Illinois Transmission		Intersegment Eliminations			Ameren Illinois
2019
External revenues	$	387	$	320	$	55	$	—		$	762
Intersegment revenues	—		—		15		(15		)	—
Net income available to common shareholder	36		57		27		—			120
Capital expenditures	124		51		92		—			267
2018
External revenues	$	400	$	311	$	49	$	—		$	760
Intersegment revenues	—		—		13		(13		)	—
Net income available to common shareholder	33		42		20		—			95
Capital expenditures	122		60		118		—			300

The following tables present disaggregated revenues by segment at Ameren and Ameren Illinois for the three months ended March 31, 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



Three Months	Ameren Missouri			Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas			Ameren Transmission		Other		Intersegment Eliminations			Ameren
2019
Residential	$	312		$	217	$	—		$	—	$	—	$	—		$	529
Commercial	239			123		—			—		—		—			362
Industrial	55			34		—			—		—		—			89
Other	98			13		—			114		—		(23		)	202
Total electric revenues	$	704		$	387	$	—		$	114	$	—	$	(23	)	$	1,182
Residential	$	38		$	—	$	246		$	—	$	—	$	—		$	284
Commercial	16			—		65			—		—		—			81
Industrial	2			—		4			—		—		—			6
Other	(2		)	—		5			—		—		—			3
Total gas revenues	$	54		$	—	$	320		$	—	$	—	$	—		$	374
Total revenues^(a)	$	758		$	387	$	320		$	114	$	—	$	(23	)	$	1,556
2018
Residential	$	332		$	219	$	—		$	—	$	—	$	—		$	551
Commercial	252			124		—			—		—		—			376
Industrial	61			35		—			—		—		—			96
Other	96			22		—			104		—		(22		)	200
Total electric revenues	$	741		$	400	$	—		$	104	$	—	$	(22	)	$	1,223
Residential	$	41		$	—	$	243		$	—	$	—	$	—		$	284
Commercial	16			—		67			—		—		—			83
Industrial	2			—		6			—		—		—			8
Other	(8		)	—		(5		)	—		—		—			(13		)
Total gas revenues	$	51		$	—	$	311		$	—	$	—	$	—		$	362
Total revenues^(a)	$	792		$	400	$	311		$	104	$	—	$	(22	)	$	1,585


(a)	The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the three months ended March 31, 2019 and 2018:



Three Months	Ameren Missouri			Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas			Ameren Transmission			Ameren
2019
Revenues from alternative revenue programs	$	15		$	22	$	(3	)	$	(5	)	$	29
Other revenues not from contracts with customers	5			3		1			—			9
2018
Revenues from alternative revenue programs	$	(4	)	$	31	$	(3	)	$	(4	)	$	20
Other revenues not from contracts with customers	14			10		1			—			25

Ameren Illinois



Three Months	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas			Ameren Illinois Transmission		Intersegment Eliminations			Total Ameren Illinois
2019
Residential	$	217	$	246		$	—	$	—		$	463
Commercial	123		65			—		—			188
Industrial	34		4			—		—			38
Other	13		5			70		(15		)	73
Total revenues^(a)	$	387	$	320		$	70	$	(15	)	$	762
2018
Residential	$	219	$	243		$	—	$	—		$	462
Commercial	124		67			—		—			191
Industrial	35		6			—		—			41
Other	22		(5		)	62		(13		)	66
Total revenues^(a)	$	400	$	311		$	62	$	(13	)	$	760


(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 months ended March 31, 2019 and 2018:



Three Months	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas			Ameren Illinois Transmission			Ameren Illinois
2019
Revenues from alternative revenue programs	$	22	$	(3	)	$	(5	)	$	14
Other revenues not from contracts with customers	3		1			—			4
2018
Revenues from alternative revenue programs	$	31	$	(3	)	$	(4	)	$	24
Other revenues not from contracts with customers	10		1			—			11

Summary Of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 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 two additional MISO-approved electric transmission projects, the Illinois Rivers and Mark Twain 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 March 31, 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 March 31, 2019, the maximum exposure to loss related to these variable interest entities was approximately $16 million, which represents 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 March 31, 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 March 31, 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 $15 million.

Company-owned Life Insurance

Ameren and Ameren Illinois have company-owned life insurance, which is recorded at the net cash surrender value. The net cash surrender value is the amount that can be realized under the insurance policies at the balance sheet date. As of March 31, 2019, the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $258 million (December 31, 2018 – $244 million) and $123 million (December 31, 2018 – $122 million), respectively, while total borrowings against the policies were $113 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.

Share-based Compensation, Option and Incentive Plans Policy

Stock-based Compensation

The following table summarizes Ameren's nonvested performance share unit and restricted stock unit activity for the three months ended March 31, 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	288,956		67.42		^(b)	124,925		65.23
Forfeitures	(2,228	)	65.06			(833	)	63.16
Vested and undistributed^(c)	(65,458	)	62.04			(14,047	)	61.77
Vested and distributed	(176,923	)	44.13			—		—
Nonvested at March 31, 2019^(d)	727,158		$	63.40		265,298		$	60.64


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


(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 333,466 vested and undistributed performance share units and 40,604 vested and undistributed restricted stock units.

Deferred Compensation

As of March 31, 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 March 31, 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.

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

3 Months Ended

Mar. 31, 2019

Debt Disclosure [Abstract]

Schedule of Short-term Debt

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



	March 31, 2019		December 31, 2018
Ameren (parent)	$	618	$	470
Ameren Missouri	55		55
Ameren Illinois	126		72
Ameren consolidated	$	799	$	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 three months ended March 31, 2019 and 2018:



	Ameren (parent)			Ameren Missouri			Ameren Illinois			Ameren Consolidated
2019
Average daily commercial paper outstanding at par value	$	480		$	246		$	89		$	815
Weighted-average interest rate	2.87		%	2.84		%	2.76		%	2.85		%
Peak commercial paper during period at par value^(a)	$	618		$	549		$	130		$	1,113
Peak interest rate	3.10		%	2.97		%	2.90		%	3.10		%
2018
Average daily commercial paper outstanding at par value	$	378		$	213		$	137		$	728
Weighted-average interest rate	1.90		%	1.88		%	1.96		%	1.90		%
Peak commercial paper during period at par value^(a)	$	454		$	282		$	238		$	960
Peak interest rate	2.35		%	2.40		%	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.

Other Income and Expenses (Tables)

3 Months Ended

Mar. 31, 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 months ended March 31, 2019 and 2018:



	Three Months
	2019	2018
Ameren:



	Three Months
	2019			2018
Allowance for equity funds used during construction	$	6		$	5
Interest income on industrial development revenue bonds	6			6
Other interest income	2			2
Non-service cost components of net periodic benefit income^(a)	22			16
Miscellaneous income	2			1
Donations	(6		)	(5		)
Miscellaneous expense	(3		)	(2		)
Total Other Income, Net	$	29		$	23
Ameren Missouri:
Allowance for equity funds used during construction	$	4		$	4
Interest income on industrial development revenue bonds	6			6
Non-service cost components of net periodic benefit income^(a)	4			5
Miscellaneous income	1			1
Donations	(2		)	(1		)
Miscellaneous expense	(1		)	(2		)
Total Other Income, Net	$	12		$	13
Ameren Illinois:
Allowance for equity funds used during construction	$	2		$	1
Interest income	2			2
Non-service cost components of net periodic benefit income	12			7
Miscellaneous income	1			—
Donations	(4		)	(4		)
Miscellaneous expense	(2		)	—
Total Other Income, Net	$	11		$	6


(a)	For the three months ended March 31, 2019 and 2018, the non-service cost components of net periodic benefit income were partially offset by a $7 million and $4 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.

