Cover Page - shares	9 Months Ended
Cover Page - shares	Sep. 30, 2021	Oct. 29, 2021
Entity Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2021
Document Transition Report	false
Entity File Number	1-14756
Entity Registrant Name	Ameren Corporation
Entity Tax Identification Number	43-1723446
Entity Incorporation, State or Country Code	MO
Entity Address, Address Line One	1901 Chouteau Avenue
Entity Address, City or Town	St. Louis
Entity Address, State or Province	MO
Entity Address, Postal Zip Code	63103
City Area Code	(314)
Local Phone Number	621-3222
Title of 12(b) Security	Common Stock, $0.01 par value per share
Trading Symbol(s)	AEE
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		257,606,191
Entity Central Index Key	0001002910
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Amendment Flag	false
Union Electric Company
Entity Information [Line Items]
Entity File Number	1-2967
Entity Registrant Name	Union Electric Company
Entity Tax Identification Number	43-0559760
Entity Incorporation, State or Country Code	MO
Entity Address, Address Line One	1901 Chouteau Avenue
Entity Address, City or Town	St. Louis
Entity Address, State or Province	MO
Entity Address, Postal Zip Code	63103
City Area Code	(314)
Local Phone Number	621-3222
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		102,123,834
Entity Central Index Key	0000100826
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Amendment Flag	false
Ameren Illinois Company
Entity Information [Line Items]
Entity File Number	1-3672
Entity Registrant Name	Ameren Illinois Company
Entity Tax Identification Number	37-0211380
Entity Incorporation, State or Country Code	IL
Entity Address, Address Line One	10 Executive Drive
Entity Address, City or Town	Collinsville
Entity Address, State or Province	IL
Entity Address, Postal Zip Code	62234
City Area Code	(618)
Local Phone Number	343-8150
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		25,452,373
Entity Central Index Key	0000018654
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Amendment Flag	false

Consolidated Statement of Income (Loss) and Comprehensive Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Operating Revenues:
Total operating revenues	$ 1,811	$ 1,628	$ 4,849	$ 4,466
Operating Expenses:
Fuel	184	141	422	400
Purchased power	159	140	479	383
Natural gas purchased for resale	45	34	275	183
Other operations and maintenance	457	418	1,289	1,240
Depreciation and amortization	290	273	856	799
Taxes other than income taxes	142	128	392	372
Total operating expenses	1,277	1,134	3,713	3,377
Operating Income	534	494	1,136	1,089
Other Income, Net	56	48	151	117
Interest Charges	94	110	290	311
Income Before Income Taxes	496	432	997	895
Income Taxes	70	63	128	134
Net Income	426	369	869	761
Less: Net Income Attributable to Noncontrolling Interests	1	2	4	5
Net Income Attributable to Ameren Common Shareholders	425	367	865	756
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	1	1	1	2
Comprehensive Income	427	370	870	763
Less: Comprehensive Income Attributable to Noncontrolling Interests	1	2	4	5
Comprehensive Income Attributable to Ameren Common Shareholders	$ 426	$ 368	$ 866	$ 758
Earnings Per Share, Basic and Diluted [Abstract]
Earnings per Common Share - Basic (in dollars per share)	$ 1.66	$ 1.48	$ 3.38	$ 3.06
Earnings per Common Share - Diluted (in dollars per share)	$ 1.65	$ 1.47	$ 3.36	$ 3.04
Weighted Average Common Shares Outstanding - Basic (in shares)	257.3	247.1	255.9	246.8
Weighted Average Number of Shares Outstanding - Diluted (in shares)	258.6	249.2	257.2	248.4
Electricity
Operating Revenues:
Total operating revenues	$ 1,668	$ 1,489	$ 4,108	$ 3,846
Natural Gas
Operating Revenues:
Total operating revenues	143	139	741	620
Union Electric Company
Operating Revenues:
Total operating revenues	1,129	1,001	2,642	2,473
Operating Expenses:
Fuel	184	141	422	400
Purchased power	57	48	195	124
Natural gas purchased for resale	4	5	40	29
Other operations and maintenance	240	221	683	662
Depreciation and amortization	161	154	474	448
Taxes other than income taxes	104	92	266	254
Total operating expenses	750	661	2,080	1,917
Operating Income	379	340	562	556
Other Income, Net	27	26	74	55
Interest Charges	32	50	107	140
Income Before Income Taxes	374	316	529	471
Income Taxes	(2)	18	(7)	29
Net Income	376	298	536	442
Preferred Stock Dividends	1	1	3	3
Net Income (Loss) Attributable to Parent	375	297	533	439
Union Electric Company \| Electricity
Operating Revenues:
Total operating revenues	1,113	984	2,543	2,386
Union Electric Company \| Natural Gas
Operating Revenues:
Total operating revenues	16	17	99	87
Ameren Illinois Company
Operating Revenues:
Total operating revenues	645	589	2,097	1,879
Operating Expenses:
Purchased power	108	97	298	271
Natural gas purchased for resale	41	29	235	154
Other operations and maintenance	217	197	604	578
Depreciation and amortization	118	109	350	323
Taxes other than income taxes	34	33	114	107
Total operating expenses	518	465	1,601	1,433
Operating Income	127	124	496	446
Other Income, Net	19	17	49	45
Interest Charges	41	39	123	116
Income Before Income Taxes	105	102	422	375
Income Taxes	26	25	107	93
Net Income	79	77	315	282
Preferred Stock Dividends	0	0	1	2
Net Income (Loss) Attributable to Parent	79	77	314	280
Ameren Illinois Company \| Electricity
Operating Revenues:
Total operating revenues	518	467	1,455	1,346
Ameren Illinois Company \| Natural Gas
Operating Revenues:
Total operating revenues	$ 127	$ 122	$ 642	$ 533

Consolidated Statement of Income (Loss) and Comprehensive Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Pension and other postretirement benefit plan activity, tax	$ 0	$ 0	$ 0	$ 0

Consolidated Balance Sheet - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Current Assets:
Cash and cash equivalents	$ 7	$ 139
Accounts receivable - trade (less allowance for doubtful accounts)	515	415
Unbilled revenue	332	269
Miscellaneous accounts receivable	111	65
Inventories	595	521
Current regulatory assets	359	109
Other current assets	285	135
Total current assets	2,204	1,653
Property, Plant, and Equipment, Net	28,559	26,807
Investments and Other Assets:
Nuclear decommissioning trust fund	1,076	982
Goodwill	411	411
Regulatory assets	1,235	1,100
Other assets	1,180	1,077
Total investments and other assets	3,902	3,570
TOTAL ASSETS	34,665	32,030
Current Liabilities:
Current maturities of long-term debt	57	8
Short-term debt	553	490
Accounts and wages payable	811	958
Taxes accrued	231	82
Interest accrued	103	114
Current regulatory liabilities	138	121
Other current liabilities	464	407
Total current liabilities	2,357	2,180
Long-term Debt, Net	12,444	11,078
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes and tax credits, net	3,465	3,211
Regulatory liabilities	5,425	5,282
Asset retirement obligations	705	696
Pension and other postretirement benefits	39	37
Other deferred credits and liabilities	416	466
Total deferred credits and other liabilities	10,050	9,692
Commitments and Contingencies
Shareholders’ Equity:
Common Stock	3	3
Other paid-in capital, principally premium on common stock	6,483	6,179
Retained earnings	3,199	2,757
Accumulated other comprehensive loss	0	(1)
Total shareholders’ equity	9,685	8,938
Noncontrolling Interests	129	142
Total equity	9,814	9,080
TOTAL LIABILITIES AND EQUITY	34,665	32,030
Union Electric Company
Current Assets:
Cash and cash equivalents	0	136
Advances to money pool	24	139
Accounts receivable - trade (less allowance for doubtful accounts)	266	166
Accounts receivable – affiliates	51	57
Unbilled revenue	208	133
Miscellaneous accounts receivable	93	36
Inventories	406	386
Current regulatory assets	172	60
Other current assets	170	79
Total current assets	1,390	1,192
Property, Plant, and Equipment, Net	14,934	13,879
Investments and Other Assets:
Nuclear decommissioning trust fund	1,076	982
Regulatory assets	483	347
Other assets	385	383
Total investments and other assets	1,944	1,712
TOTAL ASSETS	18,268	16,783
Current Liabilities:
Current maturities of long-term debt	8	8
Accounts and wages payable	380	501
Accounts payable – affiliates	47	46
Taxes accrued	206	42
Interest accrued	62	53
Mark-to-market derivative liabilities	75	10
Current asset retirement obligations	59	60
Current regulatory liabilities	56	26
Other current liabilities	111	87
Total current liabilities	1,004	833
Long-term Debt, Net	5,618	5,096
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes and tax credits, net	1,822	1,742
Regulatory liabilities	3,114	3,110
Asset retirement obligations	700	691
Pension and other postretirement benefits	30	35
Other deferred credits and liabilities	54	66
Total deferred credits and other liabilities	5,720	5,644
Commitments and Contingencies
Shareholders’ Equity:
Common Stock	511	511
Other paid-in capital, principally premium on common stock	2,701	2,518
Preferred stock	80	80
Retained earnings	2,634	2,101
Total shareholders’ equity	5,926	5,210
TOTAL LIABILITIES AND EQUITY	18,268	16,783
Ameren Illinois Company
Current Assets:
Cash and cash equivalents	0	0
Advances to money pool	41	0
Accounts receivable - trade (less allowance for doubtful accounts)	237	234
Accounts receivable – affiliates	94	64
Unbilled revenue	124	136
Miscellaneous accounts receivable	8	12
Inventories	189	135
Mark-to-market derivative assets	56	8
Current regulatory assets	183	37
Other current assets	28	21
Total current assets	960	647
Property, Plant, and Equipment, Net	11,891	11,201
Investments and Other Assets:
Goodwill	411	411
Regulatory assets	730	742
Other assets	599	534
Total investments and other assets	1,740	1,687
TOTAL ASSETS	14,591	13,535
Current Liabilities:
Borrowings from money pool	0	19
Accounts and wages payable	340	363
Accounts payable – affiliates	59	51
Customer deposits	57	74
Current environmental remediation	39	37
Current regulatory liabilities	81	88
Other current liabilities	170	184
Total current liabilities	746	816
Long-term Debt, Net	4,391	3,946
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes and tax credits, net	1,520	1,367
Regulatory liabilities	2,193	2,063
Pension and other postretirement benefits	60	69
Environmental remediation	41	57
Other deferred credits and liabilities	228	251
Total deferred credits and other liabilities	4,042	3,807
Commitments and Contingencies
Shareholders’ Equity:
Common Stock	0	0
Other paid-in capital, principally premium on common stock	2,797	2,652
Preferred stock	49	62
Retained earnings	2,566	2,252
Total shareholders’ equity	5,412	4,966
TOTAL LIABILITIES AND EQUITY	$ 14,591	$ 13,535

Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Accounts Receivable, Allowance for Credit Loss, Current	$ 36	$ 50
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	400,000,000.0	400,000,000.0
Common Stock, Shares, Outstanding	257,600,000	253,300,000
Union Electric Company
Accounts Receivable, Allowance for Credit Loss, Current	$ 14	$ 16
Common stock, par value (in dollars per share)	$ 5	$ 5
Common stock, shares authorized (in shares)	150,000,000.0	150,000,000.0
Common Stock, Shares, Outstanding	102,100,000	102,100,000
Ameren Illinois Company
Accounts Receivable, Allowance for Credit Loss, Current	$ 22	$ 34
Common stock, no par value (in dollars per share)	$ 0	$ 0
Common stock, shares authorized (in shares)	45,000,000.0	45,000,000.0
Common Stock, Shares, Outstanding	25,500,000	25,500,000

Consolidated Statement of Cash Flows - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash Flows From Operating Activities:
Net income	$ 869	$ 761
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	903	802
Amortization of nuclear fuel	37	68
Amortization of debt issuance costs and premium/discounts	17	16
Deferred income taxes and investment tax credits, net	139	125
Allowance for equity funds used during construction	(30)	(25)
Stock-based compensation costs	17	16
Other	11	14
Changes in assets and liabilities:
Receivables	(212)	(113)
Inventories	(73)	(61)
Accounts and wages payable	(143)	(190)
Taxes accrued	148	154
Regulatory assets and liabilities	(340)	(55)
Assets, other	(118)	(66)
Liabilities, other	(27)	(76)
Pension and other postretirement benefits	(6)	(41)
Net cash provided by operating activities	1,192	1,329
Cash Flows From Investing Activities:
Capital expenditures	(2,098)	(1,884)
Wind generation expenditures	(515)	0
Nuclear fuel expenditures	(19)	(61)
Purchases of securities – nuclear decommissioning trust fund	(411)	(169)
Sales and maturities of securities – nuclear decommissioning trust fund	404	135
Other	(7)	(2)
Net cash used in investing activities	(2,646)	(1,981)
Cash Flows From Financing Activities:
Dividends on common stock	(423)	(367)
Dividends paid to noncontrolling interest holders	(4)	(5)
Short-term debt, net	63	(168)
Maturities of long-term debt	0	(85)
Issuances of long-term debt	1,423	1,263
Issuances of common stock	297	37
Redemptions of Ameren Illinois preferred stock	(13)	0
Employee payroll taxes related to stock-based compensation	(17)	(20)
Debt issuance costs	(15)	(11)
Other	(13)	0
Net cash provided by financing activities	1,298	644
Net change in cash, cash equivalents, and restricted cash	(156)	(8)
Cash, cash equivalents, and restricted cash at beginning of year	301	176
Cash, cash equivalents, and restricted cash at end of period	145	168
Union Electric Company
Cash Flows From Operating Activities:
Net income	536	442
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	522	452
Amortization of nuclear fuel	37	68
Amortization of debt issuance costs and premium/discounts	5	4
Deferred income taxes and investment tax credits, net	(7)	(1)
Allowance for equity funds used during construction	(17)	(15)
Other	8	13
Changes in assets and liabilities:
Receivables	(216)	(93)
Inventories	(19)	(33)
Accounts and wages payable	(145)	(168)
Taxes accrued	146	158
Regulatory assets and liabilities	(164)	(56)
Assets, other	(40)	(10)
Liabilities, other	(7)	(46)
Pension and other postretirement benefits	6	(6)
Net cash provided by operating activities	645	709
Cash Flows From Investing Activities:
Capital expenditures	(1,052)	(778)
Wind generation expenditures	(515)	0
Nuclear fuel expenditures	(19)	(61)
Purchases of securities – nuclear decommissioning trust fund	(411)	(169)
Sales and maturities of securities – nuclear decommissioning trust fund	404	135
Payments for Advance to Affiliate		(5)
Proceeds from Collection of Advance to Affiliate	115
Other	0	1
Net cash used in investing activities	(1,478)	(877)
Cash Flows From Financing Activities:
Dividends on preferred stock	(3)	(3)
Short-term debt, net	0	(234)
Maturities of long-term debt	0	(85)
Issuances of long-term debt	524	465
Capital contributions from parent	183	0
Debt issuance costs	(5)	(4)
Net cash provided by financing activities	699	139
Net change in cash, cash equivalents, and restricted cash	(134)	(29)
Cash, cash equivalents, and restricted cash at beginning of year	145	39
Cash, cash equivalents, and restricted cash at end of period	11	10
Ameren Illinois Company
Cash Flows From Operating Activities:
Net income	315	282
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	349	322
Amortization of debt issuance costs and premium/discounts	10	9
Deferred income taxes and investment tax credits, net	134	72
Allowance for equity funds used during construction	(13)	(10)
Other	8	10
Changes in assets and liabilities:
Receivables	8	(9)
Inventories	(54)	(28)
Accounts and wages payable	12	(12)
Taxes accrued	(21)	(20)
Regulatory assets and liabilities	(167)	11
Assets, other	(59)	(52)
Liabilities, other	1	(30)
Pension and other postretirement benefits	(16)	(30)
Net cash provided by operating activities	507	515
Cash Flows From Investing Activities:
Capital expenditures	(1,026)	(1,031)
Payments for Advance to Affiliate	(41)	0
Other	(7)	1
Net cash used in investing activities	(1,074)	(1,030)
Cash Flows From Financing Activities:
Dividends on preferred stock	(1)	(2)
Short-term debt, net	0	189
Money pool borrowings, net	(19)	0
Issuances of long-term debt	449	0
Capital contributions from parent	145	350
Redemption of preferred stock	(13)	0
Debt issuance costs	(6)	0
Other	(13)	0
Net cash provided by financing activities	542	537
Net change in cash, cash equivalents, and restricted cash	(25)	22
Cash, cash equivalents, and restricted cash at beginning of year	147	125
Cash, cash equivalents, and restricted cash at end of period	$ 122	$ 147