Derivative Financial Instruments (Tables)

3 Months Ended

Mar. 31, 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 March 31, 2019, and December 31, 2018. As of March 31, 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)	65	—	65	66	—	66
Natural gas (in mmbtu)	18	156	174	19	154	173
Power (in megawatthours)	2	8	10	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 March 31, 2019, and December 31, 2018:



			March 31, 2019					December 31, 2018
	Balance Sheet Location		Ameren Missouri		Ameren Illinois		Ameren			Ameren Missouri		Ameren Illinois		Ameren
Fuel oils	Other current assets	$	5	$	—	$	5		$	3	$	—	$	3
	Other assets		4		—		4			5		—		5
Natural gas	Other current assets		—		3		3			—		1		1
	Other assets		—		3		3			—		2		2
Power	Other current assets		1		—		1			4		—		4
	Other assets		1		—		1			—		—		—
	Total assets	$	11	$	6	$	17		$	12	$	3	$	15
Fuel oils	Other current liabilities	$	3	$	—	$	3		$	4	$	—	$	4
	Other deferred credits and liabilities		4		—		4			9		—		9
Natural gas	Other current liabilities		2		6		8			4		8		12
	Other deferred credits and liabilities		1		4		5			1		6		7
Power	Other current liabilities		2		14		16			4		14		18
	Other deferred credits and liabilities		—		170		170			—		169		169
	Total liabilities	$	12	$	194	$	206		$	22	$	197	$	219

Derivative Instruments With Credit Risk-Related Contingent Features

The following table presents, as of March 31, 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 March 31, 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	$	64	$	4	$	58
Ameren Illinois	39		—		29
Ameren	$	103	$	4	$	87


(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)

3 Months Ended

Mar. 31, 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 March 31, 2019, and December 31, 2018:



		March 31, 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	$	—	$	7	$	9		$	1	$	—	$	7	$	8
	Natural gas	—		3		3		6			—		2		1		3
	Power	—		—		2		2			—		1		3		4
	Total derivative assets – commodity contracts	$	2	$	3	$	12	$	17		$	1	$	3	$	11	$	15
	Nuclear decommissioning trust fund:
	Equity securities:
	U.S. large capitalization	$	488	$	—	$	—	$	488		$	427	$	—	$	—	$	427
	Debt securities:
	U.S. Treasury and agency securities	—		151		—		151			—		148		—		148
	Corporate bonds	—		76		—		76			—		72		—		72
	Other	—		33		—		33			—		32		—		32
	Total nuclear decommissioning trust fund	$	488	$	260	$	—	$	748	^(b)	$	427	$	252	$	—	$	679	^(b)
	Total Ameren	$	490	$	263	$	12	$	765		$	428	$	255	$	11	$	694
Ameren Missouri
	Derivative assets – commodity contracts^(a):
	Fuel oils	$	2	$	—	$	7	$	9		$	1	$	—	$	7	$	8
	Power	—		—		2		2			—		1		3		4
	Total derivative assets – commodity contracts	$	2	$	—	$	9	$	11		$	1	$	1	$	10	$	12
	Nuclear decommissioning trust fund:
	Equity securities:
	U.S. large capitalization	$	488	$	—	$	—	$	488		$	427	$	—	$	—	$	427
	Debt securities:
	U.S. Treasury and agency securities	—		151		—		151			—		148		—		148
	Corporate bonds	—		76		—		76			—		72		—		72
	Other	—		33		—		33			—		32		—		32
	Total nuclear decommissioning trust fund	$	488	$	260	$	—	$	748	^(b)	$	427	$	252	$	—	$	679	^(b)
	Total Ameren Missouri	$	490	$	260	$	9	$	759		$	428	$	253	$	10	$	691
Ameren Illinois
	Derivative assets – commodity contracts^(a):
	Natural gas	$	—	$	3	$	3	$	6		$	—	$	2	$	1	$	3
Liabilities:
Ameren
	Derivative liabilities – commodity contracts^(a):
	Fuel oils	$	1	$	—	$	6	$	7		$	2	$	—	$	11	$	13
	Natural gas	—		10		3		13			—		15		4		19
	Power	—		—		186		186			—		1		186		187
	Total Ameren	$	1	$	10	$	195	$	206		$	2	$	16	$	201	$	219
Ameren Missouri
	Derivative liabilities – commodity contracts^(a):
	Fuel oils	$	1	$	—	$	6	$	7		$	2	$	—	$	11	$	13
	Natural gas	—		3		—		3			—		5		—		5
	Power	—		—		2		2			—		1		3		4
	Total Ameren Missouri	$	1	$	3	$	8	$	12		$	2	$	6	$	14	$	22
Ameren Illinois
	Derivative liabilities – commodity contracts^(a):
	Natural gas	$	—	$	7	$	3	$	10		$	—	$	10	$	4	$	14
	Power	—		—		184		184			—		—		183		183
	Total Ameren Illinois	$	—	$	7	$	187	$	194		$	—	$	10	$	187	$	197


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


(b)	Balance excludes $6 million and $5 million of cash and cash equivalents, receivables, payables, and accrued income, net, for March 31, 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 months ended March 31, 2019 and 2018:



	2019								2018
	Ameren Missouri		Ameren Illinois			Ameren			Ameren Missouri			Ameren Illinois			Ameren
Beginning balance at January 1	$	—	$	(183	)	$	(183	)	$	7		$	(195	)	$	(188	)
Realized and unrealized losses included in regulatory assets/liabilities	—		(4		)	(4		)	(2		)	1			(1		)
Settlements	—		3			3			(1		)	3			2
Ending balance at March 31	$	—	$	(184	)	$	(184	)	$	4		$	(191	)	$	(187	)
Change in unrealized losses related to assets/liabilities held at March 31	$	—	$	(4	)	$	(4	)	$	(1	)	$	1		$	—

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 March 31, 2019, and December 31, 2018:



			Fair Value^(a)						Weighted Average
	Commodity		Assets		Liabilities	Valuation Technique(s)	Unobservable Input	Range	Weighted Average
2019	Power^(b)	$	2	$	(186)	Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps ($/MWh)^(c)	24 – 38	28
							Nodal basis ($/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

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 March 31, 2019, and December 31, 2018:



	March 31, 2019
	Carrying Amount			Fair Value
	Carrying Amount			Level 1		Level 2		Level 3			Total
Ameren:
Cash, cash equivalents, and restricted cash	$	118		$	118	$	—	$	—		$	118
Investments in held-to-maturity debt securities^(a)	270			—		270		—			270
Short-term debt	799			—		799		—			799
Long-term debt (including current portion)^(a)	8,557		^(b)	—		8,623		447		^(c)	9,070
Ameren Missouri:
Cash, cash equivalents, and restricted cash	$	12		$	12	$	—	$	—		$	12
Investments in held-to-maturity debt securities^(a)	270			—		270		—			270
Short-term debt	55			—		55		—			55
Long-term debt (including current portion)^(a)	4,116		^(b)	—		4,404		—			4,404
Ameren Illinois:
Cash, cash equivalents, and restricted cash	$	93		$	93	$	—	$	—		$	93
Short-term debt	126			—		126		—			126
Long-term debt (including current portion)	3,296		^(b)	—		3,515		—			3,515
	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 March 31, 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)

Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $62 million, $25 million, and $32 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of March 31, 2019. Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $58 million, $22 million, and $31 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2018.


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

Related Party Transactions (Tables)

3 Months Ended

Mar. 31, 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 months ended March 31, 2019 and 2018:



				Three Months
Agreement	Income Statement Line Item			Ameren Missouri		Ameren Illinois
Ameren Missouri power supply	Operating Revenues	2019	$	(b)	$	(a)
agreements with Ameren Illinois		2018		3		(a)
Ameren Missouri and Ameren Illinois	Operating Revenues	2019		7		1
rent and facility services		2018		5		1
Ameren Missouri and Ameren Illinois	Operating Revenues	2019		(b)		(b)
miscellaneous support services		2018		(b)		(b)
Total Operating Revenues		2019	$	7	$	1
		2018		8		1
Ameren Illinois power supply	Purchased Power	2019	$	(a)	$	(b)
agreements with Ameren Missouri		2018		(a)		3
Ameren Illinois transmission	Purchased Power	2019		(a)		(b)
services with ATXI		2018		(a)		(b)
Total Purchased Power		2019	$	(a)	$	(b)
		2018		(a)		3
Ameren Missouri and Ameren Illinois	Other Operations and Maintenance	2019	$	(b)	$	1
rent and facility services		2018		(b)		2
Ameren Services support services	Other Operations and Maintenance	2019		32		30
agreement		2018		33		30
Total Other Operations and Maintenance		2019	$	32	$	31
		2018		33		32
Money pool borrowings (advances)	Interest Charges/ Other Income, Net	2019	$	—	$	—
		2018		(b)		(b)


(a)	Not applicable.