Consolidated Statement of Stockholders' Equity - USD ($) $ in Millions	Total	Common Stock	Other Paid-in Capital	Retained Earnings	Deferred Retirement Benefit Costs	Accumulated Other Comprehensive Income (Loss)	Total Ameren Corporation Shareholders' Equity	Noncontrolling Interests	Union Electric Company	Union Electric Company Common Stock	Union Electric Company Other Paid-in Capital	Union Electric Company Preferred Stock	Union Electric Company Retained Earnings	Ameren Illinois Company	Ameren Illinois Company Common Stock	Ameren Illinois Company Other Paid-in Capital	Ameren Illinois Company Preferred Stock	Ameren Illinois Company Retained Earnings
Beginning of period at Dec. 31, 2019			$ 5,694	$ 2,380	$ (17)			$ 142			$ 2,027		$ 1,731			$ 2,188	$ 62	$ 1,882
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Settlement of forward sale agreement through common shares issuance			0
Shares issued under the ATM program			0
Shares issued under the DRPlus and 401(k) plan			37
Stock-based compensation activity			2
Capital contributions from parent									$ 0		0			$ 350		350
Net income	$ 761								442				442	282				282
Net income attributable to Ameren common shareholders	756			756
Dividends on common stock				(367)
Preferred stock dividends													(3)					(2)
Change in deferred retirement benefit costs	2				2
Net income attributable to noncontrolling interest holders	$ (5)							5
Dividends paid to noncontrolling interest holders								(5)
Redemptions of preferred stock								0									0
Common stock shares outstanding at beginning of year (in shares) at Dec. 31, 2019	246,200,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under forward sale agreement (in shares)	0
Shares issued under the DRPlus and 401(k) plan (in shares)	0
Shares issued under the DRPlus and 401(k) plan	500,000
Shares issued for stock-based compensation (in shares)	500,000
Common stock shares outstanding at end of period (in shares) at Sep. 30, 2020	247,200,000
End of period at Sep. 30, 2020	$ 8,631	$ 2	5,733	2,769	(15)	$ (15)		142		$ 511	2,027	$ 80	2,170		$ 0	2,538	62	2,162
Shareholders' equity, end of year at Sep. 30, 2020							$ 8,489		4,788					4,762
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per common share (in dollars per share)	$ 1.485
Beginning of period at Jun. 30, 2020			5,716	2,525	(16)			142					1,873			2,538	62	2,085
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Settlement of forward sale agreement through common shares issuance			0
Shares issued under the ATM program			0
Shares issued under the DRPlus and 401(k) plan			10
Stock-based compensation activity			7
Capital contributions from parent											0					0
Net income	$ 369								298				298	77				77
Net income attributable to Ameren common shareholders	367			367
Dividends on common stock				(123)
Preferred stock dividends													(1)					0
Change in deferred retirement benefit costs	1				1
Net income attributable to noncontrolling interest holders	$ (2)							2
Dividends paid to noncontrolling interest holders								(2)
Redemptions of preferred stock								0									0
Common stock shares outstanding at beginning of year (in shares) at Jun. 30, 2020	247,100,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under forward sale agreement (in shares)	0
Shares issued under the DRPlus and 401(k) plan (in shares)	0
Shares issued under the DRPlus and 401(k) plan	100,000
Shares issued for stock-based compensation (in shares)	0
Common stock shares outstanding at end of period (in shares) at Sep. 30, 2020	247,200,000
End of period at Sep. 30, 2020	$ 8,631	2	5,733	2,769	(15)	(15)		142		511	2,027	80	2,170		0	2,538	62	2,162
Shareholders' equity, end of year at Sep. 30, 2020							8,489		4,788					4,762
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per common share (in dollars per share)	$ 0.495
Beginning of period at Dec. 31, 2020	$ 9,080		6,179	2,757	(1)			142			2,518		2,101			2,652	62	2,252
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Settlement of forward sale agreement through common shares issuance			113
Shares issued under the ATM program			148
Shares issued under the DRPlus and 401(k) plan			36
Stock-based compensation activity			7
Capital contributions from parent									183		183			145		145
Net income	869								$ 536				536	$ 315				315
Net income attributable to Ameren common shareholders	865			865
Dividends on common stock				(423)
Preferred stock dividends													(3)					(1)
Change in deferred retirement benefit costs	1				1
Net income attributable to noncontrolling interest holders	$ (4)							4
Dividends paid to noncontrolling interest holders								(4)
Redemptions of preferred stock								(13)									(13)
Common stock shares outstanding at beginning of year (in shares) at Dec. 31, 2020	253,300,000								102,100,000					25,500,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under forward sale agreement (in shares)	1,600,000
Shares issued under the DRPlus and 401(k) plan (in shares)	1,800,000
Shares issued under the DRPlus and 401(k) plan	400,000
Shares issued for stock-based compensation (in shares)	500,000
Common stock shares outstanding at end of period (in shares) at Sep. 30, 2021	257,600,000								102,100,000					25,500,000
End of period at Sep. 30, 2021	$ 9,814	3	6,483	3,199	0	0		129		511	2,701	80	2,634		0	2,797	49	2,566
Shareholders' equity, end of year at Sep. 30, 2021	$ 9,685						9,685		$ 5,926					$ 5,412
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per common share (in dollars per share)	$ 1.650
Beginning of period at Jun. 30, 2021			6,436	2,915	(1)			129			2,701		2,259			2,722	49	2,487
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Settlement of forward sale agreement through common shares issuance			0
Shares issued under the ATM program			27
Shares issued under the DRPlus and 401(k) plan			12
Stock-based compensation activity			8
Capital contributions from parent											0					75
Net income	$ 426								$ 376				376	$ 79				79
Net income attributable to Ameren common shareholders	425			425
Dividends on common stock				(141)
Preferred stock dividends													(1)					0
Change in deferred retirement benefit costs	1				1
Net income attributable to noncontrolling interest holders	$ (1)							1
Dividends paid to noncontrolling interest holders								(1)
Redemptions of preferred stock								0									0
Common stock shares outstanding at beginning of year (in shares) at Jun. 30, 2021	257,100,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under forward sale agreement (in shares)	0
Shares issued under the DRPlus and 401(k) plan (in shares)	400,000
Shares issued under the DRPlus and 401(k) plan	100,000
Shares issued for stock-based compensation (in shares)	0
Common stock shares outstanding at end of period (in shares) at Sep. 30, 2021	257,600,000								102,100,000					25,500,000
End of period at Sep. 30, 2021	$ 9,814	$ 3	$ 6,483	$ 3,199	$ 0	$ 0		$ 129		$ 511	$ 2,701	$ 80	$ 2,634		$ 0	$ 2,797	$ 49	$ 2,566
Shareholders' equity, end of year at Sep. 30, 2021	$ 9,685						$ 9,685		$ 5,926					$ 5,412
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per common share (in dollars per share)	$ 0.550

Summary Of Significant Accounting Policies	9 Months Ended
Summary Of Significant Accounting Policies	Sep. 30, 2021
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	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 in the MISO. The COVID-19 pandemic continues to affect our results of operations, financial position, and liquidity, but we continue to expect gradual improvement in sales volumes in 2021, compared to 2020. In the first nine months of 2021, our sales volumes, excluding the estimated effects of weather and customer energy-efficiency programs, increased compared to the same period in 2020. However, our accounts receivable balances that were past due or that were a part of a deferred payment arrangement are higher than normal historical levels, as customer payments have been affected. The continued effect of the COVID-19 pandemic on our results of operations, financial position, and liquidity in subsequent periods will depend on its severity and longevity, future regulatory or legislative actions with respect thereto, and the resulting impact on business, economic, and capital market conditions. In general, restrictions on social activities and nonessential businesses implemented in our service territories in 2020 have been relaxed. We continue to assess the impacts the COVID-19 pandemic is having on our businesses, including but not limited to impacts on our liquidity; demand for residential, commercial, and industrial electric and natural gas services; changes in deferred payment arrangements for customers; the timing and extent to which recovery of incremental costs incurred, net of savings, and forgone customer late fee revenues at Ameren Missouri is allowed by the MoPSC; changes in our ability to disconnect customers for nonpayment; bad debt expense; supply chain operations; the availability of our employees and contractors; counterparty credit; capital construction; infrastructure operations and maintenance; energy-efficiency programs; and pension valuations. In March 2021, the MoPSC approved accounting authority orders that allowed Ameren Missouri to accumulate $9 million of certain costs incurred related to the COVID-19 pandemic, net of savings, as well as forgone customer late fees and reconnection fee revenues from March 2020 to March 2021, for potential recovery in the current electric and natural gas service regulatory rate reviews. While the revenues from Ameren Illinois’ electric distribution business, residential and small nonresidential customers of Ameren Illinois’ natural gas distribution business, and Ameren Illinois’ and ATXI’s electric transmission businesses are decoupled from changes in sales volumes, earnings at Ameren Missouri and those associated with Ameren Illinois’ large nonresidential natural gas customers are exposed to such changes. Regarding uncollectible accounts receivable, Ameren Illinois’ electric distribution and natural gas distribution businesses have bad debt riders, which provide for recovery of bad debt write-offs, net of any subsequent recoveries. Ameren Missouri does not have a bad debt rider or tracker, and thus its earnings are exposed to increases in bad debt expense, absent regulatory relief. However, Ameren Missouri has not experienced and does not expect a material impact to earnings from increases in bad debt expense. As of September 30, 2021, accounts receivable balances that were 30 days or greater past due or that were a part of a deferred payment arrangement represented 18%, 12%, and 25%, or $97 million, $33 million, and $64 million, of Ameren’s, Ameren Missouri’s, and Ameren Illinois’ customer trade receivables before allowance for doubtful accounts, respectively. In comparison, as of September 30, 2019, these percentages were 13%, 9%, and 18%, or $65 million, $23 million, and $42 million, for Ameren, Ameren Missouri, and Ameren Illinois, respectively. For information regarding Ameren Illinois’ suspension and subsequent reinstatement of customer disconnections and late fee charges for nonpayment and Ameren Missouri’s accounting authority orders related to the COVID-19 pandemic, see Note 2 – Rate and Regulatory Matters below. 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 for an interim period may not give a true indication of results that may be expected for a full year. These financial statements should be read in conjunction with the financial statements and accompanying notes included in the Form 10-K. Variable Interest Entities As of September 30, 2021, and December 31, 2020, Ameren had unconsolidated variable interests as a limited partner in various equity method investments, totaling $53 million and $37 million, respectively, included in “Other assets” on Ameren’s consolidated balance sheet. Ameren is not the primary beneficiary of these investments because it does not have the power to direct matters that most significantly affect the activities of these variable interest entities. As of September 30, 2021, the maximum exposure to loss related to these variable interests is limited to the investment in these partnerships of $53 million plus associated outstanding funding commitments of $27 million. Company-owned Life Insurance Ameren and Ameren Illinois have company-owned life insurance, which is recorded at the net cash surrender value. The net cash surrender value is the amount that can be realized under the insurance policies at the balance sheet date. As of September 30, 2021, the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $273 million (December 31, 2020 – $272 million) and $115 million (December 31, 2020 – $115 million), respectively, while total borrowings against the policies were $109 million (December 31, 2020 – $107 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.

Rate And Regulatory Matters	9 Months Ended
Rate And Regulatory Matters	Sep. 30, 2021
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 legal proceedings. See Note 2 – Rate and Regulatory Matters under Part II, Item 8, of the Form 10-K for additional information and a summary of our regulatory frameworks. We are unable to predict the ultimate outcome of these matters, the timing of final decisions of the various agencies and courts, or the impact on our results of operations, financial position, or liquidity. Missouri 2021 Electric Service Regulatory Rate Review In March 2021, Ameren Missouri filed a request with the MoPSC seeking approval to increase its annual revenues for electric service by $299 million. The electric rate increase request is based on a 9.9% ROE, a capital structure composed of 51.9% common equity, a rate base of $10.0 billion, and a test year ended December 31, 2020, with certain pro-forma adjustments expected through a true-up date of September 30, 2021. Ameren Missouri also requested the continued use of the FAC and trackers for pension and postretirement benefits, uncertain income tax positions, and certain excess deferred income taxes that the MoPSC previously authorized in earlier electric rate orders. Additionally, Ameren Missouri requested to recover certain estimated costs associated with the Meramec Energy Center, which is expected to be retired in 2022, over a five-year period. Ameren Missouri requested the use of a tracker for any variances between certain costs collected in customer rates associated with the Meramec Energy Center and actual recoverable costs incurred after the date new rates become effective, which would be considered for recovery or refund in a future electric regulatory rate review. The electric rate increase request reflects the following: •increased infrastructure investments made under Ameren Missouri’s Smart Energy Plan; •the impact of the transition to a cleaner generation portfolio, including advancing the retirement dates of the Sioux and Rush Island energy centers consistent with Ameren Missouri’s 2020 IRP and 700 MWs of wind generation investment for the High Prairie and Atchison renewable energy centers, which are mitigated by reductions resulting from the request to recover certain Meramec Energy Center costs over a five-year period and the associated tracker; •decreased weather-normalized customer sales volumes; and •increased pension and other post-retirement benefits and tax amortization expenses, partially offset by decreased other operations and maintenance expenses. In October 2021, the MoPSC staff recommended an increase to Ameren Missouri's annual electric service revenues of $188 million based on a 9.5% ROE, a capital structure composed of 50.32% common equity, which was Ameren Missouri’s June 2021 capital structure, and a rate base of $10.0 billion. Ameren Missouri anticipates that the MoPSC staff will update its recommendation for Ameren Missouri’s September 2021 capital structure. The MoPSC staff supported the authorization of the proposed Meramec Energy Center tracker, but recommended tracking certain incremental costs. The MoPSC staff’s recommendation includes advancing the retirement dates of the Sioux and Rush Island energy centers consistent with Ameren Missouri’s request, but also includes lower depreciation, primarily on distribution investments, than reflected in Ameren Missouri’s request and a lower estimated headcount, resulting in lower payroll and benefit costs, among other things. The MoOPC and the MIEC are challenging the prudency of the High Prairie Energy Center investment as a result of the curtailed nighttime operations at the energy center to limit its impact on protected species. In September 2021, the MoOPC challenged approximately 25% of the costs and requested return associated with the High Prairie Energy Center investment included in Ameren Missouri’s requested revenue requirement. Also, in September 2021, the MIEC challenged approximately 20% of the requested return on the High Prairie Energy Center investment included in Ameren Missouri's revenue requirement and, in October 2021, challenged a portion of the return collected in 2021 through the RESRAM. Also, in October 2021, the MIEC proposed further adjustments to lower FAC and RESRAM rates, due to higher fuel costs and lower production tax credits generated as a result of the curtailed operations. See Note 9 – Commitments and Contingencies for additional information on the curtailed nighttime operations at the High Prairie Energy Center. A decision by the MoPSC is expected by mid-February 2022, with new rates effective by late February 2022. Ameren Missouri cannot predict the level of any electric service rate change the MoPSC may approve, whether the requested regulatory recovery mechanisms will be approved, or whether any rate change that may eventually be approved will be sufficient for Ameren Missouri to recover its costs and earn a reasonable return on its investments when the rate change goes into effect. Atchison Renewable Energy Center In January 2021, Ameren Missouri acquired a 300-MW wind generation project located in northwestern Missouri. As of June 30, 2021, Ameren Missouri had placed the project in service as the Atchison Renewable Energy Center. The purchase price of the energy center was approximately $500 million, including an immaterial amount of transaction costs. The Atchison Renewable Energy Center will support Ameren Missouri’s compliance with the Missouri renewable energy standard. 2021 Natural Gas Delivery Service Regulatory Rate Review In March 2021, Ameren Missouri filed a request with the MoPSC seeking approval to increase its annual revenues for natural gas delivery service by $9 million. The natural gas rate increase request is based on a 9.8% ROE, a capital structure composed of 51.9% common equity, a rate base of $310 million, and a test year ended December 31, 2020, with certain pro-forma adjustments expected through a true-up date of September 30, 2021. The request includes the continued use of the PGA, ISRS, and DCA and trackers for pension and other postretirement benefits and certain excess deferred taxes that the MoPSC previously authorized in earlier natural gas rate orders. In October 2021, the MoPSC staff recommended an increase to Ameren Missouri's annual natural gas delivery service revenues of $4 million based on a 9.5% ROE, a capital structure composed of 50.32% common equity, which was Ameren Missouri’s June 2021 capital structure, and a rate base of $301 million. Ameren Missouri anticipates that the MoPSC staff will update its recommendation for Ameren Missouri’s September 2021 capital structure. The MoPSC staff’s recommendation includes a lower estimated headcount, resulting in lower payroll and benefit costs than reflected in Ameren Missouri’s request, among other things. A decision by the MoPSC is expected by mid-February 2022, with new rates effective by late February 2022. Ameren Missouri cannot predict the level of any natural gas delivery service rate change the MoPSC may approve, or whether any rate change that may eventually be approved will be sufficient for Ameren Missouri to recover its costs and earn a reasonable return on its investments when the rate change goes into effect. Accounting Authority Orders Related to COVID-19 Pandemic Costs In March 2021, the MoPSC issued orders approving nonunanimous stipulation and agreements related to Ameren Missouri’s electric and natural gas service accounting authority order requests. The orders allowed Ameren Missouri to accumulate $9 million of certain costs incurred related to the COVID-19 pandemic, net of cost savings, as well as forgone customer late fee and reconnection fee revenues from March 2020 to March 2021, for potential recovery in the electric and natural gas service regulatory rate reviews discussed above. In March 2021, Ameren Missouri deferred other operations and maintenance expenses of $5 million as a regulatory asset related to the accounting authority orders. If approved for recovery, Ameren Missouri would recognize the remaining $4 million associated with forgone customer late fee and reconnection fee revenue when billed to customers. In the electric and natural gas regulatory rate reviews discussed above, the MoPSC staff supported the recovery of the regulatory asset and forgone customer late fee and reconnection fee revenues. MEEIA In September 2021, the MoPSC issued an order approving Ameren Missouri’s energy savings results for the second year of the MEEIA 2019 program. As a result of this order and a MoPSC order issued in August 2020, and in accordance with revenue recognition guidance, Ameren Missouri recognized revenues of $9 million and $6 million in the third quarter of 2021 and 2020, respectively. In October 2021, the MoPSC issued an order approving Ameren Missouri’s request to implement the 2023 program year of its six-year MEEIA 2019 program. The order established performance incentives that would provide Ameren Missouri an opportunity to earn additional revenues, including $13 million if Ameren Missouri achieves certain energy-efficiency goals during the 2023 program year. Ameren Missouri intends to invest $75 million in energy-efficiency programs during the 2023 program year. Extension of PGA Recovery In October 2021, the MoPSC issued an order allowing Ameren Missouri to extend the collection period for the cumulative PGA under-recovery as of August 2021, which includes the February 2021 under-recovery of $53 million, from 12 months to 36 months beginning November 2021, to lessen the impact on customer rates. Similar to other deferrals under the PGA, the deferral associated with the February 2021 under-recovery will earn carrying costs at short-term interest rates. Illinois CEJA The CEJA was enacted in September 2021. The CEJA resulted in changes to the regulatory framework applicable to Ameren Illinois’ electric distribution business by giving Ameren Illinois the option to file an MYRP with the ICC by mid-January 2023, with rates effective beginning in 2024, among other things. Ameren Illinois has the option to file for an MYRP every four years. Subject to stakeholder workshops regarding the MYRP ratemaking process, including establishing performance metrics, Ameren Illinois anticipates filing an MYRP for rates effective beginning in 2024. If it does not file then, its next opportunity to file an MYRP would be for rates effective beginning in 2028. Ameren Illinois expects to continue to use the current IEIMA performance-based formula ratemaking framework to establish annual customer rates effective through 2023 and reconcile the related revenue requirements. In order to utilize the IEIMA reconciliation, Ameren Illinois must file either a traditional regulatory rate review or an MYRP pursuant to the CEJA by mid-January 2023. Under the MYRP, the ICC would approve base rates for electric distribution service to be charged to customers for each calendar year of a four-year period. The base rates for a particular calendar year would be based on forecasted recoverable costs and an ROE approved by the ICC applied to Ameren Illinois’ forecasted average annual rate base using a forecasted capital structure, with a common equity ratio of up to 50% being deemed prudent and reasonable by law and a higher equity ratio requiring specific ICC approval. The ROE approved by the ICC is applicable to each calendar year of the four-year period. Under a traditional regulatory rate review, the revenue requirement may be based on a future test year and would include an ROE approved by the ICC. Ameren Illinois’ existing riders remain effective whether it elects to file an MYRP or a traditional regulatory rate review. Additionally, electric distribution service revenues would continue to be decoupled from sales volumes under either election. The MYRP would also allow Ameren Illinois to reconcile electric distribution service rates to its actual revenue requirement on an annual basis, subject to a reconciliation cap, which provides that the actual revenue requirement does not exceed 105% of the annual revenue requirement approved by the ICC. Certain variations from forecasted costs would be excluded from the reconciliation cap, including those associated with major storms; new business and facility relocations; changes in the timing of expenditures or investments into or out of the applicable calendar year; and changes in interest rates, income taxes, taxes other than income taxes, pension and other post-retirement benefits costs, and amortization of certain regulatory assets. The reconciliation cap would also exclude costs recovered through riders outside of base rates, such as riders for electric energy-efficiency investments, power procurement and transmission services, renewable energy credit compliance, zero emission credits, certain environmental costs, and bad debt write-offs, among others. The actual revenue requirement for a particular year would incorporate Ameren Illinois’ year-end rate base and actual capital structure for such year, provided that the common equity ratio in such capital structure may not exceed that approved by the ICC in the MYRP. Additionally, the ROE approved by the ICC in the MYRP would be subject to annual adjustments during the four-year period based on certain performance metrics relating to delivery system reliability, supplier diversity, affordability of customer delivery service cost, customer service performance, timeliness of response to customer requests for interconnection of distributed energy resources, and reductions in peak load due to demand response programs, with aggregate symmetrical performance-based ROE incentives and penalties ranging from 20 to 60 basis points annually. Excluding potential phase-in of the initial rate increase discussed below, if a given year’s revenue requirement varies from the amount collected from customers, an adjustment would be made to electric operating revenues with an offset to a regulatory asset or liability to reflect that year’s actual revenue requirement, independent of actual sales volumes. The regulatory balance would then be collected from, or refunded to, customers within two years from the end of the applicable annual period. Under an MYRP, the CEJA permits any initial rate increase to be phased in, with at least 50% of the first annual period’s approved rate increase reflected in rates in the first annual period, with the remaining portion deferred as a regulatory asset and collected from customers over a period not to exceed two years beginning within one year after the second annual period’s rates are effective. Ameren Illinois recognizes revenues when amounts are expected to be collected from customers within two years from the end of an applicable year. The CEJA contains other provisions in addition to the ratemaking impacts discussed above. The law permits Ameren Illinois to invest up to $20 million in each of two solar generation and battery storage pilot projects in Illinois. Additionally, the law increases the existing customer surcharge for renewable energy resources, which funds IPA renewable energy credit procurement events. As a result, Ameren Illinois expects additional annual revenues of approximately $100 million to be collected under the rider for renewable energy credit compliance. It also establishes an Energy Transition Assistance Fund to support economic and workforce development programs designed to assist the state of Illinois with its transition to clean energy sources. The fund will be subsidized through customer surcharges collected by electric utilities operating in the state, including Ameren Illinois, and will be remitted in the month following collection to an Illinois state agency. Ameren Illinois expects to collect up to $50 million annually related to this fund. The CEJA also requires Ameren Illinois to file a multi-year integrated grid plan with the ICC to ensure electric distribution infrastructure investments align with the state of Illinois’ renewable energy, climate, and environmental goals, and to support grid modernization, clean energy, and energy efficiency, while providing electric distribution service to customers at affordable rates, among other things. The first multi-year integrated grid plan is required to be filed by mid-January 2023, with the next filing required by mid-January 2026, and every four years thereafter. See Note 9 – Commitments and Contingencies for additional information on emission standards established by the CEJA that will limit the operations of Ameren Missouri’s natural gas-fired energy centers located in the state of Illinois. Electric Distribution Service Rate Reconciliation Tariff In March 2021, the ICC issued an order approving Ameren Illinois’ requested tariff to reconcile its electric distribution service revenue requirement once Ameren Illinois ceases to update customer rates under performance-based formula ratemaking. The last update under such ratemaking is anticipated to be for 2023 customer rates. The tariff would allow Ameren Illinois to reconcile its revenue requirement for customer rates established for 2022 and 2023. To utilize the reconciliation, Ameren Illinois is required to file a request to update its electric distribution service rates through either a traditional regulatory rate review, which may be based on a future test year and would reflect a proposed ROE subject to ICC approval, or through the filing of an MYRP, which Ameren Illinois expects to file for rates effective beginning in 2024 pursuant to the CEJA as described above. Based on AIC’s anticipated last year setting rates under IEIMA performance-based formula ratemaking, the rate update request would need to be filed by mid-January 2023. Pursuant to the order, Ameren Illinois’ 2022 and 2023 revenues would reflect each year’s actual recoverable costs, year-end rate base, and a return at the applicable WACC, with the ROE component based on the annual average of the monthly yields of the 30-year United States Treasury bonds plus 580 basis points. The revenue requirement reconciliation adjustment would be collected from, or refunded to, customers within two years from the end of the reconciled year. Electric Distribution Service Rates Under IEIMA In April 2021, Ameren Illinois filed its annual electric distribution service performance-based formula rate update to be used for 2022 rates with the ICC. In August 2021, Ameren Illinois filed a revised request seeking to increase its annual revenues for electric distribution service by $59 million. This update reflects an increase to the annual performance-based formula rate based on 2020 actual recoverable costs and expected net plant additions for 2021, an increase to include the 2020 revenue requirement reconciliation adjustment including a capital structure composed of 51% common equity, and an increase for the conclusion of the 2019 revenue requirement reconciliation adjustment, which will be fully refunded to customers in 2021, consistent with the ICC’s December 2020 annual update filing order. In August 2021, the ICC staff submitted its calculation of the revenue requirement included in Ameren Illinois’ update filing, recommending an amount comparable to Ameren Illinois’ filing. An ICC decision in this proceeding is required by December 2021, with new rates effective in January 2022. Electric Customer Energy-Efficiency Investments In May 2021, Ameren Illinois filed its annual electric customer energy-efficiency formula rate update to be used for 2022 rates with the ICC. In September 2021, Ameren Illinois filed a revised request to increase its rates by $10 million, which is comparable to a calculation submitted by the ICC staff in August 2021. An ICC decision in this proceeding is required by December 2021, with new rates effective in January 2022. In July 2021, the ICC issued an order approving Ameren Illinois’ energy-efficiency plan that includes annual investments in electric energy-efficiency programs of approximately $100 million per year from 2022 through 2025. Pursuant to the CEJA, the annual investments in electric energy-efficiency programs may increase by approximately $10 million to $40 million, with the increase depending on the election of certain customers to participate in the programs. Those customers have until late January 2022 to decide if they will participate in the four-year programs. The ICC has the ability to reduce the amount of electric energy-efficiency savings goals in future plan program years if there are insufficient cost-effective programs available, which could reduce the investments in electric energy-efficiency programs. The electric energy-efficiency program investments and the return on those investments are collected from customers through a rider and are not recovered through the electric distribution service performance-based formula ratemaking framework. QIP Reconciliation Hearings In March 2021, the ICC issued an order approving Ameren Illinois’ QIP reconciliation for 2018. The ICC also found that Ameren Illinois’ natural gas capital investments recovered under the QIP during 2018 were accurate and prudent. The ICC order effectively dismissed the Illinois Attorney General’s office challenge with respect to 2018 capital investments. In March 2020, Ameren Illinois filed a request with the ICC for a reconciliation hearing to determine the accuracy and prudence of natural gas capital investments recovered under the QIP rider during 2019. In August 2021, the Illinois Attorney General’s office challenged the recovery of capital investments that were made during 2019, alleging that the ICC should disallow approximately $70 million in natural gas capital investments as improper and imprudent, providing a potential over-recovery of approximately $3 million in 2019. In August 2021, the ICC staff filed testimony that supports the prudence and reasonableness of the capital investments made during 2019. Ameren Illinois’ 2019 QIP rate recovery request under review by the ICC is within the rate increase limitations allowed by law. The ICC is under no deadline to issue an order in this proceeding. Service Disconnection Moratorium From March 2020 through March 2021, the ICC limited disconnection activities and late fees for customer nonpayment to varying degrees based on customer class. In March 2021, the ICC issued an order allowing Ameren Illinois to resume disconnection activities for all residential customers through a phased-in approach, which began in April 2021 for customers with the largest past due balances and in June 2021 for all remaining residential customers. The March 2021 order also required Ameren Illinois to offer deferred payment arrangements extending to 18 months to all residential customers through June 2021. In addition, the order requires Ameren Illinois to extend the financial assistance program established by a June 2020 ICC order through 2021. Ameren Illinois is allowed to recover up to $4 million in costs incurred during 2021 related to this financial assistance program. These costs will be deferred as regulatory assets and the portion associated with Ameren Illinois’ electric distribution business will be recovered through its bad debt rider and the portion associated with its natural gas distribution business will be recovered through a special purpose rider. Federal Transmission Formula Rate Revisions In February 2020, the MISO, on behalf of Ameren Missouri, Ameren Illinois, and ATXI, filed requests with the FERC to revise each company’s transmission formula rate calculations with respect to the calculation used for materials and supplies inventories included in rate base. In May 2020, the FERC issued orders approving the revisions prospectively. In addition, the FERC declined to order refunds for earlier periods, as requested by intervenors in Ameren Illinois’ filing, but directed its audit staff to review historical rate recovery in connection with an ongoing FERC audit. In June 2020, Ameren Missouri, Ameren Illinois, and ATXI filed requests for rehearing arguing, among other things, the revisions should be applied retrospectively to include the period January 1, 2019, to June 1, 2020, and that the FERC should not require refunds for periods prior to 2019. In July 2020, the FERC denied the rehearing requests without addressing the issues raised. In July 2020, Ameren Missouri, Ameren Illinois, and ATXI filed an appeal of the July 2020 rehearing denials to the United States Court of Appeals for the District of Columbia Circuit, which is under no deadline to address the appeal. In October 2020, the FERC issued an order reaffirming its May 2020 order and denying the arguments raised in the rehearing requests filed by Ameren Missouri, Ameren Illinois, and ATXI. Regardless of the outcome of the appeal, the impacts of the May 2020 and October 2020 orders are not expected to be material to Ameren’s, Ameren Missouri’s, or Ameren Illinois’ results of operations, financial position, or liquidity. In March 2021, the FERC issued an order related to an intervenor challenge to Ameren Illinois’ 2020 transmission formula rate update. As a result of this order, in March 2021, Ameren Illinois recorded a regulatory liability of $9 million, largely as a reduction of electric operating revenues, to reflect expected refunds, including interest, primarily related to the historical rate recovery of materials and supplies inventories included in rate base. In April 2021, Ameren Illinois filed a request for rehearing with the FERC regarding its March 2021 order. In May 2021, the FERC denied the rehearing request without addressing the issues raised, and indicated it will address them in a future order. The FERC is under no deadline to issue an order, nor is it required to address the issues raised in the rehearing request. In July 2021, Ameren Illinois filed an appeal of the March 2021 order and the May 2021 rehearing denial to the United States Court of Appeals for the District of Columbia Circuit, which is under no deadline to address the appeal. FERC Complaint Cases In November 2013, a customer group filed a complaint case with the FERC seeking a reduction in the allowed base ROE 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 ROE to 10.32%, or a 10.82% total allowed ROE with the inclusion of a 50 basis point incentive adder for participation in an RTO, that was effective from late September 2016 forward. The September 2016 order also required refunds for the period November 2013 to February 2015, which were paid in 2017. In November 2019, the FERC issued an order addressing the November 2013 complaint case, which set the allowed base ROE at 9.88%, superseding the 10.32% previously ordered, and required refunds, with interest, for the periods November 2013 to February 2015 and from late September 2016 forward. In December 2019, the MISO transmission owners, including Ameren Missouri, Ameren Illinois, and ATXI, filed requests for rehearing with the FERC. In May 2020, the FERC issued an order addressing the requests for rehearing, which set the allowed base ROE at 10.02%, superseding the 9.88% previously ordered, and required refunds, with interest, for the periods November 2013 to February 2015 and from late September 2016 forward. In June 2020, various parties filed requests for rehearing with the FERC, challenging the new ROE methodology established by the May 2020 order. In July 2020, the FERC denied the rehearing requests without addressing the issues raised, and indicated it will address the requests for rehearing in a future order. Also in July 2020, Ameren Missouri, Ameren Illinois, and ATXI filed an appeal of the May 2020 order to the United States Court of Appeals for the District of Columbia Circuit challenging the refunds required for the period from September 2016 to May 2020. The court is under no deadline to address the appeal. As of September 30, 2021, Ameren and Ameren Illinois had substantially paid the refunds, including interest, associated with the allowed base ROE set by the May 2020 order in the November 2013 complaint case. The increase in the FERC-allowed base ROE resulting from the May 2020 order was not material to Ameren Missouri’s results of operations, financial position, or liquidity.

Short-Term Debt And Liquidity

9 Months Ended

Sep. 30, 2021

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.

Short-term Borrowings

The Missouri Credit Agreement and the Illinois Credit Agreement are available to support issuances under Ameren (parent)’s, Ameren Missouri’s, and Ameren Illinois’ commercial paper programs, respectively, subject to borrowing sublimits, and the issuance of letters of credit. As of September 30, 2021, based on commercial paper outstanding and letters of credit issued under the Credit Agreements, along with cash and cash equivalents, the net liquidity available to Ameren (parent), Ameren Missouri, and Ameren Illinois, collectively, was $1.8 billion. The Ameren Companies were in compliance with the covenants in their Credit Agreements as of September 30, 2021. As of September 30, 2021, the ratios of consolidated indebtedness to consolidated total capitalization, calculated in accordance with the provisions of the Credit Agreements, were 57%, 48%, and 45% for Ameren, Ameren Missouri, and Ameren Illinois, respectively.

As of September 30, 2021, and December 31, 2020, Ameren (parent)’s commercial paper outstanding, net of issuance discounts, was $553 million and $490 million, respectively. There were no borrowings outstanding under the Credit Agreements as of September 30, 2021, or December 31, 2020.

The following table summarizes the activity and relevant interest rates for Ameren (parent)’s, Ameren Missouri’s, and Ameren Illinois’ commercial paper issuances and borrowings under the Credit Agreements in the aggregate for the nine months ended September 30, 2021 and 2020:


	Ameren (parent)			Ameren Missouri				Ameren Illinois			Ameren Consolidated
2021
Average daily amount outstanding	$	404		$	121			$	140		$	665

Weighted-average interest rate	0.23		%	0.22		%		0.22		%	0.22		%
Peak amount outstanding during period(a)	$	650		$	546			$	485		$	1,134
Peak interest rate	0.33		%	0.25		%		0.25		%	0.33		%
2020
Average daily amount outstanding	$	62		$	142			$	53		$	257

Weighted-average interest rate	2.04		%	1.76		%		1.10		%	1.70		%
Peak amount outstanding during period(a)	$	425		$	573			$	243		$	908
Peak interest rate	3.30		%	5.05		%	(b)	3.40		%	5.05		%	(b)

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

(b)Ameren’s and Ameren Missouri’s peak interest rate was affected by temporary disruptions in the commercial paper market in the first quarter of 2020.

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 utility money pool for the three and nine months ended September 30, 2021, was 0.06% and 0.17%, respectively (2020 – 0.10% and 0.81%, respectively). See Note 8 – Related-party Transactions for the amount of interest income and expense from the utility money pool arrangements recorded by Ameren Missouri and Ameren Illinois for the three and nine months ended September 30, 2021 and 2020.

Long-Term Debt And Equity Financings	9 Months Ended
Long-Term Debt And Equity Financings	Sep. 30, 2021
Long-Term Debt And Equity Financings [Abstract]
LONG-TERM DEBT AND EQUITY FINANCINGS	LONG-TERM DEBT AND EQUITY FINANCINGS Ameren For the three and nine months ended September 30, 2021, Ameren issued a total of 0.1 million and 0.4 million shares of common stock, respectively, under its DRPlus and 401(k) plan, and received proceeds of $12 million and $36 million, respectively. In addition, in the first quarter of 2021, Ameren issued 0.5 million shares of common stock valued at $33 million upon the settlement of stock-based compensation awards. In February 2021, Ameren settled the remainder of a forward sale agreement that was entered into in August 2019, by physically delivering 1.6 million shares of common stock for cash proceeds of $113 million. The proceeds were used to fund a portion of Ameren Missouri’s wind generation investments. See Note 2 – Rate and Regulatory Matters in this report and under Part II, Item 8, in the Form 10-K for additional information about the wind generation investments. In May 2021, Ameren entered into an equity distribution sales agreement pursuant to which Ameren may offer and sell from time to time up to $750 million of its common stock through an ATM program, which includes the ability to enter into forward sales agreements. For the three and nine months ended September 30, 2021, Ameren issued 0.4 million and 1.8 million shares of common stock, respectively, and received proceeds of $27 million and $148 million, respectively. These proceeds were net of less than $1 million and $2 million in compensation paid to selling agents for the three and nine months ended September 30, 2021, respectively. In September 2021, Ameren entered into a forward sale agreement under the ATM program with a counterparty relating to 0.4 million shares of common stock. The forward sale agreement can be settled at Ameren’s discretion on or prior to May 3, 2023. On a settlement date or dates, if Ameren elects to physically settle the forward sale agreement, Ameren will issue shares of common stock to the counterparty at the then-applicable forward sale price. The forward sale price was initially $87.87 per share. The forward sale agreement will be physically settled unless Ameren elects to settle in cash or to net share settle. At September 30, 2021, Ameren could have settled the forward sale agreement with physical delivery of 0.4 million shares of common stock to the counterparty in exchange for cash of $31 million. The forward sale agreement has been classified as an equity transaction. In March 2021, Ameren (parent) issued $450 million of 1.75% senior unsecured notes due March 2028, with interest payable semiannually on March 15 and September 15 of each year, beginning September 15, 2021. Ameren received net proceeds of $447 million, which were used for general corporate purposes, including the repayment of short-term debt. Ameren Missouri In June 2021, Ameren Missouri issued $525 million of 2.15% first mortgage bonds due March 2032, with interest payable semiannually on March 15 and September 15 of each year, beginning March 15, 2022. Ameren Missouri received net proceeds of $521 million, which were used to repay short-term debt and for near-term capital expenditures. Ameren Missouri intends to allocate an amount equal to the net proceeds to sustainability projects meeting certain eligibility criteria. Ameren Missouri received capital contributions totaling $183 million from Ameren (parent) during the nine months ended September 30, 2021. Ameren Illinois In March 2021, Ameren Illinois redeemed its 6.625% and 7.75% series preferred stock at par for $12 million and $1 million, respectively. The preferred stock of Ameren Illinois is reflected in “Noncontrolling Interests” on Ameren’s consolidated balance sheet. In June 2021, Ameren Illinois issued $350 million of 2.90% first mortgage bonds due June 2051, with interest payable semiannually on June 15 and December 15 of each year, beginning December 15, 2021. Ameren Illinois received net proceeds of $345 million, which were used to repay short-term debt. Ameren Illinois intends to allocate an amount equal to the net proceeds to sustainability projects meeting certain eligibility criteria. In June 2021, Ameren Illinois issued $100 million of 0.375% first mortgage bonds due June 2023, with interest payable semiannually on June 15 and December 15 of each year, beginning December 15, 2021. Ameren Illinois received net proceeds of $100 million, which were used to repay short-term debt. Ameren Illinois received capital contributions totaling $145 million from Ameren (parent) during the nine months ended September 30, 2021. Indenture Provisions and Other Covenants See Note 5 – Long-term Debt and Equity Financings under Part II, Item 8, in the Form 10-K for a description of our indenture provisions and other covenants, as well as restrictions on the payment of dividends. At September 30, 2021, the Ameren Companies were in compliance with the provisions and covenants contained in their indentures and articles of incorporation, as applicable, and ATXI was in compliance with the provisions and covenants contained in its note purchase agreement. Off-balance-sheet Arrangements At September 30, 2021, none of the Ameren Companies had any significant off-balance-sheet financing arrangements, other than variable interest entities and the September 2021 forward sale agreement relating to common stock. See Note 1 – Summary of Significant Accounting Policies for further detail concerning variable interest entities.