(b)	Amount less than $1 million.

Commitments and Contingencies (Tables)

3 Months Ended

Mar. 31, 2019

Commitments and Contingencies Disclosure [Abstract]

Long-term Purchase Commitment

The table below presents our estimated minimum fuel, purchased power, and other commitments at March 31, 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 March 31, 2019.



	Coal		Natural Gas^(a)		Nuclear Fuel		Purchased Power^(b)(c)			Methane Gas		Other		Total
Ameren:
2019	$	279	$	154	$	25	$	88	^(d)	$	2	$	52	$	600
2020	160		158		43		54			3		42		460
2021	121		85		58		10			3		30		307
2022	72		32		13		—			3		26		146
2023	—		8		41		—			3		29		81
Thereafter	—		34		29		—			27		71		161
Total	$	632	$	471	$	209	$	152		$	41	$	250	$	1,755
Ameren Missouri:
2019	$	279	$	33	$	25	$	—		$	2	$	39	$	378
2020	160		34		43		—			3		29		269
2021	121		16		58		—			3		25		223
2022	72		5		13		—			3		26		119
2023	—		5		41		—			3		29		78
Thereafter	—		13		29		—			27		55		124
Total	$	632	$	106	$	209	$	—		$	41	$	203	$	1,191
Ameren Illinois:
2019	$	—	$	121	$	—	$	88	^(d)	$	—	$	5	$	214
2020	—		124		—		54			—		4		182
2021	—		69		—		10			—		—		79
2022	—		27		—		—			—		—		27
2023	—		3		—		—			—		—		3
Thereafter	—		21		—		—			—		—		21
Total	$	—	$	365	$	—	$	152		$	—	$	9	$	526


(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.


(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 $10 million of zero emission credits through May 2019.

Callaway Energy Center (Tables)

3 Months Ended

Mar. 31, 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 April 1, 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	13,623		^(a)	138		^(b)
	$	14,073	^(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)

3 Months Ended

Mar. 31, 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 months ended March 31, 2019 and 2018:



	Pension Benefits						Postretirement Benefits
	Three Months						Three Months
	2019			2018			2019			2018
Service cost^(a)	$	22		$	25		$	4		$	5
Non-service cost components:
Interest cost	47			42			11			11
Expected return on plan assets	(69		)	(69		)	(19		)	(19		)
Amortization of:
Prior service benefit	—			—			(1		)	(1		)
Actuarial loss (gain)	6			16			(4		)	—
Total non-service cost components^(b)	$	(16	)	$	(11	)	$	(13	)	$	(9	)
Net periodic benefit cost (income)	$	6		$	14		$	(9	)	$	(4	)


(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 months ended March 31, 2019 and 2018:



	Pension Benefits				Postretirement Benefits
	Three Months				Three Months
	2019		2018		2019			2018
Ameren Missouri^(a)	$	1	$	5	$	(2	)	$	—
Ameren Illinois	5		9		(7		)	(4		)
Ameren^(a)	$	6	$	14	$	(9	)	$	(4	)


(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)

3 Months Ended

Mar. 31, 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 months ended March 31, 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	(7)	(2)	^(a)	(12)	—	^(a)	(3)	(3)
Other depreciation differences	—	—		—	1		—	—
Amortization of deferred investment tax credit	(1)	(1)		(1)	(1)		—	—
State tax	6	7		4	4		7	7
Stock-based compensation	(7)	(3)		—	—		—	—
Other	—	—		(3)	—		—	—
Effective income tax rate	12%	22%		9%	25%		25%	25%

Supplemental Information (Tables)

3 Months Ended

Mar. 31, 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 March 31, 2019, and December 31, 2018:



	March 31, 2019						December 31, 2018
	Ameren		Ameren Missouri		Ameren Illinois		Ameren		Ameren Missouri		Ameren Illinois
Cash and cash equivalents	$	8	$	—	$	—	$	16	$	—	$	—
Restricted cash included in “Other current assets”	16		5		6		13		4		6
Restricted cash included in “Other assets”	87		—		87		74		—		74
Restricted cash included in “Nuclear decommissioning trust fund”	7		7		—		4		4		—
Total cash, cash equivalents, and restricted cash	$	118	$	12	$	93	$	107	$	8	$	80

Lease Supplemental Information

The following table provides supplemental balance sheet information related to operating leases as of March 31, 2019:



	Ameren			Ameren Missouri
Other assets	$	36		$	34
Other current liabilities	7			6
Other deferred credits and liabilities	29			28
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 March 31, 2019:



	Ameren		Ameren Missouri
2019	$	6	$	5
2020	7		7
2021	7		6
2022	6		5
2023	5		5
Thereafter	10		10
Total lease payments	41		38
Less imputed interest	5		4
Total^(a)	$	36	$	34

(a)

The amount of remaining maturities of operating lease liabilities under previous authoritative accounting guidance as of December 31, 2018, is materially consistent to the amount as of March 31, 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 three months ended March 31, 2019 and 2018:



	March 31, 2019						March 31, 2018
	Ameren		Ameren Missouri		Ameren Illinois		Ameren			Ameren Missouri			Ameren Illinois
Accrued capital expenditures	$	208	$	92	$	106	$	202		$	73		$	114
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	64		64		—		(11		)	(11		)	—

Asset Retirement Obligation Disclosure

The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the three months ended March 31, 2019:



	Ameren Missouri				Ameren Illinois^(a)			Ameren
Balance at December 31, 2018	$	646		^(a)	$	4	^(b)	$	650		^(a)
Liabilities settled	(3		)		—			(3		)
Accretion	6			^(c)	—			6			^(c)
Balance at March 31, 2019	$	649		^(a)	$	4	^(b)	$	653		^(a)


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

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 three months ended March 31, 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	288,956		67.42		^(b)	124,925		65.23
Forfeitures	(2,228	)	65.06			(833	)	63.16
Vested and undistributed^(c)	(65,458	)	62.04			(14,047	)	61.77
Vested and distributed	(176,923	)	44.13			—		—
Nonvested at March 31, 2019^(d)	727,158		$	63.40		265,298		$	60.64


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


(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 333,466 vested and undistributed performance share units and 40,604 vested and undistributed restricted stock units.

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 months ended March 31, 2019 and 2018:



	Three Months
	2019		2018
Ameren Missouri	$	31	$	34
Ameren Illinois	39		35		^(a)
Ameren	$	70	$	69	^(a)


(a)	Amounts have been adjusted from those previously reported to reflect additional excise taxes for the three months ended March 31, 2018.

Segment Information (Tables)

3 Months Ended

Mar. 31, 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 months ended March 31, 2019 and 2018. Ameren, Ameren Missouri, and Ameren Illinois management review segment capital expenditure information rather than any individual or total asset amount.

Ameren



Three Months	Ameren Missouri		Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Transmission			Other		Intersegment Eliminations			Ameren
2019
External revenues	$	751	$	386	$	320	$	99		$	—	$	—		$	1,556
Intersegment revenues	7		1		—		15			—		(23		)	—
Net income attributable to Ameren common shareholders	39		36		57		44		^(a)	15		—			191
Capital expenditures	240		124		51		121			10		(2		)	544
2018
External revenues	$	784	$	399	$	311	$	91		$	—	$	—		$	1,585
Intersegment revenues	8		1		—		13			—		(22		)	—
Net income attributable to Ameren common shareholders	38		33		42		37		^(a)	1		—			151
Capital expenditures	249		122		60		145			7		(4		)	579


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

Ameren Illinois



Three Months	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Illinois Transmission		Intersegment Eliminations			Ameren Illinois
2019
External revenues	$	387	$	320	$	55	$	—		$	762
Intersegment revenues	—		—		15		(15		)	—
Net income available to common shareholder	36		57		27		—			120
Capital expenditures	124		51		92		—			267
2018
External revenues	$	400	$	311	$	49	$	—		$	760
Intersegment revenues	—		—		13		(13		)	—
Net income available to common shareholder	33		42		20		—			95
Capital expenditures	122		60		118		—			300

Disaggregation of Revenue

The following tables present disaggregated revenues by segment at Ameren and Ameren Illinois for the three months ended March 31, 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



Three Months	Ameren Missouri			Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas			Ameren Transmission		Other		Intersegment Eliminations			Ameren
2019
Residential	$	312		$	217	$	—		$	—	$	—	$	—		$	529
Commercial	239			123		—			—		—		—			362
Industrial	55			34		—			—		—		—			89
Other	98			13		—			114		—		(23		)	202
Total electric revenues	$	704		$	387	$	—		$	114	$	—	$	(23	)	$	1,182
Residential	$	38		$	—	$	246		$	—	$	—	$	—		$	284
Commercial	16			—		65			—		—		—			81
Industrial	2			—		4			—		—		—			6
Other	(2		)	—		5			—		—		—			3
Total gas revenues	$	54		$	—	$	320		$	—	$	—	$	—		$	374
Total revenues^(a)	$	758		$	387	$	320		$	114	$	—	$	(23	)	$	1,556
2018
Residential	$	332		$	219	$	—		$	—	$	—	$	—		$	551
Commercial	252			124		—			—		—		—			376
Industrial	61			35		—			—		—		—			96
Other	96			22		—			104		—		(22		)	200
Total electric revenues	$	741		$	400	$	—		$	104	$	—	$	(22	)	$	1,223
Residential	$	41		$	—	$	243		$	—	$	—	$	—		$	284
Commercial	16			—		67			—		—		—			83
Industrial	2			—		6			—		—		—			8
Other	(8		)	—		(5		)	—		—		—			(13		)
Total gas revenues	$	51		$	—	$	311		$	—	$	—	$	—		$	362
Total revenues^(a)	$	792		$	400	$	311		$	104	$	—	$	(22	)	$	1,585


(a)	The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the three months ended March 31, 2019 and 2018:



Three Months	Ameren Missouri			Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas			Ameren Transmission			Ameren
2019
Revenues from alternative revenue programs	$	15		$	22	$	(3	)	$	(5	)	$	29
Other revenues not from contracts with customers	5			3		1			—			9
2018
Revenues from alternative revenue programs	$	(4	)	$	31	$	(3	)	$	(4	)	$	20
Other revenues not from contracts with customers	14			10		1			—			25

Ameren Illinois



Three Months	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas			Ameren Illinois Transmission		Intersegment Eliminations			Total Ameren Illinois
2019
Residential	$	217	$	246		$	—	$	—		$	463
Commercial	123		65			—		—			188
Industrial	34		4			—		—			38
Other	13		5			70		(15		)	73
Total revenues^(a)	$	387	$	320		$	70	$	(15	)	$	762
2018
Residential	$	219	$	243		$	—	$	—		$	462
Commercial	124		67			—		—			191
Industrial	35		6			—		—			41
Other	22		(5		)	62		(13		)	66
Total revenues^(a)	$	400	$	311		$	62	$	(13	)	$	760


(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 months ended March 31, 2019 and 2018:



Three Months	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas			Ameren Illinois Transmission			Ameren Illinois
2019
Revenues from alternative revenue programs	$	22	$	(3	)	$	(5	)	$	14
Other revenues not from contracts with customers	3		1			—			4
2018
Revenues from alternative revenue programs	$	31	$	(3	)	$	(4	)	$	24
Other revenues not from contracts with customers	10		1			—			11

Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 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	258	244
Corporate owned life insurance, borrowings	113	113
Unrecorded Unconditional Purchase Obligation	1,755
Ameren Illinois Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Cash Surrender Value of Life Insurance	123	122
Corporate owned life insurance, borrowings	113	$ 113
Unrecorded Unconditional Purchase Obligation	526
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	16
Partnership Funding Commitment
Basis Of Presentation And Significant Accounting Policies [Line Items]
Unrecorded Unconditional Purchase Obligation	$ 15

Rate And Regulatory Matters (Narrative-Missouri) (Detail) $ in Millions	3 Months Ended
	Jun. 30, 2019 USD ($) MWh	Mar. 31, 2019 USD ($)	Dec. 31, 2018 MWh	Jun. 30, 2018 MWh	Mar. 31, 2018 USD ($)
Rate And Regulatory Matters [Line Items]
Total operating revenues		$ 1,556			$ 1,585
Union Electric Company
Rate And Regulatory Matters [Line Items]
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			157	400
Estimated Capital Project Costs		$ 1,400
Final Rate Order \| Natural Gas \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Interim Rate Increase (Decrease), Amount		2
Pending Rate Case [Member] \| Natural Gas \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 4
Public Utilities, Requested Return on Equity, Percentage		10.30%
Public Utilities, Requested Equity Capital Structure, Percentage		51.84%
Rate Base		$ 259
Public Utilities, Requested Rate Increase (Decrease) on Interim Rates, Amount		6
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 \| Wind Generation Facility \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Amount of Megawatts \| MWh	300
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 Return on Equity, Percentage	9.50%
Public Utilities, Requested Equity Capital Structure, Percentage	50.00%

Rate And Regulatory Matters (Narrative-Illinois) (Detail) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Rate And Regulatory Matters [Line Items]
Regulatory Assets, Noncurrent		$ 1,134	$ 1,127
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory Assets, Noncurrent		776	$ 759
Ameren Transmission Company of Illinois
Rate And Regulatory Matters [Line Items]
Estimated Capital Project Costs		81
Property, Plant and Equipment, Additions		38
2019 IEIMA Revenue Requirement Reconciliation \| IEIMA \| Ameren Illinois Company \| Electric Distribution
Rate And Regulatory Matters [Line Items]
Regulatory Assets, Noncurrent		$ 41
Subsequent Event \| Pending Rate Case [Member] \| IEIMA \| Ameren Illinois Company \| Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 7

Rate And Regulatory Matters (Narrative-Federal) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 161	$ 149
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 79	$ 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	$ 44
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
Short-Term Debt And Liquidity (Narrative) (Detail)	Mar. 31, 2019 USD ($)	Mar. 31, 2018
Credit Agreements
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,292,000,000
Actual debt-to-capital ratio	0.54
Utilities
Short-term Debt [Line Items]
Short-term Debt, Weighted Average Interest Rate, over Time	2.87%	1.90%
Union Electric Company \| Missouri Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.48
Ameren Illinois Company \| Illinois Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.47

Short-Term Debt and Liquidity (Commercial Paper outstanding) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Commercial paper outstanding	$ 799	$ 597
Ameren (parent)
Short-term Debt [Line Items]
Commercial paper outstanding	618	470
Union Electric Company
Short-term Debt [Line Items]
Commercial paper outstanding	55	55
Ameren Illinois Company
Short-term Debt [Line Items]
Commercial paper outstanding	$ 126	$ 72

Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 815	$ 728
Weighted-average interest rate	2.85%	1.90%
Short-term Debt, Maximum Amount Outstanding During Period	$ 1,113	$ 960
Peak interest rate	3.10%	2.55%
Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 480	$ 378
Weighted-average interest rate	2.87%	1.90%
Short-term Debt, Maximum Amount Outstanding During Period	$ 618	$ 454
Peak interest rate	3.10%	2.35%
Union Electric Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 246	$ 213
Weighted-average interest rate	2.84%	1.88%
Short-term Debt, Maximum Amount Outstanding During Period	$ 549	$ 282
Peak interest rate	2.97%	2.40%
Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 89	$ 137
Weighted-average interest rate	2.76%	1.96%
Short-term Debt, Maximum Amount Outstanding During Period	$ 130	$ 238
Peak interest rate	2.90%	2.55%

Long-Term debt and Equity Financings (Narrative) (Details) - USD ($) shares in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Long-Term Debt And Equity Financings [Line Items]
Stock Issued During Period, Shares, New Issues	0.3	0.3
Issuance of common stock	$ 19,000,000	$ 17,000,000
Stock Issued During Period, Shares, Other	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]
Total principal amount of notes	$ 450,000,000
State interest rate	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]
State interest rate	6.70%
Repayments of other long-term debt	$ 329,000,000

Other Income, Net (Detail) - USD ($) $ in Millions	3 Months Ended
Other Income, Net (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Mar. 31, 2018
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 6	$ 5
Interest income on industrial development revenue bonds	6	6
Interest income	2	2
Non-service cost components of net periodic benefit income	22	16
Other income	2	1
Donations	(6)	(5)
Other expense	(3)	(2)
Other Income (Expense), Net	29	23
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	4	4
Interest income on industrial development revenue bonds	6	6
Non-service cost components of net periodic benefit income	4	5
Other income	1	1
Donations	(2)	(1)
Other expense	(1)	(2)
Other Income (Expense), Net	12	13
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	7	4
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	2	1
Interest income	2	2
Non-service cost components of net periodic benefit income	12	7
Other income	1	0
Donations	(4)	(4)
Other expense	(2)	0
Other Income (Expense), Net	$ 11	$ 6

Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) gal in Millions, MWh in Millions, MMBTU in Millions	Mar. 31, 2019 MWh gal MMBTU	Dec. 31, 2018 MWh gal MMBTU
Fuel Oils
Derivative [Line Items]
Quantity \| gal	65	66
Natural Gas
Derivative [Line Items]
Quantity \| MMBTU	174	173
Power
Derivative [Line Items]
Quantity \| MWh	10	9
Union Electric Company \| Fuel Oils
Derivative [Line Items]
Quantity \| gal	65	66
Union Electric Company \| Natural Gas
Derivative [Line Items]
Quantity \| MMBTU	18	19
Union Electric Company \| Power
Derivative [Line Items]
Quantity \| MWh	2	1
Ameren Illinois Company \| Natural Gas
Derivative [Line Items]
Quantity \| MMBTU	156	154
Ameren Illinois Company \| Power
Derivative [Line Items]
Quantity \| MWh	8	8

Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative liabilities	$ 206
Not Designated As Hedging Instrument
Derivative [Line Items]
Derivative assets	17	$ 15
Derivative liabilities		219
Not Designated As Hedging Instrument \| Fuel Oils \| Other Current Assets
Derivative [Line Items]
Derivative assets	5	3
Not Designated As Hedging Instrument \| Fuel Oils \| Other Assets
Derivative [Line Items]
Derivative assets	4	5
Not Designated As Hedging Instrument \| Fuel Oils \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	3	4
Not Designated As Hedging Instrument \| Fuel Oils \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	4	9
Not Designated As Hedging Instrument \| Natural Gas \| Other Current Assets
Derivative [Line Items]
Derivative assets	3	1
Not Designated As Hedging Instrument \| Natural Gas \| Other Assets
Derivative [Line Items]
Derivative assets	3	2
Not Designated As Hedging Instrument \| Natural Gas \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	8	12
Not Designated As Hedging Instrument \| Natural Gas \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	5	7
Not Designated As Hedging Instrument \| Power \| Other Current Assets
Derivative [Line Items]
Derivative assets	1	4
Not Designated As Hedging Instrument \| Power \| Other Assets
Derivative [Line Items]
Derivative assets	1
Not Designated As Hedging Instrument \| Power \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	16	18
Not Designated As Hedging Instrument \| Power \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	170	169
Union Electric Company \| Not Designated As Hedging Instrument
Derivative [Line Items]
Derivative assets	11	12
Derivative liabilities	12	22
Union Electric Company \| Not Designated As Hedging Instrument \| Fuel Oils \| Other Current Assets
Derivative [Line Items]
Derivative assets	5	3
Union Electric Company \| Not Designated As Hedging Instrument \| Fuel Oils \| Other Assets
Derivative [Line Items]
Derivative assets	4	5
Union Electric Company \| Not Designated As Hedging Instrument \| Fuel Oils \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	3	4
Union Electric Company \| Not Designated As Hedging Instrument \| Fuel Oils \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	4	9
Union Electric Company \| Not Designated As Hedging Instrument \| Natural Gas \| Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Union Electric Company \| Not Designated As Hedging Instrument \| Natural Gas \| Other Assets
Derivative [Line Items]
Derivative assets	0	0
Union Electric Company \| Not Designated As Hedging Instrument \| Natural Gas \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	2	4
Union Electric Company \| Not Designated As Hedging Instrument \| Natural Gas \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	1	1
Union Electric Company \| Not Designated As Hedging Instrument \| Power \| Other Current Assets
Derivative [Line Items]
Derivative assets	1	4
Union Electric Company \| Not Designated As Hedging Instrument \| Power \| Other Assets
Derivative [Line Items]
Derivative assets	1
Union Electric Company \| Not Designated As Hedging Instrument \| Power \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	2	4
Union Electric Company \| Not Designated As Hedging Instrument \| Power \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company \| Not Designated As Hedging Instrument
Derivative [Line Items]
Derivative assets	6	3
Derivative liabilities	194	197
Ameren Illinois Company \| Not Designated As Hedging Instrument \| Fuel Oils \| Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company \| Not Designated As Hedging Instrument \| Fuel Oils \| Other Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company \| Not Designated As Hedging Instrument \| Fuel Oils \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company \| Not Designated As Hedging Instrument \| Fuel Oils \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company \| Not Designated As Hedging Instrument \| Natural Gas \| Other Current Assets
Derivative [Line Items]
Derivative assets	3	1
Ameren Illinois Company \| Not Designated As Hedging Instrument \| Natural Gas \| Other Assets
Derivative [Line Items]
Derivative assets	3	2
Ameren Illinois Company \| Not Designated As Hedging Instrument \| Natural Gas \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	6	8
Ameren Illinois Company \| Not Designated As Hedging Instrument \| Natural Gas \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	4	6
Ameren Illinois Company \| Not Designated As Hedging Instrument \| Power \| Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company \| Not Designated As Hedging Instrument \| Power \| Other Assets
Derivative [Line Items]
Derivative assets	0
Ameren Illinois Company \| Not Designated As Hedging Instrument \| Power \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	14	14
Ameren Illinois Company \| Not Designated As Hedging Instrument \| Power \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	$ 170	$ 169

Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Detail) $ in Millions	Mar. 31, 2019 USD ($)
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	$ 103
Cash Collateral Posted	4
Potential Aggregate Amount of Additional Collateral Required	87
Union Electric Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	64
Cash Collateral Posted	4
Potential Aggregate Amount of Additional Collateral Required	58
Ameren Illinois Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	39
Cash Collateral Posted	0
Potential Aggregate Amount of Additional Collateral Required	$ 29

Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	$ 748	$ 679
Assets fair value	765	694
Derivative liabilities	(206)
Excluded receivables, payables, and accrued income, net	6	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	488	427
Assets fair value	490	428
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	260	252
Assets fair value	263	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	12	11
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	17	15
Derivative liabilities	(206)	(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	(1)	(2)
Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	3
Derivative liabilities	(10)	(16)
Commodity Contract \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	12	11
Derivative liabilities	(195)	(201)
Fuel Oils \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	9	8
Derivative liabilities	(7)	(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	(1)	(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	7	7
Derivative liabilities	(6)	(11)
Power \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	4
Derivative liabilities	(186)	(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	2	3
Derivative liabilities	(186)	(186)
Natural Gas \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	6	3
Derivative liabilities	(13)	(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	0	0
Natural Gas \| Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	2
Derivative liabilities	(10)	(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	488	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	488	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	76	72
Debt Securities \| US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	151	148
Debt Securities \| Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	33	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	76	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	151	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	33	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	748	679
Assets fair value	759	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	488	427
Assets fair value	490	428
Union Electric Company \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	260	252
Assets fair value	260	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	9	10
Union Electric Company \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	11	12
Derivative liabilities	(12)	(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	(1)	(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	9	10
Derivative liabilities	(8)	(14)
Union Electric Company \| Fuel Oils \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	9	8
Derivative liabilities	(7)	(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	(1)	(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	7	7
Derivative liabilities	(6)	(11)
Union Electric Company \| Power \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	4
Derivative liabilities	(2)	(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	2	3
Derivative liabilities	(2)	(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	488	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	488	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	76	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	151	148
Union Electric Company \| Debt Securities \| Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	33	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	76	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	151	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	33	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	(194)	(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	0	0
Ameren Illinois Company \| Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	(7)	(10)
Ameren Illinois Company \| Commodity Contract \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	(187)	(187)
Ameren Illinois Company \| Power \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	(184)	(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	(184)	(183)
Ameren Illinois Company \| Natural Gas \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	6	3
Derivative liabilities	(10)	(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	0	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	3	2
Derivative liabilities	(7)	(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
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	$ (184)	$ (187)	$ (183)	$ (188)
Included in regulatory assets/liabilities	(4)	(1)
Settlements	3	2
Change in unrealized gains (losses) related to assets/liabilities held at period end	(4)	0
Union Electric Company
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	0	4	0	7
Included in regulatory assets/liabilities	0	(2)
Settlements	0	(1)
Change in unrealized gains (losses) related to assets/liabilities held at period end	0	(1)
Ameren Illinois Company
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	(184)	(191)	$ (183)	$ (195)
Included in regulatory assets/liabilities	(4)	1
Settlements	3	3
Change in unrealized gains (losses) related to assets/liabilities held at period end	$ (4)	$ 1

Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) $ in Millions	Mar. 31, 2019 USD ($) $ / MMBTU $ / MWh	Dec. 31, 2018 USD ($)	Mar. 31, 2018 $ / MMBTU $ / MWh
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities \| $	$ (206)
Power \| Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets \| $	2	$ 3
Power \| Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities \| $	$ (186)	$ (186)
Commodity Forward Price \| Power \| Discounted Cash Flow \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input			23
Commodity Forward Price \| Power \| Discounted Cash Flow \| Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input			39
Commodity Forward Price \| Power \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input			28
Nodal Basis \| Power \| Discounted Cash Flow \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	(9)		(9)
Nodal Basis \| Power \| Discounted Cash Flow \| Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0		0
Nodal Basis \| Power \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	(2)		(2)
Quoted Price \| Power \| Discounted Cash Flow \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	24
Quoted Price \| Power \| Discounted Cash Flow \| Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	38
Quoted Price \| Power \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	28
Commodity Future Price \| Power \| Fundamental Energy Production Model \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input \| $ / MMBTU	3		3
Commodity Future Price \| Power \| Fundamental Energy Production Model \| Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input \| $ / MMBTU	4		4
Commodity Future Price \| Power \| 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	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt	$ 799	$ 597
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	118	107	$ 89	$ 68
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt	55	55
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	12	8	10	7
Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt	126	72
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	93	80	$ 52	$ 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	799	597
Long-term debt (including current portion)	8,557	8,439
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	118	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	55	55
Long-term debt (including current portion)	4,116	3,998
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	12	8
Debt Issuance Costs, Net	25	22
Carrying Amount \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt	126	72
Long-term debt (including current portion)	3,296	3,296
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	93	80
Debt Issuance Costs, Net	32	31
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	799	597
Investments, Fair Value Disclosure	270	270
Long-term Debt, Fair Value	9,070	8,669
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	118	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	118	107
Fair Value \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	799	597
Investments, Fair Value Disclosure	270	270
Long-term Debt, Fair Value	8,623	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	447	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	55	55
Investments, Fair Value Disclosure	270	270
Long-term Debt, Fair Value	4,404	4,156
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	12	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	12	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	55	55
Investments, Fair Value Disclosure	270	270
Long-term Debt, Fair Value	4,404	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	126	72
Long-term Debt, Fair Value	3,515	3,391
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	93	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	93	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	126	72
Long-term Debt, Fair Value	3,515	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
	Mar. 31, 2019	Mar. 31, 2018
Related Party Transaction [Line Items]
Operating Revenues	$ 0	$ 0
Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	32	33
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	0	0
Operating Expenses	31	32
Ameren Missouri Power Supply Agreements with Ameren Illinois \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	1	3
Ameren Missouri and Ameren Illinois Rent and Facility Services \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	7	5
Operating Expenses	1	1
Ameren Missouri and Ameren Illinois Rent and Facility Services \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	1
Operating Expenses	1	2
Total Related Party Operating Revenues [Member] \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	7	8
Total Related Party Operating Revenues [Member] \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	1
Ameren Missouri and Ameren Illinois miscellaneous support services \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	1	1
Ameren Missouri and Ameren Illinois miscellaneous support services \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	1
Ameren Illinois Power Supply Agreements with Ameren Missouri \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	1	3
Ameren Illinois transmission services from ATXI \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	1	1
Purchased Power \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	1	3
Ameren Services Support Services Agreement \| Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	32	33
Ameren Services Support Services Agreement \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	30	30
Money Pool [Member] \| Union Electric Company
Related Party Transaction [Line Items]
Interest Charges	0	1
Money Pool [Member] \| Ameren Illinois Company
Related Party Transaction [Line Items]
Interest Charges	$ 0	$ 1

Related Party Transactions Narrative (Details) - April 2019 Procurement - Ameren Illinois Company - Ameren Illinois Power Supply Agreements with Ameren Missouri	3 Months Ended
	Mar. 31, 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	3 Months Ended
	Mar. 31, 2019 USD ($) MWh
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	$ 600
Unrecorded Unconditional Purchase Obligation, Due within Two Years	460
Unrecorded Unconditional Purchase Obligation, Due within Three Years	307
Unrecorded Unconditional Purchase Obligation, Due within Four Years	146
Unrecorded Unconditional Purchase Obligation, Due within Five Years	81
Unrecorded Unconditional Purchase Obligation, Due after Five Years	161
Unrecorded Unconditional Purchase Obligation	1,755
Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	279
Unrecorded Unconditional Purchase Obligation, Due within Two Years	160
Unrecorded Unconditional Purchase Obligation, Due within Three Years	121
Unrecorded Unconditional Purchase Obligation, Due within Four Years	72
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	632
Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	154
Unrecorded Unconditional Purchase Obligation, Due within Two Years	158
Unrecorded Unconditional Purchase Obligation, Due within Three Years	85
Unrecorded Unconditional Purchase Obligation, Due within Four Years	32
Unrecorded Unconditional Purchase Obligation, Due within Five Years	8
Unrecorded Unconditional Purchase Obligation, Due after Five Years	34
Unrecorded Unconditional Purchase Obligation	471
Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	25
Unrecorded Unconditional Purchase Obligation, Due within Two Years	43
Unrecorded Unconditional Purchase Obligation, Due within Three Years	58
Unrecorded Unconditional Purchase Obligation, Due within Four Years	13
Unrecorded Unconditional Purchase Obligation, Due within Five Years	41
Unrecorded Unconditional Purchase Obligation, Due after Five Years	29
Unrecorded Unconditional Purchase Obligation	209
Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	88
Unrecorded Unconditional Purchase Obligation, Due within Two Years	54
Unrecorded Unconditional Purchase Obligation, Due within Three Years	10
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	$ 152
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	27
Unrecorded Unconditional Purchase Obligation	41
Other Purchased Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	52
Unrecorded Unconditional Purchase Obligation, Due within Two Years	42
Unrecorded Unconditional Purchase Obligation, Due within Three Years	30
Unrecorded Unconditional Purchase Obligation, Due within Four Years	26
Unrecorded Unconditional Purchase Obligation, Due within Five Years	29
Unrecorded Unconditional Purchase Obligation, Due after Five Years	71
Unrecorded Unconditional Purchase Obligation	250
Union Electric Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	378
Unrecorded Unconditional Purchase Obligation, Due within Two Years	269
Unrecorded Unconditional Purchase Obligation, Due within Three Years	223
Unrecorded Unconditional Purchase Obligation, Due within Four Years	119
Unrecorded Unconditional Purchase Obligation, Due within Five Years	78
Unrecorded Unconditional Purchase Obligation, Due after Five Years	124
Unrecorded Unconditional Purchase Obligation	1,191
Union Electric Company \| Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	279
Unrecorded Unconditional Purchase Obligation, Due within Two Years	160
Unrecorded Unconditional Purchase Obligation, Due within Three Years	121
Unrecorded Unconditional Purchase Obligation, Due within Four Years	72
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	632
Union Electric Company \| Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	33
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	106
Union Electric Company \| Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	25
Unrecorded Unconditional Purchase Obligation, Due within Two Years	43
Unrecorded Unconditional Purchase Obligation, Due within Three Years	58
Unrecorded Unconditional Purchase Obligation, Due within Four Years	13
Unrecorded Unconditional Purchase Obligation, Due within Five Years	41
Unrecorded Unconditional Purchase Obligation, Due after Five Years	29
Unrecorded Unconditional Purchase Obligation	209
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	27
Unrecorded Unconditional Purchase Obligation	41
Union Electric Company \| Other Purchased Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	39
Unrecorded Unconditional Purchase Obligation, Due within Two Years	29
Unrecorded Unconditional Purchase Obligation, Due within Three Years	25
Unrecorded Unconditional Purchase Obligation, Due within Four Years	26
Unrecorded Unconditional Purchase Obligation, Due within Five Years	29
Unrecorded Unconditional Purchase Obligation, Due after Five Years	55
Unrecorded Unconditional Purchase Obligation	203
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	182
Unrecorded Unconditional Purchase Obligation, Due within Three Years	79
Unrecorded Unconditional Purchase Obligation, Due within Four Years	27
Unrecorded Unconditional Purchase Obligation, Due within Five Years	3
Unrecorded Unconditional Purchase Obligation, Due after Five Years	21
Unrecorded Unconditional Purchase Obligation	526
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	121
Unrecorded Unconditional Purchase Obligation, Due within Two Years	124
Unrecorded Unconditional Purchase Obligation, Due within Three Years	69
Unrecorded Unconditional Purchase Obligation, Due within Four Years	27
Unrecorded Unconditional Purchase Obligation, Due within Five Years	3
Unrecorded Unconditional Purchase Obligation, Due after Five Years	21
Unrecorded Unconditional Purchase Obligation	365
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	88
Unrecorded Unconditional Purchase Obligation, Due within Two Years	54
Unrecorded Unconditional Purchase Obligation, Due within Three Years	10
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	152
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	5
Unrecorded Unconditional Purchase Obligation, Due within Two Years	4
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	9
Ameren Illinois Company \| Zero Emission Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation	$ 10
Ameren Illinois Company \| April 2019 Procurement
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Amount of Megawatthours \| MWh	3,704,000
Long-term Purchase Commitment, Amount	$ 108