Other Income and Expenses

9 Months Ended

Sep. 30, 2021

Other Nonoperating Income (Expense) [Abstract]

OTHER INCOME AND EXPENSES

OTHER INCOME, NET

The following table presents the components of “Other Income, Net” in the Ameren Companies’ statements of income for the three and nine months ended September 30, 2021 and 2020:


	Three Months				Nine Months
	2021		2020		2021		2020
Ameren:

Allowance for equity funds used during construction	$	14	$	12	$	30	$	25
Interest income on industrial development revenue bonds	7		7		19		19
Other interest income	—		—		1		2

Non-service cost components of net periodic benefit income(a)	34		32		102		85
Miscellaneous income	3		1		14		10
Donations	(1)		—		(5)		(14)		(b)
Miscellaneous expense	(1)		(4)		(10)		(10)
Total Other Income, Net	$	56	$	48	$	151	$	117
Ameren Missouri:

Allowance for equity funds used during construction	$	7	$	7	$	17	$	15
Interest income on industrial development revenue bonds	7		7		19		19

Non-service cost components of net periodic benefit income(a)	13		13		41		32
Miscellaneous income	2		1		3		3
Donations	—		—		(1)		(9)		(b)
Miscellaneous expense	(2)		(2)		(5)		(5)
Total Other Income, Net	$	27	$	26	$	74	$	55
Ameren Illinois:

Allowance for equity funds used during construction	$	7	$	5	$	13	$	10
Interest income	—		—		1		2

Non-service cost components of net periodic benefit income	13		12		41		36
Miscellaneous income	—		1		3		6
Donations	(1)		—		(4)		(5)
Miscellaneous expense	—		(1)		(5)		(4)
Total Other Income, Net	$	19	$	17	$	49	$	45

(a)For the three and nine months ended September 30, 2021, the non-service cost components of net periodic benefit income were adjusted by amounts deferred of $(3) million and $(6) million, 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. The deferral was $(3) million and less than $(1) million for the three and nine months ended September 30, 2020.

(b)Includes $8 million pursuant to Ameren Missouri’s March 2020 electric rate order. See Note 2 – Rate and Regulatory Matters under Part II, Item 8, in the Form 10-K for additional information.

Derivative Financial Instruments

9 Months Ended

Sep. 30, 2021

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, power, and uranium, 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 and uranium inventories that differ from the cost of those commodities in inventory;

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

•actual off-system sales revenues that differ from anticipated revenues.

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

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

If we determine that a contract meets the definition of a derivative and is not eligible for the NPNS exception, we review the contract to determine whether the resulting gains or losses qualify for regulatory deferral. Derivative contracts that qualify for regulatory deferral are recorded at fair value, with changes in fair value recorded as regulatory assets or liabilities in the period in which the change occurs. We believe derivative losses and gains deferred as regulatory assets and liabilities are probable of recovery, or refund, through future rates charged to customers. Regulatory assets and liabilities are amortized to operating income as related losses and gains are reflected in rates charged to customers. Therefore, gains and losses on these derivatives have no effect on operating income. As of September 30, 2021, and December 31, 2020, all contracts that met the definition of a derivative and were not eligible for the NPNS exception received regulatory deferral. Cash flows for all derivative financial instruments are classified in cash flows from operating activities.

The following table presents open gross commodity contract volumes by commodity type for derivative assets and liabilities as of September 30, 2021, and December 31, 2020. As of September 30, 2021, these contracts extended through October 2024, October 2026, May 2032 and March 2023 for fuel oils, natural gas, power and uranium, respectively.


	Quantity (in millions)
	September 30, 2021			December 31, 2020
Commodity	Ameren Missouri	Ameren Illinois	Ameren	Ameren Missouri	Ameren Illinois	Ameren
Fuel oils (in gallons)	28	—	28	43	—	43
Natural gas (in mmbtu)	33	136	169	33	114	147
Power (in MWhs)	6	6	12	6	7	13
Uranium (pounds in thousands)	400	—	400	365	—	365

The following table presents the carrying value and balance sheet location of all derivative commodity contracts, none of which were designated as hedging instruments, as of September 30, 2021, and December 31, 2020:


			September 30, 2021					December 31, 2020
	Balance Sheet Location		Ameren Missouri		Ameren Illinois		Ameren			Ameren Missouri		Ameren Illinois		Ameren









Fuel oils	Other current assets	$	9	$	—	$	9		$	2	$	—	$	2
	Other assets		5		—		5			—		—		—
Natural gas	Mark-to-market derivative assets		(a)		55		(a)			(a)		8		(a)

	Other current assets		13		—		68			1		—		9
	Other assets		8		20		28			2		2		4
Power	Mark-to-market derivative assets		(a)		1		(a)			(a)		—		(a)

	Other current assets		31		—		32			7		—		7
	Other assets		1		—		1			—		—		—

Uranium	Other assets		2		—		2			—		—		—
	Total assets	$	69	$	76	$	145		$	12	$	10	$	22



Fuel oils	Mark-to-market derivative liabilities	$	—	$	(a)	$	(a)		$	7	$	(a)	$	(a)

	Other current liabilities		—		—		—			—		—		7
	Other deferred credits and liabilities		—		—		—			2		—		2


Natural gas	Other current liabilities		—		4		4			1		1		2
	Other deferred credits and liabilities		—		2		2			—		1		1
Power	Mark-to-market derivative liabilities		75		(a)		(a)			3		(a)		(a)

	Other current liabilities		—		5		80			—		17		20
	Other deferred credits and liabilities		22		140		162			8		181		189

Uranium	Other current liabilities		1		—		1			—		—		—

	Total liabilities	$	98	$	151	$	249		$	21	$	200	$	221

(a)Balance sheet line item not applicable to registrant.

We believe that entering into master netting arrangements or similar agreements mitigates the level of financial loss that could result from default by allowing net settlement of derivative assets and liabilities. These master netting arrangements allow the counterparties to net settle sale and purchase transactions. Further, collateral requirements are calculated at the master netting arrangement or similar agreement level by counterparty.

The following table provides the recognized gross derivative balances and the net amounts of those derivatives subject to an enforceable master netting arrangement or similar agreement as of September 30, 2021. If the gross amounts recognized on the balance sheet were netted with derivative instruments and cash collateral received or posted at December 31, 2020, the net amounts would not be materially different from the gross amounts.


			Gross Amounts Not Offset in the Balance Sheet
Commodity Contracts Eligible to be Offset	Gross Amounts Recognized in the Balance Sheet		Derivative Instruments		Cash Collateral Received/Posted(a)		Net Amount
2021
Assets:
Ameren Missouri	$	69	$	14	$	2	$	53
Ameren Illinois	76		2		3		71
Ameren	$	145	$	16	$	5	$	124
Liabilities:
Ameren Missouri	$	98	$	14	$	57	$	27
Ameren Illinois	151		2		—		149
Ameren	$	249	$	16	$	57	$	176

(a)Cash collateral received reduces gross asset balances and is included in “Other current liabilities” and “Other deferred credits and liabilities” on the balance sheet. Cash collateral posted reduces gross liability balances and is included in “Other current assets” and “Other assets” on the balance sheet.

Credit Risk

In determining our concentrations of credit risk related to derivative instruments, we review our individual counterparties and categorize each counterparty into groupings according to the primary business in which each engages. As of September 30, 2021, if counterparty groups were to fail completely to perform on contracts, Ameren, Ameren Missouri, and Ameren Illinois' maximum exposure related to derivative assets, predominantly from financial institutions, was $127 million, $51 million, and $76 million, respectively. The potential loss on

counterparty exposures may be reduced or eliminated by the application of master netting arrangements or similar agreements and collateral held. As of September 30, 2021, the potential loss after consideration of the application of master netting arrangements or similar agreements and collateral held for Ameren, Ameren Missouri, and Ameren Illinois was $114 million, $39 million, and $75 million, respectively.

Certain of our derivative instruments contain collateral provisions tied to the Ameren Companies’ credit ratings. If our credit ratings were downgraded below investment grade, or if a counterparty with reasonable grounds for uncertainty regarding our ability to satisfy an obligation requested adequate assurance of performance, additional collateral postings might be required. The additional collateral required is the net liability position allowed under master netting arrangements or similar agreements, assuming (1) the credit risk-related contingent features underlying these arrangements were triggered and (2) those counterparties with rights to do so requested collateral. The following table presents, as of September 30, 2021, 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.



	Aggregate Fair Value of Derivative Liabilities(a)		Cash Collateral Posted		Potential Aggregate Amount of Additional Collateral Required(b)
Ameren Missouri	$	45	$	3	$	28
Ameren Illinois	6		—		4

Ameren	$	51	$	3	$	32

(a)Before consideration of master netting arrangements or similar agreements.

(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

9 Months Ended

Sep. 30, 2021

Fair Value Disclosures [Abstract]

Fair Value Disclosures

FAIR VALUE MEASUREMENTS

Fair value is defined as the price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Fair value measurements are classified in three levels based on the fair value hierarchy as defined by GAAP. See Note 8 – Fair Value Measurements under Part II, Item 8, of the Form 10-K for information related to hierarchy levels and valuation techniques.

We consider nonperformance risk in our valuation of derivative instruments by analyzing our own credit standing and the credit standing of our counterparties, and by considering any credit enhancements (e.g., collateral). Included in our valuation, and based on current market conditions, is a valuation adjustment for counterparty default derived from market data such as the price of credit default swaps, bond yields, and credit ratings. No material gains or losses related to valuation adjustments for counterparty default risk were recorded at Ameren, Ameren Missouri, or Ameren Illinois in the three and nine months ended September 30, 2021 or 2020. At September 30, 2021, and December 31, 2020, the counterparty default risk valuation adjustment related to derivative contracts was immaterial for Ameren, Ameren Missouri, and Ameren Illinois.

The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of September 30, 2021, and December 31, 2020:


		September 30, 2021									December 31, 2020
		Level 1		Level 2		Level 3		Total			Level 1		Level 2		Level 3		Total
Assets:
Ameren Missouri
	Derivative assets – commodity contracts:
	Fuel oils	$	14	$	—	$	—	$	14		$	—	$	—	$	2	$	2
	Natural gas	—		20		1		21			—		3		—		3
	Power	12		—		20		32			2		—		5		7
	Uranium	—		—		2		2			—		—		—		—
	Total derivative assets – commodity contracts	$	26	$	20	$	23	$	69		$	2	$	3	$	7	$	12
	Nuclear decommissioning trust fund:

	Equity securities:
	U.S. large capitalization	$	743	$	—	$	—	$	743		$	680	$	—	$	—	$	680
	Debt securities:
	U.S. Treasury and agency securities	—		141		—		141			—		115		—		115
	Corporate bonds	—		128		—		128			—		115		—		115


	Other	—		56		—		56			—		67		—		67
	Total nuclear decommissioning trust fund	$	743	$	325	$	—	$	1,068	(a)	$	680	$	297	$	—	$	977	(a)
	Total Ameren Missouri	$	769	$	345	$	23	$	1,137		$	682	$	300	$	7	$	989


		September 30, 2021									December 31, 2020
		Level 1		Level 2		Level 3		Total			Level 1		Level 2		Level 3		Total
Ameren Illinois
	Derivative assets – commodity contracts:
	Natural gas	$	3	$	62	$	10	$	75		$	—	$	6	$	4	$	10
	Power	—		—		1		1			—		—		—		—
	Total Ameren Illinois	$	3	$	62	$	11	$	76		$	—	$	6	$	4	$	10
Ameren
	Derivative assets – commodity contracts(b)	$	29	$	82	$	34	$	145		$	2	$	9	$	11	$	22
	Nuclear decommissioning trust fund(c)	743		325		—		1,068		(a)	680		297		—		977		(a)
	Total Ameren	$	772	$	407	$	34	$	1,213		$	682	$	306	$	11	$	999
Liabilities:
Ameren Missouri
	Derivative liabilities – commodity contracts:
	Fuel oils	$	—	$	—	$	—	$	—		$	6	$	—	$	3	$	9
	Natural gas	—		—		—		—			—		1		—		1
	Power	54		—		43		97			8		—		3		11
	Uranium	—		—		1		1			—		—		—		—
	Total Ameren Missouri	$	54	$	—	$	44	$	98		$	14	$	1	$	6	$	21
Ameren Illinois
	Derivative liabilities – commodity contracts:
	Natural gas	$	—	$	2	$	4	$	6		$	—	$	1	$	1	$	2
	Power	—		—		145		145			—		—		198		198
	Total Ameren Illinois	$	—	$	2	$	149	$	151		$	—	$	1	$	199	$	200
Ameren
	Derivative liabilities – commodity contracts(b)	$	54	$	2	$	193	$	249		$	14	$	2	$	205	$	221

(a)Balance excludes $8 million and $5 million of cash and cash equivalents, receivables, payables, and accrued income, net, for September 30, 2021, and December 31, 2020, respectively.

(b)See the Ameren Missouri and Ameren Illinois sections of the table for a breakout of the fair value of Ameren’s derivative assets and liabilities by type of commodity.

(c)See the Ameren Missouri section of the table for a breakout of the fair value of Ameren's nuclear decommissioning trust fund by investment type.

Level 3 fuel oils, natural gas, and uranium derivative contract assets and liabilities measured at fair value on a recurring basis were immaterial for all periods presented. The following table presents the fair value reconciliation of Level 3 power derivative contract assets and liabilities measured at fair value on a recurring basis for the three and nine months ended September 30, 2021 and 2020:


	2021						2020
	Ameren Missouri		Ameren Illinois		Ameren			Ameren Missouri		Ameren Illinois		Ameren
For the three months ended September 30:
Beginning balance at July 1	$	(5)	$	(166)	$	(171)		$	16	$	(229)	$	(213)
Realized and unrealized gains/(losses) included in regulatory assets/liabilities	(16)		19		3			(2)		12		10


Settlements	(2)		3		1			(4)		4		—


Ending balance at September 30	$	(23)	$	(144)	$	(167)		$	10	$	(213)	$	(203)
Change in unrealized gains/(losses) related to assets/liabilities held at September 30	$	(17)	$	19	$	2		$	(2)	$	11	$	9
For the nine months ended September 30:
Beginning balance at January 1	$	2	$	(198)	$	(196)		$	13	$	(224)	$	(211)
Realized and unrealized gains/(losses) included in regulatory assets/liabilities	(22)		43		21			18		(2)		16


Settlements	(3)		11		8			(21)		13		(8)


Ending balance at September 30	$	(23)	$	(144)	$	(167)		$	10	$	(213)	$	(203)
Change in unrealized gains/(losses) related to assets/liabilities held at September 30	$	(21)	$	42	$	21		$	6	$	(1)	$	5

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 either net income or other comprehensive income resulting from changes in the fair value of these instruments.

The following table describes the valuation techniques and significant unobservable inputs utilized for the fair value of our Level 3 power derivative contract assets and liabilities as of September 30, 2021, and December 31, 2020:


			Fair Value						Weighted Average(b)
	Commodity		Assets		Liabilities	Valuation Technique(s)	Unobservable Input(a)	Range









2021	Power(c)	$	21	$	(188)	Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps ($/MWh)	28 – 77	40

							Nodal basis ($/MWh)	(4) – 0	(2)


							Trend rate (%)	(4) – (2)	(3)








































2020	Power(c)	$	5	$	(201)	Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps ($/MWh)	23 – 37	29

							Nodal basis ($/MWh)	(6) – 0	(2)


							Trend rate (%)	2 – 6	3

(a)Generally, significant increases (decreases) in these inputs in isolation would result in a significantly higher (lower) fair value measurement.

(b)Unobservable inputs were weighted by relative fair value.

(c)Valuations through 2029 use visible forward prices adjusted for nodal-to-hub basis differentials. Valuations beyond 2029 use a trend rate factor and are similarly adjusted for nodal-to-hub basis differentials.

The following table sets forth the carrying amount and, by level within the fair value hierarchy, the fair value of financial assets and liabilities disclosed, but not recorded, at fair value as of September 30, 2021, and December 31, 2020:


	Carrying Amount			Fair Value
				Level 1		Level 2		Level 3			Total
	September 30, 2021
Ameren:
Cash, cash equivalents, and restricted cash	$	145		$	145	$	—	$	—		$	145
Investments in industrial development revenue bonds(a)	256			—		256		—			256
Short-term debt	553			—		553		—			553
Long-term debt (including current portion)(a)	12,501		(b)	—		13,576		520		(c)	14,096
Ameren Missouri:
Cash, cash equivalents, and restricted cash	$	11		$	11	$	—	$	—		$	11
Advances to money pool	24			—		24		—			24
Investments in industrial development revenue bonds(a)	256			—		256		—			256


Long-term debt (including current portion)(a)	5,626		(b)	—		6,392		—			6,392
Ameren Illinois:
Cash, cash equivalents, and restricted cash	$	122		$	122	$	—	$	—		$	122
Advances to money pool	41			—		41		—			41


Long-term debt (including current portion)	4,391		(b)	—		5,019		—			5,019
	December 31, 2020
Ameren:
Cash, cash equivalents, and restricted cash	$	301		$	301	$	—	$	—		$	301
Investments in industrial development revenue bonds(a)	256			—		256		—			256
Short-term debt	490			—		490		—			490
Long-term debt (including current portion)(a)	11,086		(b)	—		12,778		537		(c)	13,315
Ameren Missouri:
Cash, cash equivalents, and restricted cash	$	145		$	145	$	—	$	—		$	145
Advances to money pool	139			—		139		—			139
Investments in industrial development revenue bonds(a)	256			—		256		—			256

Long-term debt (including current portion)(a)	5,104		(b)	—		6,160		—			6,160
Ameren Illinois:
Cash, cash equivalents, and restricted cash	$	147		$	147	$	—	$	—		$	147
Borrowings from money pool	19			—		19		—			19
Long-term debt (including current portion)	3,946		(b)	—		4,822		—			4,822

(a)Ameren and Ameren Missouri have investments in industrial development revenue bonds, classified as held-to-maturity and recorded in “Other Assets,” that are equal to the finance obligations for the Peno Creek and Audrain CT energy centers. As of September 30, 2021, and December 31, 2020, 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 $93 million, $39 million, and $40 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of September 30, 2021. Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $84 million, $36 million, and $36 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2020.

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

- Definition

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

+ References + Details

Related Party Transactions

9 Months Ended

Sep. 30, 2021

RELATED PARTY TRANSACTIONS

RELATED-PARTY TRANSACTIONS

In the ordinary 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. For information on Ameren Missouri’s and Ameren Illinois’ capital contributions, see Note 4 – Long-term Debt and Equity Financings.

Electric Power and Capacity Supply Agreement

In April and September 2021, Ameren Illinois conducted procurement events, administered by the IPA, to purchase energy products and acquire capacity. Ameren Missouri was among the winning suppliers in this event. As a result, in April 2021, Ameren Missouri and Ameren Illinois entered into an energy product agreement by which Ameren Missouri agreed to sell, and Ameren Illinois agreed to purchase, 33,600 MWhs at an average price of $34 per MWh during the period of July 2022 through November 2022. In September 2021, Ameren Missouri contracted to supply a portion of Ameren Illinois’ capacity requirements for $2 million from June 2022 through May 2023. Additionally, in September 2021, Ameren Missouri and Ameren Illinois entered into an energy product agreement by which Ameren Missouri agreed to sell, and Ameren Illinois agreed to purchase, 136,000 MWhs at an average price of $37 per MWh during the period of January 2022 through September 2023.