Commitments And Contingencies (Environmental Matters) (Detail) $ in Millions	Mar. 31, 2019 USD ($) scrubber site	Dec. 31, 2018 USD ($)
Loss Contingencies [Line Items]
Percentage of Rate Base Related to Carbon Dioxide Energy Centers		16.00%
Asset Retirement Obligation	$ 653	$ 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%
Asset Retirement Obligation	$ 649	$ 646
Number of Energy Center Scrubbers \| scrubber	2
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	215
New CCR Rules Estimate
Loss Contingencies [Line Items]
Asset Retirement Obligation	131
New CCR Rules Estimate \| Union Electric Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 131

Callaway Energy Center (Narrative) (Detail) $ in Millions	3 Months Ended
Callaway Energy Center (Narrative) (Detail) $ in Millions	Mar. 31, 2019 USD ($)
Nuclear Plant
Nuclear Waste Matters [Line Items]
Annual decommissioning costs included in costs of service	$ 7

Callaway Energy Center (Insurance Disclosure) (Detail)	3 Months Ended
Callaway Energy Center (Insurance Disclosure) (Detail)	Mar. 31, 2019 USD ($)
Nuclear Waste Matters [Line Items]
Number of weeks of coverage after the first twelve weeks of an outage	1 year
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
Amount of weekly indemnity coverage commencing eight weeks after power outage	4,500,000
Amount of additional weekly indemnity coverage commencing after initial indemnity coverage	$ 3,600,000
Inflationary adjustment prescribed by most recent Price-Anderson Act renewal, in years	5 years
Aggregate nuclear power industry insurance policy limit for losses from terrorist attacks within twelve month period	$ 3,200,000,000
Number Of Additional Weeks After Initial Indemnity Coverage For Power Outage	1 year 4 months 10 days
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	13,623,000,000
Maximum Assessments for Single Incidents	138,000,000
Public Liability
Nuclear Waste Matters [Line Items]
Maximum Coverages	14,073,000,000
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 thereafter not exceeding policy limit	490,000,000
Sub-limit of for non-nuclear events	328,000,000
Property Damage European Mutual Association for Nuclear Insurance
Nuclear Waste Matters [Line Items]
Maximum Coverages	490,000,000
Radiation Event
Nuclear Waste Matters [Line Items]
Maximum Coverages	2,700,000,000
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

Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	$ (22)	$ (16)
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	22	25
Interest cost	47	42
Expected return on plan assets	(69)	(69)
Prior service cost (benefit)	0	0
Actuarial loss	6	16
Total non-service cost components	(16)	(11)
Net periodic benefit cost	6	14
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	4	5
Interest cost	11	11
Expected return on plan assets	(19)	(19)
Prior service cost (benefit)	(1)	(1)
Actuarial loss	(4)	0
Total non-service cost components	(13)	(9)
Net periodic benefit cost	(9)	(4)
Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(4)	(5)
Union Electric Company \| Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	1	5
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	(2)	0
Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(12)	(7)
Ameren Illinois Company \| Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	5	9
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	$ (7)	$ (4)

Retirement Benefits Retirement Benefits (Summary of Benefit Plan Costs Incurred) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 6	$ 14
Pension Plan \| Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	1	5
Pension Plan \| Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	5	9
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(9)	(4)
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	(2)	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	$ (7)	$ (4)

Income Taxes Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details)	3 Months Ended
	Mar. 31, 2019 Rate	Mar. 31, 2018 Rate
Income Taxes [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	21.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent	(7.00%)	(2.00%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent	(1.00%)	(1.00%)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	6.00%	7.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	(7.00%)	(3.00%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Percent	12.00%	22.00%
Union Electric Company
Income Taxes [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	21.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent	(12.00%)	0.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent	0.00%	1.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent	(1.00%)	(1.00%)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	4.00%	4.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent	(3.00%)	0.00%
Effective Income Tax Rate Reconciliation, Percent	9.00%	25.00%
Ameren Illinois Company
Income Taxes [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	21.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent	(3.00%)	(3.00%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent	(0.00%)	(0.00%)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	7.00%	7.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Percent	25.00%	25.00%

Supplemental Information (Narrative) (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Supplemental Information [Abstract]
Payables for purchased receivables	$ 38		$ 33
Deferred Compensation Liability, Classified, Noncurrent	$ 78		$ 80
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements (in shares)	1.5	1.5

Supplemental Information (Cash and Cash Equivalents) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	$ 8	$ 16
Restricted Cash and Cash Equivalents, Current	16	13
Restricted Cash and Cash Equivalents, Noncurrent	87	74
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	7	4
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	118	107	$ 89	$ 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	5	4
Restricted Cash and Cash Equivalents, Noncurrent	0	0
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	7	4
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	12	8	10	7
Ameren Illinois Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	0	0
Restricted Cash and Cash Equivalents, Current	6	6
Restricted Cash and Cash Equivalents, Noncurrent	87	74
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 93	$ 80	$ 52	$ 41

Supplemental Information (Leases - Supplemental Balance Sheet Information) (Details) $ in Millions	3 Months Ended
	Mar. 31, 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	36
Operating Lease, Liability, Current	7
Operating Lease, Liability, Noncurrent	$ 29
Operating Lease, Weighted Average Remaining Lease Term	6 years
Operating Lease, Weighted Average Discount Rate, Percent	3.60%
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	$ 34
Operating Lease, Liability, Current	6
Operating Lease, Liability, Noncurrent	$ 28
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	Mar. 31, 2019 USD ($)
Lessee, Lease, Description [Line Items]
2019	$ 6
2020	7
2021	7
2022	6
2023	5
Thereafter	10
Total lease payments	41
Less imputed interest	5
Total	36
Union Electric Company
Lessee, Lease, Description [Line Items]
2019	5
2020	7
2021	6
2022	5
2023	5
Thereafter	10
Total lease payments	38
Less imputed interest	4
Total	$ 34

Supplemental Information (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	$ 208	$ 202
Net Realized and Unrealized Gain (Loss) on Trading Securities	64	(11)
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	92	73
Net Realized and Unrealized Gain (Loss) on Trading Securities	64	(11)
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	$ 106	$ 114