Tax Allocation Agreement

See Note 1 – Summary of Significant Accounting Policies under Part II, Item 8, of the Form 10-K for a discussion of the tax allocation agreement. The following table presents the affiliate balances related to income taxes for Ameren Missouri and Ameren Illinois as of September 30, 2021, and December 31, 2020:


	September 30, 2021				December 31, 2020
	Ameren Missouri		Ameren Illinois		Ameren Missouri		Ameren Illinois
Income taxes payable to parent(a)	$	—	$	12	$	—	$	6
Income taxes receivable from parent(b)	—		44		9		15

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

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

Effects of Related-party Transactions on the Statement of Income

The following table presents the effect on Ameren Missouri and Ameren Illinois of related-party transactions for the three and nine months ended September 30, 2021 and 2020:


				Three Months				Nine Months
Agreement	Income Statement Line Item			Ameren Missouri		Ameren Illinois		Ameren Missouri		Ameren Illinois
Ameren Missouri power supply	Operating Revenues	2021	$	5	$	(a)	$	10	$	(a)
agreements with Ameren Illinois		2020		5		(a)		11		(a)
Ameren Missouri and Ameren Illinois	Operating Revenues	2021	$	6	$	(b)	$	20	$	(b)
rent and facility services		2020		6		(b)		19		1
Ameren Missouri and Ameren Illinois miscellaneous	Operating Revenues	2021	$	(b)	$	3	$	(b)	$	5
support services and services provided to ATXI		2020		(b)		1		1		1
Total Operating Revenues		2021	$	11	$	3	$	30	$	5
		2020		11		1		31		2
Ameren Illinois power supply	Purchased Power	2021	$	(a)	$	5	$	(a)	$	10
agreements with Ameren Missouri		2020		(a)		5		(a)		11
Ameren Missouri and Ameren Illinois	Purchased Power	2021	$	1	$	(b)	$	3	$	1
transmission services from ATXI		2020		(a)		(b)		(a)		1
Total Purchased Power		2021	$	1	$	5	$	3	$	11
		2020		(a)		5		(a)		12
Ameren Missouri and Ameren Illinois	Other Operations and Maintenance	2021	$	(b)	$	1	$	(b)	$	3
rent and facility services		2020		(b)		1		(b)		3


				Three Months				Nine Months
Agreement	Income Statement Line Item			Ameren Missouri		Ameren Illinois		Ameren Missouri		Ameren Illinois
Ameren Services support services	Other Operations and Maintenance	2021	$	41	$	37	$	110	$	101
agreement		2020		36		34		103		98
Total Other Operations and		2021	$	41	$	38	$	110	$	104
Maintenance		2020		36		35		103		101
Money pool borrowings (advances)	(Interest Charges)/Other Income, Net	2021	$	(b)	$	(b)	$	(b)	$	(b)
		2020		(b)		(b)		(b)		(b)

(a)Not applicable.

(b)Amount less than $1 million.

Commitments And Contingencies	9 Months Ended
Commitments And Contingencies	Sep. 30, 2021
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 9 – Callaway Energy Center, 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. Environmental Matters Our electric and natural gas generation, transmission, distribution and natural gas storage operations must comply with a variety of environmental statutory and regulatory requirements, including permitting programs implemented via federal, state, and local authorities. Such laws address air emissions; discharges to water bodies; the storage, handling, and disposal of hazardous substances and waste materials; siting and land use requirements; and potential ecological impacts. 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. We employ dedicated personnel knowledgeable in environmental matters to oversee our business activities’ compliance with regulatory requirements. Environmental regulations have a significant impact on the electric utility industry and compliance with these regulations could be costly for Ameren Missouri, which operates coal-fired power plants. Clean Air Act regulations that apply to 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, NOx, mercury, toxic metals and acid gases, and CO2 emissions from new power plants. Regulations implementing the Clean Water Act govern both intake and discharges of water, and may require evaluation of the ecological and biological impact of our operations and could require modifications to water intake structures or more stringent limitations on wastewater discharges. Depending upon the scope of modifications ultimately required by state regulators, these capital expenditures could be significant. The management and disposal of coal ash is regulated under the Resource Conservation and Recovery Act and the CCR rule, which require the closure of our surface impoundments at Ameren Missouri’s coal-fired energy centers. The individual or combined effects of existing and new 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 $175 million to $225 million from 2021 through 2025 in order to comply with existing environmental regulations. Additional environmental controls beyond 2025 could be required. This estimate of capital expenditures includes ash pond closure and corrective action measures required by the CCR regulations and the effluent limitation guidelines applicable to steam electric generating units, and potential modifications to cooling water intake structures at existing power plants under Clean Water Act rules, all of which are discussed below. This estimate does not include capital expenditures that may be required as a result of the NSR and Clean Air Act litigation discussed below. In addition to planned retirements of coal-fired energy centers as set forth in the 2020 IRP, Ameren Missouri’s current plan for compliance with existing air emission regulations includes burning low-sulfur coal and installing new or optimizing existing air pollution control equipment. The actual amount of capital expenditures required to comply with existing environmental regulations may vary substantially from the above estimates because of uncertainty as to future permitting requirements made by state regulators and the EPA, potential revisions to regulatory obligations, and the cost of potential compliance strategies, 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. Additionally, Ameren Missouri’s wind generation facilities may be subject to operating restrictions to limit the impact on protected species. From April 2021 through October 2021, Ameren Missouri's High Prairie Renewable Energy Center curtailed nighttime operations to limit impacts on protected species. Ameren Missouri expects to resume nighttime operations in November 2021 as the critical biological season for the year had ended, but seasonal nighttime curtailment may occur in 2022 as assessment of mitigation technologies to reduce such impacts is ongoing. Ameren Missouri does not anticipate material adverse effects resulting from these operating curtailments. See Note 2 – Rate and Regulatory Matters for information on intervenors’ challenges resulting from the High Prairie Renewable Energy Center’s curtailment in Ameren Missouri’s 2021 electric service regulatory rate review. Clean Air Act Federal and state laws, including CSAPR, regulate emissions of SO2 and NOx through the reduction of emissions at their source and the use and retirement of emission allowances. CSAPR is implemented through a series of phases, and the second phase became effective in 2017. Additional emission reduction requirements may apply in subsequent years. Ameren Missouri complies with current CSAPR requirements by minimizing emissions through the use of low-sulfur coal, operation of two scrubbers at its Sioux Energy Center, and optimization of other existing air pollution control equipment. Ameren Missouri could incur additional costs to lower its emissions at one or more of its energy centers to comply with additional CSAPR requirements in future years. These additional costs for compliance are expected to be recovered from customers through the FAC or higher base rates. CO2 Emissions Standards The EPA’s Affordable Clean Energy Rule repealed the Clean Power Plan and replaced it with a new rule that established emission guidelines for states to follow in developing plans to limit CO2 emissions and identified certain efficiency measures as the best system of emission reduction for coal-fired electric generating units. In January 2021, the United States Court of Appeals for the District of Columbia Circuit vacated the Affordable Clean Energy Rule, and ruled that the EPA had the discretion to consider emission reduction measures that include efficiency measures and generation shifting to lower carbon emissions. In October 2021, the United States Supreme Court agreed to review the circuit court’s ruling, and a decision is expected to occur in mid-2022. Actions by the United States Supreme Court could impact the EPA’s development of new regulations to address carbon emissions from coal and natural gas electric generating units. At this time, Ameren Missouri cannot predict the outcome of legal challenges to the vacated Affordable Clean Energy Rule or future rulemakings. As such, the impact on the results of operations, financial position, and liquidity of Ameren and Ameren Missouri is uncertain. NSR and Clean Air Litigation In January 2011, the United States Department of Justice, on behalf of the EPA, filed a complaint against Ameren Missouri in the United States District Court for the Eastern District of Missouri alleging that in performing projects at its coal-fired Rush Island Energy Center in 2007 and 2010, Ameren Missouri violated provisions of the Clean Air Act and Missouri law. In January 2017, the district court issued a liability ruling against Ameren Missouri and in September 2019, entered a remedy order that required Ameren Missouri to install a flue gas desulfurization system at the Rush Island Energy Center and a dry sorbent injection system at the Labadie Energy Center. Ameren Missouri appealed both the liability and remedy orders and, in August 2021, a three-judge panel of the United States Court of Appeals for the Eighth Circuit issued a decision that affirmed the liability ruling and the district court’s remedy order as it related to the installation of a flue gas desulfurization system at the Rush Island Energy Center, but reversed the order as it related to the installation of a dry sorbent injection system at the Labadie Energy Center. Ameren Missouri believes that both the court of appeals and the district court have misinterpreted and misapplied the law with respect to the Rush Island Energy Center in their respective rulings. In October 2021, Ameren Missouri sought reconsideration of the panel’s decision before the entire court of appeals, and the United States Department of Justice sought reconsideration of the panel’s decision rejecting the requirement to install a dry sorbent injection system at the Labadie Energy Center. Under the terms of the remedy order, Ameren Missouri is required to complete the installation of the flue gas desulfurization system at the Rush Island Energy Center within four and one-half years from the conclusion of the appeal process and entry of a final judgement. Based upon engineering studies from October 2019, capital expenditures to comply with the district court’s order for installation of a flue gas desulfurization system at the Rush Island Energy Center were estimated at up to $1 billion along with additional operation and maintenance expenses of $30 million to $50 million annually for the life of the energy center. Ameren Missouri’s 2020 IRP reflected an expected retirement date of 2039 for the Rush Island Energy Center and did not reflect the installation of a flue gas desulfurization system at the Rush Island Energy Center. As of September 30, 2021, the Rush Island Energy Center represented a rate base of approximately $0.4 billion. Ameren Missouri is assessing several alternatives to effectively address the court of appeals decision, including legal, operational, and regulatory measures. To the extent the resolution of the proceedings described above results in changes to the planned timing of energy center retirements, the timing and level of new generation resources, transmission infrastructure needs, or reliability considerations, Ameren Missouri would file an update to its 2020 IRP with the MoPSC. Ameren Missouri is unable to predict the ultimate resolution of this matter; however, such resolution could have a material adverse effect on the results of operations, financial position, and liquidity of Ameren and Ameren Missouri. Clean Water Act The EPA’s regulations implementing Section 316(b) of the Clean Water Act require power plant operators to evaluate cooling water intake structures and identify measures for reducing the number of aquatic organisms impinged on a power plant’s cooling water intake screens or entrained through the plant’s cooling water system. All of Ameren Missouri’s coal-fired and nuclear energy centers are subject to the cooling water intake structures rule. Requirements of the rule are implemented by state regulators through the permit renewal process of each power plant’s water discharge permit, which is expected to be completed by 2023 for Ameren Missouri. 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, prohibit effluent discharges of certain waste streams, and impose more stringent limitations on certain water discharges from power plants. To meet the requirements of the guidelines, Ameren Missouri installed dry ash handling systems and in 2020 completed construction of wastewater treatment facilities at three of its four coal-fired energy centers. The Meramec Energy Center is scheduled to close permanently in 2022 and, as a result, does not require new wastewater and dry ash handling systems. Estimated capital expenditures to complete these projects are included in the CCR management compliance plan, discussed below. CCR Management The EPA’s CCR rule establishes requirements for the management and disposal of CCR from coal-fired power plants and will result in the closure of surface impoundments at Ameren Missouri’s energy centers. Ameren Missouri has completed or expects to complete closure of surface impoundments at three of its facilities in 2021, and is scheduled to complete the last of such closures at a fourth facility in 2023. The EPA has issued a series of revisions to the CCR rule; however, none of those revisions is expected to materially impact our closure schedule. Ameren and Ameren Missouri have AROs of $88 million recorded on their respective balance sheets as of September 30, 2021, associated with CCR storage facilities. Ameren Missouri estimates it will need to make capital expenditures of $75 million to $100 million from 2021 through 2025 to implement its CCR management compliance plan, which includes installation of groundwater monitoring equipment and groundwater treatment facilities. 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. As of September 30, 2021, Ameren Illinois has remediated the majority of the 44 former MGP sites in Illinois and could substantially conclude remediation efforts at the remaining sites by 2023. The ICC allows Ameren Illinois to recover such remediation and related litigation costs from its electric and natural gas utility customers through environmental cost riders that are subject to annual prudence reviews by the ICC. As of September 30, 2021, Ameren Illinois estimated the remaining obligation related to these former MGP sites at $80 million to $145 million. Ameren and Ameren Illinois recorded a liability of $80 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 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 historical practices will result in future environmental commitments or will affect our results of operations, financial position, or liquidity. Illinois Emission Standards The CEJA established emission standards that will limit the operations of Ameren Missouri's five natural gas-fired energy centers located in the state of Illinois, including the possibility of one or more energy center closures earlier than anticipated. These energy centers are utilized to support peak loads. The emission limitations will become effective between 2030 and 2040. Ameren Missouri is reviewing the emission standards and the resulting effect they may have on its generation strategy, including any increases in capital expenditures or operating costs, or limits to the useful lives of the five natural gas-fired energy centers.

Callaway Energy Center

9 Months Ended

Sep. 30, 2021

Nuclear Waste Matters [Abstract]

CALLAWAY ENERGY CENTER

See Note 9 – Callaway Energy Center under Part II, Item 8, of the Form 10-K for information regarding spent nuclear fuel recovery, recovery of decommissioning costs, and the nuclear decommissioning trust fund. The fair value of the trust fund for Ameren Missouri’s Callaway Energy Center is reported as “Nuclear decommissioning trust fund” in Ameren’s and Ameren Missouri’s balance sheets. This amount is legally restricted and may be used only to fund the costs of nuclear decommissioning. Changes in the fair value of the trust fund are recorded as an increase or decrease to the nuclear decommissioning trust fund, with an offsetting adjustment to the related regulatory liability. 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. Every three years, the MoPSC requires Ameren Missouri to file an updated cost study and funding analysis for decommissioning its Callaway Energy Center. An updated cost study and funding analysis was filed with the MoPSC in November 2020 and reflected within the ARO. In February 2021, the MoPSC approved no change in electric rates for decommissioning costs based on Ameren Missouri’s updated cost study funding analysis. See Note 13 – Supplemental Information for more information on Ameren Missouri’s AROs.

Maintenance Outage

During its return to full power after the completion of the last refueling and maintenance outage in late December 2020, the Callaway Energy Center experienced a non-nuclear operating issue related to its generator. After replacement of certain key components of the generator, the energy center returned to service on August 4, 2021. The cost of generator repairs was approximately $60 million, which was largely capital expenditures. In April 2021, Ameren Missouri’s insurance claims were accepted by NEIL, which are expected to cover a significant portion of the capital expenditures and covered lost sales of up to $4.5 million weekly after March 17, 2021. Insurance recoveries related to lost sales were reflected in electric operating revenues and included in net energy costs under the FAC. Expected insurance recoveries related to the capital expenditures were reflected as a reduction to property, plant, and equipment. As of September 30, 2021, a $35 million insurance receivable was included in “Miscellaneous accounts receivable” on Ameren’s and Ameren Missouri’s balance sheets.

Insurance

The following table presents insurance coverage at Ameren Missouri’s Callaway Energy Center at September 30, 2021:


Type and Source of Coverage	Most Recent Renewal Date	Maximum Coverages			Maximum Assessments for Single Incidents
Public liability and nuclear worker liability:
American Nuclear Insurers	January 1, 2021	$	450		$	—
Pool participation	(a)	13,073		(a)	138		(b)
		$	13,523	(c)	$	138
Property damage:
NEIL and EMANI	April 1, 2021	$	3,200	(d)	$	25	(e)
Accidental outage:
NEIL	April 1, 2021	$	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 power 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)Accidental outage insurance provides for lost sales in the event of a prolonged accidental outage. Weekly indemnity up to $4.5 million for 52 weeks, which commences after the first 12 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 a nuclear reactor 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, resulting from terrorist attacks. The EMANI policies are not subject to industrywide aggregates in the event of terrorist attacks on nuclear facilities.

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

Retirement Benefits

9 Months Ended

Sep. 30, 2021

Retirement Benefits [Abstract]

RETIREMENT BENEFITS

The following table presents the components of the net periodic benefit cost (income) incurred for Ameren’s pension and postretirement benefit plans for the three and nine months ended September 30, 2021 and 2020:


	Pension Benefits								Postretirement Benefits
	Three Months				Nine Months				Three Months				Nine Months
	2021		2020		2021		2020		2021		2020		2021		2020
Service cost(a)	$	33	$	28	$	100	$	83	$	5	$	4	$	17	$	14
Non-service cost components:
Interest cost	38		43		114		130		9		10		25		29
Expected return on plan assets	(74)		(73)		(223)		(218)		(20)		(20)		(60)		(60)
Amortization of:

Prior service benefit	—		—		—		(1)		(1)		(1)		(3)		(3)
Actuarial loss (gain)	18		15		55		45		(1)		(3)		(4)		(7)
Total non-service cost components(b)	$	(18)	$	(15)	$	(54)	$	(44)	$	(13)	$	(14)	$	(42)	$	(41)

Net periodic benefit cost (income)	$	15	$	13	$	46	$	39	$	(8)	$	(10)	$	(25)	$	(27)

(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 share of Ameren’s pension and other postretirement costs. The following table presents the respective share of net periodic pension and other postretirement benefit costs (income) incurred for the three and nine months ended September 30, 2021 and 2020:


	Pension Benefits								Postretirement Benefits
	Three Months				Nine Months				Three Months				Nine Months
	2021		2020		2021		2020		2021		2020		2021		2020
Ameren Missouri(a)	$	7	$	5	$	22	$	16	$	(1)	$	(2)	$	(3)	$	(4)
Ameren Illinois	8		8		25		24		(8)		(8)		(23)		(24)
Other	—		—		(1)		(1)		1		—		1		1
Ameren(a)	$	15	$	13	$	46	$	39	$	(8)	$	(10)	$	(25)	$	(27)

(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.

- Definition

The entire disclosure for retirement benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section S99
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=21916913&loc=d3e273930-122802

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
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+ Details

Income Taxes

9 Months Ended

Sep. 30, 2021

Income Tax Disclosure [Abstract]

Income Taxes

INCOME TAXES

The following table presents a reconciliation of the federal statutory corporate income tax rate to the effective income tax rate for the three and nine months ended September 30, 2021 and 2020:


	Ameren					Ameren Missouri					Ameren Illinois
	2021			2020		2021			2020		2021		2020
Three Months
Federal statutory corporate income tax rate	21	%		21	%	21	%		21	%	21	%	21	%
Increases (decreases) from:
Amortization of deferred investment tax credit	(1)			(1)		(1)			(1)		—		—
Amortization of excess deferred taxes	(8)			(9)		(17)			(16)		(2)		(3)
Depreciation differences	—			—		—			—		(1)		(1)

Renewable and other tax credits	(2)		(a)	(1)		(8)		(a)	(2)		—		—
State tax	4			4		4			4		7		7
Stock-based compensation	—			1		—			—		—		—

Effective income tax rate	14	%		15	%	(1)	%		6	%	25	%	24	%
Nine Months
Federal statutory corporate income tax rate	21	%		21	%	21	%		21	%	21	%	21	%
Increases (decreases) from:
Amortization of deferred investment tax credit	—			—		—			(1)		—		—
Amortization of excess deferred taxes	(9)			(9)		(17)			(16)		(3)		(3)


Renewable and other tax credits	(3)		(a)	(1)		(8)		(a)	(1)		—		—
State tax	5			5		3			3		7		7
Stock-based compensation	(1)			(1)		—			—		—		—

Effective income tax rate	13	%		15	%	(1)	%		6	%	25	%	25	%

(a)Includes credits associated with the High Prairie and Atchison renewable energy centers. Ameren Missouri placed the High Prairie renewable energy center in service in December 2020. Additionally, Ameren Missouri placed in service the wind turbines at its Atchison renewable energy center throughout the first half of 2021. The benefit of the credits associated with Missouri renewable energy standard compliance is refunded to customers through the RESRAM.

- Definition

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References + Details

Supplemental Information

9 Months Ended

Sep. 30, 2021

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 at September 30, 2021, and December 31, 2020:


	September 30, 2021						December 31, 2020
	Ameren		Ameren Missouri		Ameren Illinois		Ameren		Ameren Missouri		Ameren Illinois
“Cash and cash equivalents”	$	7	$	—	$	—	$	139	$	136	$	—

Restricted cash included in “Other current assets”	16		5		6		17		5		6
Restricted cash included in “Other assets”	116		—		116		141		—		141
Restricted cash included in “Nuclear decommissioning trust fund”	6		6		—		4		4		—
Total cash, cash equivalents, and restricted cash	$	145	$	11	$	122	$	301	$	145	$	147

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. As of September 30, 2021, the amounts collected under the cost recovery rider restricted for use in Ameren Illinois’ procurement of renewable energy credits were classified as non-current because the amount is not expected to be refunded to customers within a year due to changes to the rider pursuant to the CEJA.

Accounts Receivable

“Accounts receivable – trade” on Ameren’s and Ameren Illinois’ balance sheets include certain receivables purchased at a discount from alternative retail electric suppliers that elect to participate in the utility consolidated billing program. At September 30, 2021, and December 31, 2020, “Other current liabilities” on Ameren’s and Ameren Illinois’ balance sheets included payables for purchased receivables of $35 million and $28 million, respectively.

The following table provides a reconciliation of the beginning and ending amount of the allowance for doubtful accounts for the three and nine months ended September 30, 2021 and 2020:


	Three Months				Nine Months
	2021		2020		2021			2020
Ameren:
Beginning of period	$	42	$	25	$	50		$	17
Bad debt expense	7		21		8			31

Net write-offs	(13)		(2)		(22)			(4)

End of period	$	36	$	44	$	36		$	44
Ameren Missouri:
Beginning of period	$	16	$	9	$	16		$	7
Bad debt expense	2		5		5			9

Net write-offs	(4)		—		(7)			(2)
End of period	$	14	$	14	$	14		$	14
Ameren Illinois:(a)
Beginning of period	$	26	$	16	$	34		$	10
Bad debt expense	5		16		3		(b)	22

Net write-offs	(9)		(2)		(15)			(2)

End of period	$	22	$	30	$	22		$	30

(a)Ameren Illinois has rate-adjustment mechanisms that allow it to recover the difference between its actual net bad debt write-offs under GAAP, including those associated with receivables purchased from alternative retail electric suppliers, and the amount of net bad debt write-offs included in its base rates.

(b)In the nine months ended September 30, 2021, Ameren Illinois’ bad debt expense was reduced as a result of incremental state funding received for customer bill assistance. The incremental state funding granted relief to low-income customers at risk of service disconnection resulting from the impacts of the COVID-19 pandemic.

Net write-offs increased for the three and nine months ended September 30, 2021, compared with the year-ago periods, due to the resumption of disconnection activities for nonpayment. See Note 2 – Rate and Regulatory Matters in this report and under Part II, Item 8, in the Form 10-K for additional information.

Supplemental Cash Flow Information

The following table provides noncash financing and investing activity excluded from the statements of cash flows for the nine months ended September 30, 2021 and 2020:


	September 30, 2021						September 30, 2020
	Ameren		Ameren Missouri				Ameren Illinois	Ameren		Ameren Missouri		Ameren Illinois
Investing
Accrued capital expenditures, including wind generation expenditures	$	396	$	209	$	182		$	311	$	115	$	191

Net realized and unrealized gain – nuclear decommissioning trust fund	85		85		—			43		43		—
Financing
Issuance of common stock for stock-based compensation	$	33	$	—	$	—		$	38	$	—	$	—

Asset Retirement Obligations

The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the nine months ended September 30, 2021:


	Ameren Missouri			Ameren Illinois			Ameren
Balance at December 31, 2020	$	751		$	5	(a)	$	756	(b)
Liabilities incurred	18		(c)	—			18		(c)
Liabilities settled	(23)			—			(23)
Accretion	23		(d)	—			23		(d)
Change in estimates	(10)		(e)	—			(10)		(e)
Balance at September 30, 2021	$	759		$	5	(a)	$	764	(b)

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

(b)Balance included $59 million and $60 million in “Other current liabilities” on the balance sheet as of September 30, 2021, and December 31, 2020, respectively.

(c)During the first nine months of 2021, Ameren Missouri recorded an ARO related to the decommissioning of the Atchison Renewable Energy Center.

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

(e)Ameren Missouri changed its fair value estimate primarily due to a decrease in the cost estimate for closure of certain CCR storage facilities.

Stock-based Compensation

On January 1, 2021, Ameren granted 293,058 performance share units with a grant date fair value of $25 million and 125,562 restricted share units with a grant date fair value of $10 million. Awards vest approximately 38 months after the grant date or on a pro-rata basis upon death or eligible retirement. The performance share units vest based on the achievement of certain specified market performance measures (251,177 performance share units) or based on the achievement of renewable generation and energy storage installation targets (41,881 performance share units). The exact number of shares issued pursuant to a performance share unit varies from 0% to 200% of the target award, depending on actual company performance relative to the performance goals.

For the nine months ended September 30, 2021 and 2020, excess tax benefits associated with the settlement of stock-based compensation awards reduced income tax expense by $5 million and $8 million, respectively.

Deferred Compensation

As of September 30, 2021, and December 31, 2020, the present value of benefits to be paid for deferred compensation obligations was $91 million and $90 million, respectively, which was primarily reflected in “Other deferred credits and liabilities” on Ameren's consolidated balance sheet.

Operating Revenues

As of September 30, 2021 and 2020, our remaining performance obligations for contracts with a term greater than one year were immaterial. The Ameren Companies elected not to disclose the aggregate amount of the transaction price allocated to the performance obligations that are unsatisfied as of the end of the reporting period for contracts with an initial expected term of one year or less.

See Note 14 – Segment Information for disaggregated revenue information.

Excise Taxes

Ameren Missouri and Ameren Illinois collect from their customers excise taxes, including municipal and state excise taxes and gross receipts taxes that are levied on the sale or distribution of natural gas and electricity. The following table presents the excise taxes recorded on a gross basis in “Operating Revenues – Electric,” “Operating Revenues – Natural gas” and “Operating Expenses – Taxes other than income taxes” on the statements of income for the three and nine months ended September 30, 2021 and 2020:


	Three Months				Nine Months
	2021		2020		2021		2020
Ameren Missouri	$	54	$	45	$	120	$	111
Ameren Illinois	27		26		93		87
Ameren	$	81	$	71	$	213	$	198

Earnings per Share

The following table reconciles the basic weighted-average number of common shares outstanding to the diluted weighted-average number of common shares outstanding for the three and nine months ended September 30, 2021 and 2020:


	Three Months		Nine Months
	2021	2020	2021	2020


Weighted-average Common Shares Outstanding – Basic	257.3	247.1	255.9	246.8
Assumed settlement of performance share units and restricted stock units	1.3	1.5	1.3	1.2
Dilutive effect of forward sale agreements	—	0.6	—	0.4
Weighted-average Common Shares Outstanding – Diluted(a)	258.6	249.2	257.2	248.4

(a)There was an immaterial number of anti-dilutive securities excluded from the earnings per diluted share calculations for the three and nine months ended September 30, 2021.There were no anti-dilutive securities excluded from the earnings per diluted share calculations for the three and nine months ended September 30, 2020.

Segment Information

9 Months Ended

Sep. 30, 2021

Segment Reporting [Abstract]

SEGMENT INFORMATION

The following tables present revenues, net income (loss) attributable to common shareholders, and capital expenditures by segment at Ameren and Ameren Illinois for the three and nine months ended September 30, 2021 and 2020. Ameren, Ameren Missouri, and Ameren Illinois management review segment capital expenditure information rather than any individual or total asset amount. For additional information about our segments, see Note 16 – Segment Information under Part II, Item 8, of the Form 10-K.

Ameren


	Ameren Missouri			Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Transmission			Other		Intersegment Eliminations		Ameren
Three Months 2021:
External revenues	$	1,118		$	425	$	127	$	141		$	—	$	—	$	1,811
Intersegment revenues	11			3		—		19			—		(33)		—
Net income (loss) attributable to Ameren common shareholders	375			36		(8)		73		(a)	(51)		—		425
Capital expenditures	466		(b)	143		93		154			2		(8)		850		(b)
Three Months 2020:
External revenues	$	990		$	390	$	122	$	126		$	—	$	—	$	1,628
Intersegment revenues	11			1		—		15			—		(27)		—
Net income (loss) attributable to Ameren common shareholders	297			34		2		62		(a)	(28)		—		367
Capital expenditures	262			129		81		185			2		(3)		656
Nine Months 2021:
External revenues	$	2,612		$	1,222	$	642	$	373		$	—	$	—	$	4,849
Intersegment revenues	30			5		—		53			—		(88)		—
Net income (loss) attributable to Ameren common shareholders	533			123		75		175		(a)	(41)		—		865
Capital expenditures	1,567		(b)	429		202		426			3		(14)		2,613		(b)
Nine Months 2020:
External revenues	$	2,442		$	1,131	$	533	$	360		$	—	$	—	$	4,466
Intersegment revenues	31			2		—		40			—		(73)		—
Net income (loss) attributable to Ameren common shareholders	439			107		66		168		(a)	(24)		—		756
Capital expenditures	778			391		221		490			4		—		1,884

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

(b)Includes $98 million and $515 million at Ameren and Ameren Missouri for wind generation expenditures for the three and nine months ended September 30, 2021, respectively.

Ameren Illinois


	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Illinois Transmission		Intersegment Eliminations		Ameren Illinois
Three Months 2021:
External revenues	$	428	$	127	$	90	$	—	$	645
Intersegment revenues	—		—		18		(18)		—
Net income (loss) available to common shareholder	36		(8)		51		—		79
Capital expenditures	143		93		144		—		380
Three Months 2020:
External revenues	$	391	$	122	$	76	$	—	$	589
Intersegment revenues	—		—		15		(15)		—
Net income available to common shareholder	34		2		41		—		77
Capital expenditures	129		81		160		—		370
Nine Months 2021:
External revenues	$	1,227	$	642	$	228	$	—	$	2,097
Intersegment revenues	—		—		49		(49)		—
Net income available to common shareholder	123		75		116		—		314
Capital expenditures	429		202		395		—		1,026
Nine Months 2020:
External revenues	$	1,133	$	533	$	213	$	—	$	1,879
Intersegment revenues	—		—		39		(39)		—
Net income available to common shareholder	107		66		107		—		280
Capital expenditures	391		221		419		—		1,031

The following tables present disaggregated revenues by segment at Ameren and Ameren Illinois for the three and nine months ended September 30, 2021 and 2020. Economic factors affect the nature, timing, amount, and uncertainty of revenues and cash flows in a similar manner across customer classes. Revenues from alternative revenue programs have a similar distribution among customer classes as revenues from contracts with customers. Other revenues not associated with contracts with customers are presented in the Other customer classification, along with electric transmission and off-system revenues.

Ameren


	Ameren Missouri		Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Transmission		Intersegment Eliminations		Ameren
Three Months 2021:
Residential	$	537	$	258	$	—	$	—	$	—	$	795
Commercial	412		143		—		—		—		555
Industrial	98		26		—		—		—		124
Other	66		1		—		160		(33)		194
Total electric revenues	$	1,113	$	428	$	—	$	160	$	(33)	$	1,668
Residential	$	9	$	—	$	75	$	—	$	—	$	84
Commercial	4		—		23		—		—		27
Industrial	—		—		3		—		—		3
Other	3		—		26		—		—		29
Total natural gas revenues	$	16	$	—	$	127	$	—	$	—	$	143
Total revenues(a)	$	1,129	$	428	$	127	$	160	$	(33)	$	1,811


	Ameren Missouri		Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Transmission		Intersegment Eliminations		Ameren
Three Months 2020:
Residential	$	455	$	234	$	—	$	—	$	—	$	689
Commercial	343		127		—		—		—		470
Industrial	87		26		—		—		—		113
Other	99		4		—		141		(27)		217
Total electric revenues	$	984	$	391	$	—	$	141	$	(27)	$	1,489
Residential	$	8	$	—	$	68	$	—	$	—	$	76
Commercial	3		—		18		—		—		21
Industrial	1		—		5		—		—		6
Other	5		—		31		—		—		36
Total natural gas revenues	$	17	$	—	$	122	$	—	$	—	$	139
Total revenues(a)	$	1,001	$	391	$	122	$	141	$	(27)	$	1,628
Nine Months 2021:
Residential	$	1,177	$	705	$	—	$	—	$	—	$	1,882
Commercial	899		402		—		—		—		1,301
Industrial	221		94		—		—		—		315
Other	246		26		—		426		(88)		610
Total electric revenues	$	2,543	$	1,227	$	—	$	426	$	(88)	$	4,108
Residential	$	54	$	—	$	438	$	—	$	—	$	492
Commercial	23		—		116		—		—		139
Industrial	2		—		20		—		—		22
Other	20		—		68		—		—		88
Total natural gas revenues	$	99	$	—	$	642	$	—	$	—	$	741
Total revenues(a)	$	2,642	$	1,227	$	642	$	426	$	(88)	$	4,849
Nine Months 2020:
Residential	$	1,111	$	664	$	—	$	—	$	—	$	1,775
Commercial	828		365		—		—		—		1,193
Industrial	207		91		—		—		—		298
Other	240		13		—		400		(73)		580
Total electric revenues	$	2,386	$	1,133	$	—	$	400	$	(73)	$	3,846
Residential	$	52	$	—	$	375	$	—	$	—	$	427
Commercial	20		—		94		—		—		114
Industrial	3		—		11		—		—		14
Other	12		—		53		—		—		65
Total natural gas revenues	$	87	$	—	$	533	$	—	$	—	$	620
Total revenues(a)	$	2,473	$	1,133	$	533	$	400	$	(73)	$	4,466

(a)The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the three and nine months ended September 30, 2021 and 2020:


	Ameren Missouri			Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Transmission		Ameren
Three Months 2021:
Revenues from alternative revenue programs	$	6		$	(97)	$	(5)	$	1	$	(95)
Other revenues not from contracts with customers	7		(a)	6		—		—		13		(a)

Three Months 2020:
Revenues from alternative revenue programs	$	1		$	(110)	$	3	$	4	$	(102)
Other revenues not from contracts with customers	6			5		—		—		11

Nine Months 2021:
Revenues from alternative revenue programs	$	(9)		$	(2)	$	—	$	5	$	(6)
Other revenues not from contracts with customers	71		(a)	9		2		—		82		(a)

Nine Months 2020:
Revenues from alternative revenue programs	$	(8)		$	(59)	$	17	$	34	$	(16)
Other revenues not from contracts with customers	21			6		1		—		28

(a)Includes insurance recoveries related to lost sales associated with the Callaway Energy Center maintenance outage. See Note 10 – Callaway Energy Center for additional information.

Ameren Illinois


	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Illinois Transmission		Intersegment Eliminations		Ameren Illinois
Three Months 2021:
Residential	$	258	$	75	$	—	$	—	$	333
Commercial	143		23		—		—		166
Industrial	26		3		—		—		29
Other	1		26		108		(18)		117
Total revenues(a)	$	428	$	127	$	108	$	(18)	$	645
Three Months 2020:
Residential	$	234	$	68	$	—	$	—	$	302
Commercial	127		18		—		—		145
Industrial	26		5		—		—		31
Other	4		31		91		(15)		111
Total revenues(a)	$	391	$	122	$	91	$	(15)	$	589
Nine Months 2021:
Residential	$	705	$	438	$	—	$	—	$	1,143
Commercial	402		116		—		—		518
Industrial	94		20		—		—		114
Other	26		68		277		(49)		322
Total revenues(a)	$	1,227	$	642	$	277	$	(49)	$	2,097
Nine Months 2020:
Residential	$	664	$	375	$	—	$	—	$	1,039
Commercial	365		94		—		—		459
Industrial	91		11		—		—		102
Other	13		53		252		(39)		279
Total revenues(a)	$	1,133	$	533	$	252	$	(39)	$	1,879

(a)The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the Ameren Illinois segments for the three and nine months ended September 30, 2021 and 2020:


	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Illinois Transmission		Ameren Illinois
Three Months 2021:
Revenues from alternative revenue programs	$	(97)	$	(5)	$	3	$	(99)
Other revenues not from contracts with customers	6		—		—		6

Three Months 2020:
Revenues from alternative revenue programs	$	(110)	$	3	$	5	$	(102)
Other revenues not from contracts with customers	5		—		—		5

Nine Months 2021:
Revenues from alternative revenue programs	$	(2)	$	—	$	4	$	2
Other revenues not from contracts with customers	9		2		—		11

Nine Months 2020:
Revenues from alternative revenue programs	$	(59)	$	17	$	29	$	(13)
Other revenues not from contracts with customers	6		1		—		7

- Definition

The entire disclosure for reporting segments including data and tables. Reportable segments include those that meet any of the following quantitative thresholds a) it's reported revenue, including sales to external customers and intersegment sales or transfers is 10 percent or more of the combined revenue, internal and external, of all operating segments b) the absolute amount of its reported profit or loss is 10 percent or more of the greater, in absolute amount of 1) the combined reported profit of all operating segments that did not report a loss or 2) the combined reported loss of all operating segments that did report a loss c) its assets are 10 percent or more of the combined assets of all operating segments.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Summary Of Significant Accounting Policies (Policies)	9 Months Ended
Summary Of Significant Accounting Policies (Policies)	Sep. 30, 2021
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 in the MISO.
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 for 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.
Consolidation, Variable Interest Entity, Policy	Variable Interest Entities As of September 30, 2021, and December 31, 2020, Ameren had unconsolidated variable interests as a limited partner in various equity method investments, totaling $53 million and $37 million, respectively, included in “Other assets” on Ameren’s consolidated balance sheet. Ameren is not the primary beneficiary of these investments because it does not have the power to direct matters that most significantly affect the activities of these variable interest entities. As of September 30, 2021, the maximum exposure to loss related to these variable interests is limited to the investment in these partnerships of $53 million plus associated outstanding funding commitments of $27 million.
Derivatives, Policy	We use derivatives to manage the risk of changes in market prices for natural gas, power, and uranium, 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 and uranium inventories that differ from the cost of those commodities in inventory; •actual cash outlays for the purchase of these commodities that differ from anticipated cash outlays; and •actual off-system sales revenues that differ from anticipated revenues. 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. We believe that entering into master netting arrangements or similar agreements mitigates the level of financial loss that could result from default by allowing net settlement of derivative assets and liabilities. These master netting arrangements allow the counterparties to net settle sale and purchase transactions. Further, collateral requirements are calculated at the master netting arrangement or similar agreement level by counterparty.
Deferred Compensation	Deferred Compensation As of September 30, 2021, and December 31, 2020, the present value of benefits to be paid for deferred compensation obligations was $91 million and $90 million, respectively, which was primarily reflected in “Other deferred credits and liabilities” on Ameren's consolidated balance sheet.
Revenue from Contract with Customer	Operating Revenues As of September 30, 2021 and 2020, 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	Excise TaxesAmeren 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.