Supplemental Information (Schedule of Asset Retirement Obligations) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	$ 653	$ 650
Liabilities settled	(3)
Accretion	6
Other current liabilities	301	282
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	649	646
Liabilities settled	(3)
Accretion	6
Other current liabilities	128	123
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	4	4
Liabilities settled	0
Accretion	0
Other current liabilities	129	130
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	3 Months Ended
	Mar. 31, 2019	Mar. 31, 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)	288,956
Forfeitures (in shares)	(2,228)
Undistributed vested units (in shares)	(65,458)
Vested and distributed (in shares)	(176,923)
Nonvested at the end of the period (in shares)	727,158
Fully Vested Undistributed to Retirement-eligible employees (in shares)	333,466		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)	65.06
Weighted-average Fair Value per Unit, Undistributed Vested Units (in dollars per share)	62.04
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)	$ 63.40
Performance period	3 years
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)	124,925
Forfeitures (in shares)	(833)
Undistributed vested units (in shares)	(14,047)
Vested and distributed (in shares)	0
Nonvested at the end of the period (in shares)	265,298
Fully Vested Undistributed to Retirement-eligible employees (in shares)	40,604		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)	65.23
Weighted-average Fair Value per Unit, Forfeitures (in dollars per share)	63.16
Weighted-average Fair Value per Unit, Undistributed Vested Units (in dollars per share)	61.77
Weighted-average Fair Value per Unit, Vested (in dollars per share)	0.00
Weighted-average Fair Value per Unit, Nonvested as of Current period end (in dollars per share)	$ 60.64

Supplemental Information (Schedule Of Excise Taxes) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accounting Policies [Line Items]
Excise tax expense	$ 70	$ 69
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	31	34
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 39	$ 35

Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) segment	Mar. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments \| segment	4
External revenues	$ 1,556	$ 1,585
Intersegment revenues	0	0
Net Income (Loss) Available to Common Stockholders, Basic	191	151
Capital expenditures	$ 544	579
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	$ 762	760
Intersegment revenues	0	0
Net Income Available to Common Shareholder	120	95
Capital expenditures	267	300
Operating Segments \| Union Electric Company
Segment Reporting Information [Line Items]
External revenues	751	784
Intersegment revenues	7	8
Net Income Available to Common Shareholder	39	38
Capital expenditures	240	249
Operating Segments \| Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	386	399
Intersegment revenues	1	1
Net Income Available to Common Shareholder	36	33
Capital expenditures	124	122
Operating Segments \| Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues	320	311
Intersegment revenues	0	0
Net Income Available to Common Shareholder	57	42
Capital expenditures	51	60
Operating Segments \| Ameren Transmission
Segment Reporting Information [Line Items]
External revenues	99	91
Intersegment revenues	15	13
Net Income Available to Common Shareholder	44	37
Capital expenditures	121	145
Operating Segments \| Other
Segment Reporting Information [Line Items]
External revenues	0	0
Intersegment revenues	0	0
Net Income Available to Common Shareholder	15	1
Capital expenditures	10	7
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	387	400
Intersegment revenues	0	0
Net Income Available to Common Shareholder	36	33
Capital expenditures	124	122
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues	320	311
Intersegment revenues	0	0
Net Income Available to Common Shareholder	57	42
Capital expenditures	51	60
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Transmission
Segment Reporting Information [Line Items]
External revenues	55	49
Intersegment revenues	15	13
Net Income Available to Common Shareholder	27	20
Capital expenditures	92	118
Intersegment Eliminations
Segment Reporting Information [Line Items]
External revenues	0	0
Intersegment revenues	(23)	(22)
Net Income Available to Common Shareholder	0	0
Capital expenditures	(2)	(4)
Intersegment Eliminations \| Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	0	0
Intersegment revenues	(15)	(13)
Net Income Available to Common Shareholder	0	0
Capital expenditures	$ 0	$ 0

Segment Information (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 1,556	$ 1,585
Revenues from alternative revenue programs	29	20
Other revenues not from contracts with customers	9	25
Electricity
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,182	1,223
Electricity \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	529	551
Electricity \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	362	376
Electricity \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	89	96
Electricity \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	202	200
Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	374	362
Natural Gas \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	284	284
Natural Gas \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	81	83
Natural Gas \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	6	8
Natural Gas \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	3	(13)
Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	762	760
Ameren Illinois Company \| Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	762	760
Revenues from alternative revenue programs	14	24
Other revenues not from contracts with customers	4	11
Ameren Illinois Company \| Ameren Illinois Company \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	463	462
Ameren Illinois Company \| Ameren Illinois Company \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	188	191
Ameren Illinois Company \| Ameren Illinois Company \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	38	41
Ameren Illinois Company \| Ameren Illinois Company \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	73	66
Ameren Illinois Company \| Electricity
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	442	449
Ameren Illinois Company \| Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	320	311
Operating Segments \| Union Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	758	792
Revenues from alternative revenue programs	15	(4)
Other revenues not from contracts with customers	5	14
Operating Segments \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	387	400
Revenues from alternative revenue programs	22	31
Other revenues not from contracts with customers	3	10
Operating Segments \| Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	320	311
Revenues from alternative revenue programs	(3)	(3)
Other revenues not from contracts with customers	1	1
Operating Segments \| Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	114	104
Revenues from alternative revenue programs	(5)	(4)
Other revenues not from contracts with customers	0	0
Operating Segments \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Electricity \| Union Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	704	741
Operating Segments \| Electricity \| Union Electric Company \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	312	332
Operating Segments \| Electricity \| Union Electric Company \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	239	252
Operating Segments \| Electricity \| Union Electric Company \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	55	61
Operating Segments \| Electricity \| Union Electric Company \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	98	96
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	387	400
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	217	219
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	123	124
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	34	35
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	13	22
Operating Segments \| Electricity \| Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Electricity \| Ameren Illinois Gas \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Electricity \| Ameren Illinois Gas \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Electricity \| Ameren Illinois Gas \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Electricity \| Ameren Illinois Gas \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Electricity \| Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	114	104
Operating Segments \| Electricity \| Ameren Transmission \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Electricity \| Ameren Transmission \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Electricity \| Ameren Transmission \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Electricity \| Ameren Transmission \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	114	104
Operating Segments \| Electricity \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Electricity \| Other \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Electricity \| Other \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Electricity \| Other \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Electricity \| Other \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Natural Gas \| Union Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	54	51
Operating Segments \| Natural Gas \| Union Electric Company \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	38	41
Operating Segments \| Natural Gas \| Union Electric Company \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	16	16
Operating Segments \| Natural Gas \| Union Electric Company \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	2	2
Operating Segments \| Natural Gas \| Union Electric Company \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(2)	(8)
Operating Segments \| Natural Gas \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	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
Operating Segments \| Natural Gas \| Ameren Illinois Electric Distribution \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	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
Operating Segments \| Natural Gas \| Ameren Illinois Electric Distribution \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Natural Gas \| Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	320	311
Operating Segments \| Natural Gas \| Ameren Illinois Gas \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	246	243
Operating Segments \| Natural Gas \| Ameren Illinois Gas \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	65	67
Operating Segments \| Natural Gas \| Ameren Illinois Gas \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	4	6
Operating Segments \| Natural Gas \| Ameren Illinois Gas \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	5	(5)
Operating Segments \| Natural Gas \| Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Natural Gas \| Ameren Transmission \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Natural Gas \| Ameren Transmission \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Natural Gas \| Ameren Transmission \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Natural Gas \| Ameren Transmission \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Natural Gas \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Natural Gas \| Other \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Natural Gas \| Other \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Natural Gas \| Other \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Natural Gas \| Other \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	22	31
Other revenues not from contracts with customers	3	10
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(3)	(3)
Other revenues not from contracts with customers	1	1
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(5)	(4)
Other revenues not from contracts with customers	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	387	400
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	217	219
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	123	124
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	34	35
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	13	22
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	70	62
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
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
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
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Transmission \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	70	62
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	320	311
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Gas \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	246	243
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Gas \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	65	67
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Gas \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	4	6
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Gas \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	5	(5)
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(23)	(22)
Intersegment Eliminations \| Electricity
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(23)	(22)
Intersegment Eliminations \| Electricity \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Intersegment Eliminations \| Electricity \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Intersegment Eliminations \| Electricity \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Intersegment Eliminations \| Electricity \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(23)	(22)
Intersegment Eliminations \| Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Intersegment Eliminations \| Natural Gas \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Intersegment Eliminations \| Natural Gas \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Intersegment Eliminations \| Natural Gas \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Intersegment Eliminations \| Natural Gas \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Intersegment Eliminations \| Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(15)	(13)
Intersegment Eliminations \| Ameren Illinois Company \| Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Intersegment Eliminations \| Ameren Illinois Company \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Intersegment Eliminations \| Ameren Illinois Company \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Intersegment Eliminations \| Ameren Illinois Company \| Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ (15)	$ (13)