- Definition

Disclosure of accounting policy for revenue from contract with customer.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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+ Details

Short-Term Debt and Liquidity (Tables)

9 Months Ended

Sep. 30, 2021

Debt Disclosure [Abstract]

Schedule of Short-term Debt

The following table summarizes the activity and relevant interest rates for Ameren (parent)’s, Ameren Missouri’s, and Ameren Illinois’ commercial paper issuances and borrowings under the Credit Agreements in the aggregate for the nine months ended September 30, 2021 and 2020:


	Ameren (parent)			Ameren Missouri				Ameren Illinois			Ameren Consolidated
2021
Average daily amount outstanding	$	404		$	121			$	140		$	665

Weighted-average interest rate	0.23		%	0.22		%		0.22		%	0.22		%
Peak amount outstanding during period(a)	$	650		$	546			$	485		$	1,134
Peak interest rate	0.33		%	0.25		%		0.25		%	0.33		%
2020
Average daily amount outstanding	$	62		$	142			$	53		$	257

Weighted-average interest rate	2.04		%	1.76		%		1.10		%	1.70		%
Peak amount outstanding during period(a)	$	425		$	573			$	243		$	908
Peak interest rate	3.30		%	5.05		%	(b)	3.40		%	5.05		%	(b)

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

(b)Ameren’s and Ameren Missouri’s peak interest rate was affected by temporary disruptions in the commercial paper market in the first quarter of 2020.

Other Income and Expenses (Tables)

9 Months Ended

Sep. 30, 2021

Other Nonoperating Income (Expense) [Abstract]

Other Income And Expenses

The following table presents the components of “Other Income, Net” in the Ameren Companies’ statements of income for the three and nine months ended September 30, 2021 and 2020:


	Three Months				Nine Months
	2021		2020		2021		2020
Ameren:

Allowance for equity funds used during construction	$	14	$	12	$	30	$	25
Interest income on industrial development revenue bonds	7		7		19		19
Other interest income	—		—		1		2

Non-service cost components of net periodic benefit income(a)	34		32		102		85
Miscellaneous income	3		1		14		10
Donations	(1)		—		(5)		(14)		(b)
Miscellaneous expense	(1)		(4)		(10)		(10)
Total Other Income, Net	$	56	$	48	$	151	$	117
Ameren Missouri:

Allowance for equity funds used during construction	$	7	$	7	$	17	$	15
Interest income on industrial development revenue bonds	7		7		19		19

Non-service cost components of net periodic benefit income(a)	13		13		41		32
Miscellaneous income	2		1		3		3
Donations	—		—		(1)		(9)		(b)
Miscellaneous expense	(2)		(2)		(5)		(5)
Total Other Income, Net	$	27	$	26	$	74	$	55
Ameren Illinois:

Allowance for equity funds used during construction	$	7	$	5	$	13	$	10
Interest income	—		—		1		2

Non-service cost components of net periodic benefit income	13		12		41		36
Miscellaneous income	—		1		3		6
Donations	(1)		—		(4)		(5)
Miscellaneous expense	—		(1)		(5)		(4)
Total Other Income, Net	$	19	$	17	$	49	$	45

(a)For the three and nine months ended September 30, 2021, the non-service cost components of net periodic benefit income were adjusted by amounts deferred of $(3) million and $(6) million, 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. The deferral was $(3) million and less than $(1) million for the three and nine months ended September 30, 2020.

(b)Includes $8 million pursuant to Ameren Missouri’s March 2020 electric rate order. See Note 2 – Rate and Regulatory Matters under Part II, Item 8, in the Form 10-K for additional information.

Derivative Financial Instruments (Tables)

9 Months Ended

Sep. 30, 2021

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Open Gross Derivative Volumes By Commodity Type

The following table presents open gross commodity contract volumes by commodity type for derivative assets and liabilities as of September 30, 2021, and December 31, 2020. As of September 30, 2021, these contracts extended through October 2024, October 2026, May 2032 and March 2023 for fuel oils, natural gas, power and uranium, respectively.


	Quantity (in millions)
	September 30, 2021			December 31, 2020
Commodity	Ameren Missouri	Ameren Illinois	Ameren	Ameren Missouri	Ameren Illinois	Ameren
Fuel oils (in gallons)	28	—	28	43	—	43
Natural gas (in mmbtu)	33	136	169	33	114	147
Power (in MWhs)	6	6	12	6	7	13
Uranium (pounds in thousands)	400	—	400	365	—	365

Derivative Instruments Carrying Value

The following table presents the carrying value and balance sheet location of all derivative commodity contracts, none of which were designated as hedging instruments, as of September 30, 2021, and December 31, 2020:


			September 30, 2021					December 31, 2020
	Balance Sheet Location		Ameren Missouri		Ameren Illinois		Ameren			Ameren Missouri		Ameren Illinois		Ameren









Fuel oils	Other current assets	$	9	$	—	$	9		$	2	$	—	$	2
	Other assets		5		—		5			—		—		—
Natural gas	Mark-to-market derivative assets		(a)		55		(a)			(a)		8		(a)

	Other current assets		13		—		68			1		—		9
	Other assets		8		20		28			2		2		4
Power	Mark-to-market derivative assets		(a)		1		(a)			(a)		—		(a)

	Other current assets		31		—		32			7		—		7
	Other assets		1		—		1			—		—		—

Uranium	Other assets		2		—		2			—		—		—
	Total assets	$	69	$	76	$	145		$	12	$	10	$	22



Fuel oils	Mark-to-market derivative liabilities	$	—	$	(a)	$	(a)		$	7	$	(a)	$	(a)

	Other current liabilities		—		—		—			—		—		7
	Other deferred credits and liabilities		—		—		—			2		—		2


Natural gas	Other current liabilities		—		4		4			1		1		2
	Other deferred credits and liabilities		—		2		2			—		1		1
Power	Mark-to-market derivative liabilities		75		(a)		(a)			3		(a)		(a)

	Other current liabilities		—		5		80			—		17		20
	Other deferred credits and liabilities		22		140		162			8		181		189

Uranium	Other current liabilities		1		—		1			—		—		—

	Total liabilities	$	98	$	151	$	249		$	21	$	200	$	221

(a)Balance sheet line item not applicable to registrant.

Offsetting Assets and Liabilities

The following table provides the recognized gross derivative balances and the net amounts of those derivatives subject to an enforceable master netting arrangement or similar agreement as of September 30, 2021. If the gross amounts recognized on the balance sheet were netted with derivative instruments and cash collateral received or posted at December 31, 2020, the net amounts would not be materially different from the gross amounts.


			Gross Amounts Not Offset in the Balance Sheet
Commodity Contracts Eligible to be Offset	Gross Amounts Recognized in the Balance Sheet		Derivative Instruments		Cash Collateral Received/Posted(a)		Net Amount
2021
Assets:
Ameren Missouri	$	69	$	14	$	2	$	53
Ameren Illinois	76		2		3		71
Ameren	$	145	$	16	$	5	$	124
Liabilities:
Ameren Missouri	$	98	$	14	$	57	$	27
Ameren Illinois	151		2		—		149
Ameren	$	249	$	16	$	57	$	176

(a)Cash collateral received reduces gross asset balances and is included in “Other current liabilities” and “Other deferred credits and liabilities” on the balance sheet. Cash collateral posted reduces gross liability balances and is included in “Other current assets” and “Other assets” on the balance sheet.

Derivative Instruments With Credit Risk-Related Contingent Features

The following table presents, as of September 30, 2021, 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.



	Aggregate Fair Value of Derivative Liabilities(a)		Cash Collateral Posted		Potential Aggregate Amount of Additional Collateral Required(b)
Ameren Missouri	$	45	$	3	$	28
Ameren Illinois	6		—		4

Ameren	$	51	$	3	$	32

(a)Before consideration of master netting arrangements or similar agreements.

(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)

9 Months Ended

Sep. 30, 2021

Fair Value Disclosures [Abstract]

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

The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of September 30, 2021, and December 31, 2020:


		September 30, 2021									December 31, 2020
		Level 1		Level 2		Level 3		Total			Level 1		Level 2		Level 3		Total
Assets:
Ameren Missouri
	Derivative assets – commodity contracts:
	Fuel oils	$	14	$	—	$	—	$	14		$	—	$	—	$	2	$	2
	Natural gas	—		20		1		21			—		3		—		3
	Power	12		—		20		32			2		—		5		7
	Uranium	—		—		2		2			—		—		—		—
	Total derivative assets – commodity contracts	$	26	$	20	$	23	$	69		$	2	$	3	$	7	$	12
	Nuclear decommissioning trust fund:

	Equity securities:
	U.S. large capitalization	$	743	$	—	$	—	$	743		$	680	$	—	$	—	$	680
	Debt securities:
	U.S. Treasury and agency securities	—		141		—		141			—		115		—		115
	Corporate bonds	—		128		—		128			—		115		—		115


	Other	—		56		—		56			—		67		—		67
	Total nuclear decommissioning trust fund	$	743	$	325	$	—	$	1,068	(a)	$	680	$	297	$	—	$	977	(a)
	Total Ameren Missouri	$	769	$	345	$	23	$	1,137		$	682	$	300	$	7	$	989


		September 30, 2021									December 31, 2020
		Level 1		Level 2		Level 3		Total			Level 1		Level 2		Level 3		Total
Ameren Illinois
	Derivative assets – commodity contracts:
	Natural gas	$	3	$	62	$	10	$	75		$	—	$	6	$	4	$	10
	Power	—		—		1		1			—		—		—		—
	Total Ameren Illinois	$	3	$	62	$	11	$	76		$	—	$	6	$	4	$	10
Ameren
	Derivative assets – commodity contracts(b)	$	29	$	82	$	34	$	145		$	2	$	9	$	11	$	22
	Nuclear decommissioning trust fund(c)	743		325		—		1,068		(a)	680		297		—		977		(a)
	Total Ameren	$	772	$	407	$	34	$	1,213		$	682	$	306	$	11	$	999
Liabilities:
Ameren Missouri
	Derivative liabilities – commodity contracts:
	Fuel oils	$	—	$	—	$	—	$	—		$	6	$	—	$	3	$	9
	Natural gas	—		—		—		—			—		1		—		1
	Power	54		—		43		97			8		—		3		11
	Uranium	—		—		1		1			—		—		—		—
	Total Ameren Missouri	$	54	$	—	$	44	$	98		$	14	$	1	$	6	$	21
Ameren Illinois
	Derivative liabilities – commodity contracts:
	Natural gas	$	—	$	2	$	4	$	6		$	—	$	1	$	1	$	2
	Power	—		—		145		145			—		—		198		198
	Total Ameren Illinois	$	—	$	2	$	149	$	151		$	—	$	1	$	199	$	200
Ameren
	Derivative liabilities – commodity contracts(b)	$	54	$	2	$	193	$	249		$	14	$	2	$	205	$	221

(a)Balance excludes $8 million and $5 million of cash and cash equivalents, receivables, payables, and accrued income, net, for September 30, 2021, and December 31, 2020, respectively.

(b)See the Ameren Missouri and Ameren Illinois sections of the table for a breakout of the fair value of Ameren’s derivative assets and liabilities by type of commodity.

(c)See the Ameren Missouri section of the table for a breakout of the fair value of Ameren's nuclear decommissioning trust fund by investment type.

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

The following table presents the fair value reconciliation of Level 3 power derivative contract assets and liabilities measured at fair value on a recurring basis for the three and nine months ended September 30, 2021 and 2020:


	2021						2020
	Ameren Missouri		Ameren Illinois		Ameren			Ameren Missouri		Ameren Illinois		Ameren
For the three months ended September 30:
Beginning balance at July 1	$	(5)	$	(166)	$	(171)		$	16	$	(229)	$	(213)
Realized and unrealized gains/(losses) included in regulatory assets/liabilities	(16)		19		3			(2)		12		10


Settlements	(2)		3		1			(4)		4		—


Ending balance at September 30	$	(23)	$	(144)	$	(167)		$	10	$	(213)	$	(203)
Change in unrealized gains/(losses) related to assets/liabilities held at September 30	$	(17)	$	19	$	2		$	(2)	$	11	$	9
For the nine months ended September 30:
Beginning balance at January 1	$	2	$	(198)	$	(196)		$	13	$	(224)	$	(211)
Realized and unrealized gains/(losses) included in regulatory assets/liabilities	(22)		43		21			18		(2)		16


Settlements	(3)		11		8			(21)		13		(8)


Ending balance at September 30	$	(23)	$	(144)	$	(167)		$	10	$	(213)	$	(203)
Change in unrealized gains/(losses) related to assets/liabilities held at September 30	$	(21)	$	42	$	21		$	6	$	(1)	$	5

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

The following table describes the valuation techniques and significant unobservable inputs utilized for the fair value of our Level 3 power derivative contract assets and liabilities as of September 30, 2021, and December 31, 2020:


			Fair Value						Weighted Average(b)
	Commodity		Assets		Liabilities	Valuation Technique(s)	Unobservable Input(a)	Range









2021	Power(c)	$	21	$	(188)	Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps ($/MWh)	28 – 77	40

							Nodal basis ($/MWh)	(4) – 0	(2)


							Trend rate (%)	(4) – (2)	(3)








































2020	Power(c)	$	5	$	(201)	Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps ($/MWh)	23 – 37	29

							Nodal basis ($/MWh)	(6) – 0	(2)


							Trend rate (%)	2 – 6	3

(a)Generally, significant increases (decreases) in these inputs in isolation would result in a significantly higher (lower) fair value measurement.

(b)Unobservable inputs were weighted by relative fair value.

(c)Valuations through 2029 use visible forward prices adjusted for nodal-to-hub basis differentials. Valuations beyond 2029 use a trend rate factor and are similarly adjusted for nodal-to-hub basis differentials.

Schedule of Financial Assets and Liabilities

The following table sets forth the carrying amount and, by level within the fair value hierarchy, the fair value of financial assets and liabilities disclosed, but not recorded, at fair value as of September 30, 2021, and December 31, 2020:


	Carrying Amount			Fair Value
				Level 1		Level 2		Level 3			Total
	September 30, 2021
Ameren:
Cash, cash equivalents, and restricted cash	$	145		$	145	$	—	$	—		$	145
Investments in industrial development revenue bonds(a)	256			—		256		—			256
Short-term debt	553			—		553		—			553
Long-term debt (including current portion)(a)	12,501		(b)	—		13,576		520		(c)	14,096
Ameren Missouri:
Cash, cash equivalents, and restricted cash	$	11		$	11	$	—	$	—		$	11
Advances to money pool	24			—		24		—			24
Investments in industrial development revenue bonds(a)	256			—		256		—			256


Long-term debt (including current portion)(a)	5,626		(b)	—		6,392		—			6,392
Ameren Illinois:
Cash, cash equivalents, and restricted cash	$	122		$	122	$	—	$	—		$	122
Advances to money pool	41			—		41		—			41


Long-term debt (including current portion)	4,391		(b)	—		5,019		—			5,019
	December 31, 2020
Ameren:
Cash, cash equivalents, and restricted cash	$	301		$	301	$	—	$	—		$	301
Investments in industrial development revenue bonds(a)	256			—		256		—			256
Short-term debt	490			—		490		—			490
Long-term debt (including current portion)(a)	11,086		(b)	—		12,778		537		(c)	13,315
Ameren Missouri:
Cash, cash equivalents, and restricted cash	$	145		$	145	$	—	$	—		$	145
Advances to money pool	139			—		139		—			139
Investments in industrial development revenue bonds(a)	256			—		256		—			256

Long-term debt (including current portion)(a)	5,104		(b)	—		6,160		—			6,160
Ameren Illinois:
Cash, cash equivalents, and restricted cash	$	147		$	147	$	—	$	—		$	147
Borrowings from money pool	19			—		19		—			19
Long-term debt (including current portion)	3,946		(b)	—		4,822		—			4,822

(a)Ameren and Ameren Missouri have investments in industrial development revenue bonds, classified as held-to-maturity and recorded in “Other Assets,” that are equal to the finance obligations for the Peno Creek and Audrain CT energy centers. As of September 30, 2021, and December 31, 2020, 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 $93 million, $39 million, and $40 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of September 30, 2021. Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $84 million, $36 million, and $36 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2020.

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

Related Party Transactions (Tables)

9 Months Ended

Sep. 30, 2021

Schedule of Affiliate Receivables and Payables

The following table presents the affiliate balances related to income taxes for Ameren Missouri and Ameren Illinois as of September 30, 2021, and December 31, 2020:


	September 30, 2021				December 31, 2020
	Ameren Missouri		Ameren Illinois		Ameren Missouri		Ameren Illinois
Income taxes payable to parent(a)	$	—	$	12	$	—	$	6
Income taxes receivable from parent(b)	—		44		9		15

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

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

Schedule of Related Party Transactions

The following table presents the effect on Ameren Missouri and Ameren Illinois of related-party transactions for the three and nine months ended September 30, 2021 and 2020:


				Three Months				Nine Months
Agreement	Income Statement Line Item			Ameren Missouri		Ameren Illinois		Ameren Missouri		Ameren Illinois
Ameren Missouri power supply	Operating Revenues	2021	$	5	$	(a)	$	10	$	(a)
agreements with Ameren Illinois		2020		5		(a)		11		(a)
Ameren Missouri and Ameren Illinois	Operating Revenues	2021	$	6	$	(b)	$	20	$	(b)
rent and facility services		2020		6		(b)		19		1
Ameren Missouri and Ameren Illinois miscellaneous	Operating Revenues	2021	$	(b)	$	3	$	(b)	$	5
support services and services provided to ATXI		2020		(b)		1		1		1
Total Operating Revenues		2021	$	11	$	3	$	30	$	5
		2020		11		1		31		2
Ameren Illinois power supply	Purchased Power	2021	$	(a)	$	5	$	(a)	$	10
agreements with Ameren Missouri		2020		(a)		5		(a)		11
Ameren Missouri and Ameren Illinois	Purchased Power	2021	$	1	$	(b)	$	3	$	1
transmission services from ATXI		2020		(a)		(b)		(a)		1
Total Purchased Power		2021	$	1	$	5	$	3	$	11
		2020		(a)		5		(a)		12
Ameren Missouri and Ameren Illinois	Other Operations and Maintenance	2021	$	(b)	$	1	$	(b)	$	3
rent and facility services		2020		(b)		1		(b)		3


				Three Months				Nine Months
Agreement	Income Statement Line Item			Ameren Missouri		Ameren Illinois		Ameren Missouri		Ameren Illinois
Ameren Services support services	Other Operations and Maintenance	2021	$	41	$	37	$	110	$	101
agreement		2020		36		34		103		98
Total Other Operations and		2021	$	41	$	38	$	110	$	104
Maintenance		2020		36		35		103		101
Money pool borrowings (advances)	(Interest Charges)/Other Income, Net	2021	$	(b)	$	(b)	$	(b)	$	(b)
		2020		(b)		(b)		(b)		(b)

(a)Not applicable.

(b)Amount less than $1 million.

Callaway Energy Center (Tables)

9 Months Ended

Sep. 30, 2021

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 at September 30, 2021:


Type and Source of Coverage	Most Recent Renewal Date	Maximum Coverages			Maximum Assessments for Single Incidents
Public liability and nuclear worker liability:
American Nuclear Insurers	January 1, 2021	$	450		$	—
Pool participation	(a)	13,073		(a)	138		(b)
		$	13,523	(c)	$	138
Property damage:
NEIL and EMANI	April 1, 2021	$	3,200	(d)	$	25	(e)
Accidental outage:
NEIL	April 1, 2021	$	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 power 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)Accidental outage insurance provides for lost sales in the event of a prolonged accidental outage. Weekly indemnity up to $4.5 million for 52 weeks, which commences after the first 12 weeks of an outage, plus up to $3.6 million per week for a minimum of 71 weeks thereafter for a total not exceeding the policy limit of $490 million. Nonradiation events are limited to $328 million.

Retirement Benefits (Tables)

9 Months Ended

Sep. 30, 2021

Retirement Benefits [Abstract]

Components Of Net Periodic Benefit Cost

The following table presents the components of the net periodic benefit cost (income) incurred for Ameren’s pension and postretirement benefit plans for the three and nine months ended September 30, 2021 and 2020:


	Pension Benefits								Postretirement Benefits
	Three Months				Nine Months				Three Months				Nine Months
	2021		2020		2021		2020		2021		2020		2021		2020
Service cost(a)	$	33	$	28	$	100	$	83	$	5	$	4	$	17	$	14
Non-service cost components:
Interest cost	38		43		114		130		9		10		25		29
Expected return on plan assets	(74)		(73)		(223)		(218)		(20)		(20)		(60)		(60)
Amortization of:

Prior service benefit	—		—		—		(1)		(1)		(1)		(3)		(3)
Actuarial loss (gain)	18		15		55		45		(1)		(3)		(4)		(7)
Total non-service cost components(b)	$	(18)	$	(15)	$	(54)	$	(44)	$	(13)	$	(14)	$	(42)	$	(41)

Net periodic benefit cost (income)	$	15	$	13	$	46	$	39	$	(8)	$	(10)	$	(25)	$	(27)

(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 share of Ameren’s pension and other postretirement costs. The following table presents the respective share of net periodic pension and other postretirement benefit costs (income) incurred for the three and nine months ended September 30, 2021 and 2020:


	Pension Benefits								Postretirement Benefits
	Three Months				Nine Months				Three Months				Nine Months
	2021		2020		2021		2020		2021		2020		2021		2020
Ameren Missouri(a)	$	7	$	5	$	22	$	16	$	(1)	$	(2)	$	(3)	$	(4)
Ameren Illinois	8		8		25		24		(8)		(8)		(23)		(24)
Other	—		—		(1)		(1)		1		—		1		1
Ameren(a)	$	15	$	13	$	46	$	39	$	(8)	$	(10)	$	(25)	$	(27)

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

Income Taxes Income Taxes (Tables)

9 Months Ended

Sep. 30, 2021

Income Tax Disclosure [Abstract]

Schedule of Effective Income Tax Rate Reconciliation

The following table presents a reconciliation of the federal statutory corporate income tax rate to the effective income tax rate for the three and nine months ended September 30, 2021 and 2020:


	Ameren					Ameren Missouri					Ameren Illinois
	2021			2020		2021			2020		2021		2020
Three Months
Federal statutory corporate income tax rate	21	%		21	%	21	%		21	%	21	%	21	%
Increases (decreases) from:
Amortization of deferred investment tax credit	(1)			(1)		(1)			(1)		—		—
Amortization of excess deferred taxes	(8)			(9)		(17)			(16)		(2)		(3)
Depreciation differences	—			—		—			—		(1)		(1)

Renewable and other tax credits	(2)		(a)	(1)		(8)		(a)	(2)		—		—
State tax	4			4		4			4		7		7
Stock-based compensation	—			1		—			—		—		—

Effective income tax rate	14	%		15	%	(1)	%		6	%	25	%	24	%
Nine Months
Federal statutory corporate income tax rate	21	%		21	%	21	%		21	%	21	%	21	%
Increases (decreases) from:
Amortization of deferred investment tax credit	—			—		—			(1)		—		—
Amortization of excess deferred taxes	(9)			(9)		(17)			(16)		(3)		(3)


Renewable and other tax credits	(3)		(a)	(1)		(8)		(a)	(1)		—		—
State tax	5			5		3			3		7		7
Stock-based compensation	(1)			(1)		—			—		—		—

Effective income tax rate	13	%		15	%	(1)	%		6	%	25	%	25	%

(a)Includes credits associated with the High Prairie and Atchison renewable energy centers. Ameren Missouri placed the High Prairie renewable energy center in service in December 2020. Additionally, Ameren Missouri placed in service the wind turbines at its Atchison renewable energy center throughout the first half of 2021. The benefit of the credits associated with Missouri renewable energy standard compliance is refunded to customers through the RESRAM.

Supplemental Information (Tables)

9 Months Ended

Sep. 30, 2021

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 at September 30, 2021, and December 31, 2020:


	September 30, 2021						December 31, 2020
	Ameren		Ameren Missouri		Ameren Illinois		Ameren		Ameren Missouri		Ameren Illinois
“Cash and cash equivalents”	$	7	$	—	$	—	$	139	$	136	$	—

Restricted cash included in “Other current assets”	16		5		6		17		5		6
Restricted cash included in “Other assets”	116		—		116		141		—		141
Restricted cash included in “Nuclear decommissioning trust fund”	6		6		—		4		4		—
Total cash, cash equivalents, and restricted cash	$	145	$	11	$	122	$	301	$	145	$	147

Schedule of Accounts, Notes, Loans and Financing Receivable

The following table provides a reconciliation of the beginning and ending amount of the allowance for doubtful accounts for the three and nine months ended September 30, 2021 and 2020:


	Three Months				Nine Months
	2021		2020		2021			2020
Ameren:
Beginning of period	$	42	$	25	$	50		$	17
Bad debt expense	7		21		8			31

Net write-offs	(13)		(2)		(22)			(4)

End of period	$	36	$	44	$	36		$	44
Ameren Missouri:
Beginning of period	$	16	$	9	$	16		$	7
Bad debt expense	2		5		5			9

Net write-offs	(4)		—		(7)			(2)
End of period	$	14	$	14	$	14		$	14
Ameren Illinois:(a)
Beginning of period	$	26	$	16	$	34		$	10
Bad debt expense	5		16		3		(b)	22

Net write-offs	(9)		(2)		(15)			(2)

End of period	$	22	$	30	$	22		$	30

(a)Ameren Illinois has rate-adjustment mechanisms that allow it to recover the difference between its actual net bad debt write-offs under GAAP, including those associated with receivables purchased from alternative retail electric suppliers, and the amount of net bad debt write-offs included in its base rates.

(b)In the nine months ended September 30, 2021, Ameren Illinois’ bad debt expense was reduced as a result of incremental state funding received for customer bill assistance. The incremental state funding granted relief to low-income customers at risk of service disconnection resulting from the impacts of the COVID-19 pandemic.

Schedule of Cash Flow, Supplemental Disclosures

The following table provides noncash financing and investing activity excluded from the statements of cash flows for the nine months ended September 30, 2021 and 2020:


	September 30, 2021						September 30, 2020
	Ameren		Ameren Missouri				Ameren Illinois	Ameren		Ameren Missouri		Ameren Illinois
Investing
Accrued capital expenditures, including wind generation expenditures	$	396	$	209	$	182		$	311	$	115	$	191

Net realized and unrealized gain – nuclear decommissioning trust fund	85		85		—			43		43		—
Financing
Issuance of common stock for stock-based compensation	$	33	$	—	$	—		$	38	$	—	$	—

Asset Retirement Obligation Disclosure

The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the nine months ended September 30, 2021:


	Ameren Missouri			Ameren Illinois			Ameren
Balance at December 31, 2020	$	751		$	5	(a)	$	756	(b)
Liabilities incurred	18		(c)	—			18		(c)
Liabilities settled	(23)			—			(23)
Accretion	23		(d)	—			23		(d)
Change in estimates	(10)		(e)	—			(10)		(e)
Balance at September 30, 2021	$	759		$	5	(a)	$	764	(b)

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

(b)Balance included $59 million and $60 million in “Other current liabilities” on the balance sheet as of September 30, 2021, and December 31, 2020, respectively.

(c)During the first nine months of 2021, Ameren Missouri recorded an ARO related to the decommissioning of the Atchison Renewable Energy Center.

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

(e)Ameren Missouri changed its fair value estimate primarily due to a decrease in the cost estimate for closure of certain CCR storage facilities.

Schedule of excise taxes

The following table presents the excise taxes recorded on a gross basis in “Operating Revenues – Electric,” “Operating Revenues – Natural gas” and “Operating Expenses – Taxes other than income taxes” on the statements of income for the three and nine months ended September 30, 2021 and 2020:


	Three Months				Nine Months
	2021		2020		2021		2020
Ameren Missouri	$	54	$	45	$	120	$	111
Ameren Illinois	27		26		93		87
Ameren	$	81	$	71	$	213	$	198

Schedule of Earnings Per Share, Basic and Diluted

The following table reconciles the basic weighted-average number of common shares outstanding to the diluted weighted-average number of common shares outstanding for the three and nine months ended September 30, 2021 and 2020:


	Three Months		Nine Months
	2021	2020	2021	2020


Weighted-average Common Shares Outstanding – Basic	257.3	247.1	255.9	246.8
Assumed settlement of performance share units and restricted stock units	1.3	1.5	1.3	1.2
Dilutive effect of forward sale agreements	—	0.6	—	0.4
Weighted-average Common Shares Outstanding – Diluted(a)	258.6	249.2	257.2	248.4

(a)There was an immaterial number of anti-dilutive securities excluded from the earnings per diluted share calculations for the three and nine months ended September 30, 2021.There were no anti-dilutive securities excluded from the earnings per diluted share calculations for the three and nine months ended September 30, 2020.

Segment Information (Tables)

9 Months Ended

Sep. 30, 2021

Segment Reporting [Abstract]

Schedule Of Segment Reporting Information By Segment

The following tables present revenues, net income (loss) attributable to common shareholders, and capital expenditures by segment at Ameren and Ameren Illinois for the three and nine months ended September 30, 2021 and 2020. Ameren, Ameren Missouri, and Ameren Illinois management review segment capital expenditure information rather than any individual or total asset amount. For additional information about our segments, see Note 16 – Segment Information under Part II, Item 8, of the Form 10-K.

Ameren


	Ameren Missouri			Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Transmission			Other		Intersegment Eliminations		Ameren
Three Months 2021:
External revenues	$	1,118		$	425	$	127	$	141		$	—	$	—	$	1,811
Intersegment revenues	11			3		—		19			—		(33)		—
Net income (loss) attributable to Ameren common shareholders	375			36		(8)		73		(a)	(51)		—		425
Capital expenditures	466		(b)	143		93		154			2		(8)		850		(b)
Three Months 2020:
External revenues	$	990		$	390	$	122	$	126		$	—	$	—	$	1,628
Intersegment revenues	11			1		—		15			—		(27)		—
Net income (loss) attributable to Ameren common shareholders	297			34		2		62		(a)	(28)		—		367
Capital expenditures	262			129		81		185			2		(3)		656
Nine Months 2021:
External revenues	$	2,612		$	1,222	$	642	$	373		$	—	$	—	$	4,849
Intersegment revenues	30			5		—		53			—		(88)		—
Net income (loss) attributable to Ameren common shareholders	533			123		75		175		(a)	(41)		—		865
Capital expenditures	1,567		(b)	429		202		426			3		(14)		2,613		(b)
Nine Months 2020:
External revenues	$	2,442		$	1,131	$	533	$	360		$	—	$	—	$	4,466
Intersegment revenues	31			2		—		40			—		(73)		—
Net income (loss) attributable to Ameren common shareholders	439			107		66		168		(a)	(24)		—		756
Capital expenditures	778			391		221		490			4		—		1,884

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

(b)Includes $98 million and $515 million at Ameren and Ameren Missouri for wind generation expenditures for the three and nine months ended September 30, 2021, respectively.

Ameren Illinois


	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Illinois Transmission		Intersegment Eliminations		Ameren Illinois
Three Months 2021:
External revenues	$	428	$	127	$	90	$	—	$	645
Intersegment revenues	—		—		18		(18)		—
Net income (loss) available to common shareholder	36		(8)		51		—		79
Capital expenditures	143		93		144		—		380
Three Months 2020:
External revenues	$	391	$	122	$	76	$	—	$	589
Intersegment revenues	—		—		15		(15)		—
Net income available to common shareholder	34		2		41		—		77
Capital expenditures	129		81		160		—		370
Nine Months 2021:
External revenues	$	1,227	$	642	$	228	$	—	$	2,097
Intersegment revenues	—		—		49		(49)		—
Net income available to common shareholder	123		75		116		—		314
Capital expenditures	429		202		395		—		1,026
Nine Months 2020:
External revenues	$	1,133	$	533	$	213	$	—	$	1,879
Intersegment revenues	—		—		39		(39)		—
Net income available to common shareholder	107		66		107		—		280
Capital expenditures	391		221		419		—		1,031

Disaggregation of Revenue

The following tables present disaggregated revenues by segment at Ameren and Ameren Illinois for the three and nine months ended September 30, 2021 and 2020. Economic factors affect the nature, timing, amount, and uncertainty of revenues and cash flows in a similar manner across customer classes. Revenues from alternative revenue programs have a similar distribution among customer classes as revenues from contracts with customers. Other revenues not associated with contracts with customers are presented in the Other customer classification, along with electric transmission and off-system revenues.

Ameren


	Ameren Missouri		Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Transmission		Intersegment Eliminations		Ameren
Three Months 2021:
Residential	$	537	$	258	$	—	$	—	$	—	$	795
Commercial	412		143		—		—		—		555
Industrial	98		26		—		—		—		124
Other	66		1		—		160		(33)		194
Total electric revenues	$	1,113	$	428	$	—	$	160	$	(33)	$	1,668
Residential	$	9	$	—	$	75	$	—	$	—	$	84
Commercial	4		—		23		—		—		27
Industrial	—		—		3		—		—		3
Other	3		—		26		—		—		29
Total natural gas revenues	$	16	$	—	$	127	$	—	$	—	$	143
Total revenues(a)	$	1,129	$	428	$	127	$	160	$	(33)	$	1,811


	Ameren Missouri		Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Transmission		Intersegment Eliminations		Ameren
Three Months 2020:
Residential	$	455	$	234	$	—	$	—	$	—	$	689
Commercial	343		127		—		—		—		470
Industrial	87		26		—		—		—		113
Other	99		4		—		141		(27)		217
Total electric revenues	$	984	$	391	$	—	$	141	$	(27)	$	1,489
Residential	$	8	$	—	$	68	$	—	$	—	$	76
Commercial	3		—		18		—		—		21
Industrial	1		—		5		—		—		6
Other	5		—		31		—		—		36
Total natural gas revenues	$	17	$	—	$	122	$	—	$	—	$	139
Total revenues(a)	$	1,001	$	391	$	122	$	141	$	(27)	$	1,628
Nine Months 2021:
Residential	$	1,177	$	705	$	—	$	—	$	—	$	1,882
Commercial	899		402		—		—		—		1,301
Industrial	221		94		—		—		—		315
Other	246		26		—		426		(88)		610
Total electric revenues	$	2,543	$	1,227	$	—	$	426	$	(88)	$	4,108
Residential	$	54	$	—	$	438	$	—	$	—	$	492
Commercial	23		—		116		—		—		139
Industrial	2		—		20		—		—		22
Other	20		—		68		—		—		88
Total natural gas revenues	$	99	$	—	$	642	$	—	$	—	$	741
Total revenues(a)	$	2,642	$	1,227	$	642	$	426	$	(88)	$	4,849
Nine Months 2020:
Residential	$	1,111	$	664	$	—	$	—	$	—	$	1,775
Commercial	828		365		—		—		—		1,193
Industrial	207		91		—		—		—		298
Other	240		13		—		400		(73)		580
Total electric revenues	$	2,386	$	1,133	$	—	$	400	$	(73)	$	3,846
Residential	$	52	$	—	$	375	$	—	$	—	$	427
Commercial	20		—		94		—		—		114
Industrial	3		—		11		—		—		14
Other	12		—		53		—		—		65
Total natural gas revenues	$	87	$	—	$	533	$	—	$	—	$	620
Total revenues(a)	$	2,473	$	1,133	$	533	$	400	$	(73)	$	4,466

(a)The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the three and nine months ended September 30, 2021 and 2020:


	Ameren Missouri			Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Transmission		Ameren
Three Months 2021:
Revenues from alternative revenue programs	$	6		$	(97)	$	(5)	$	1	$	(95)
Other revenues not from contracts with customers	7		(a)	6		—		—		13		(a)

Three Months 2020:
Revenues from alternative revenue programs	$	1		$	(110)	$	3	$	4	$	(102)
Other revenues not from contracts with customers	6			5		—		—		11

Nine Months 2021:
Revenues from alternative revenue programs	$	(9)		$	(2)	$	—	$	5	$	(6)
Other revenues not from contracts with customers	71		(a)	9		2		—		82		(a)

Nine Months 2020:
Revenues from alternative revenue programs	$	(8)		$	(59)	$	17	$	34	$	(16)
Other revenues not from contracts with customers	21			6		1		—		28

(a)Includes insurance recoveries related to lost sales associated with the Callaway Energy Center maintenance outage. See Note 10 – Callaway Energy Center for additional information.

Ameren Illinois


	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Illinois Transmission		Intersegment Eliminations		Ameren Illinois
Three Months 2021:
Residential	$	258	$	75	$	—	$	—	$	333
Commercial	143		23		—		—		166
Industrial	26		3		—		—		29
Other	1		26		108		(18)		117
Total revenues(a)	$	428	$	127	$	108	$	(18)	$	645
Three Months 2020:
Residential	$	234	$	68	$	—	$	—	$	302
Commercial	127		18		—		—		145
Industrial	26		5		—		—		31
Other	4		31		91		(15)		111
Total revenues(a)	$	391	$	122	$	91	$	(15)	$	589
Nine Months 2021:
Residential	$	705	$	438	$	—	$	—	$	1,143
Commercial	402		116		—		—		518
Industrial	94		20		—		—		114
Other	26		68		277		(49)		322
Total revenues(a)	$	1,227	$	642	$	277	$	(49)	$	2,097
Nine Months 2020:
Residential	$	664	$	375	$	—	$	—	$	1,039
Commercial	365		94		—		—		459
Industrial	91		11		—		—		102
Other	13		53		252		(39)		279
Total revenues(a)	$	1,133	$	533	$	252	$	(39)	$	1,879

(a)The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the Ameren Illinois segments for the three and nine months ended September 30, 2021 and 2020:


	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Illinois Transmission		Ameren Illinois
Three Months 2021:
Revenues from alternative revenue programs	$	(97)	$	(5)	$	3	$	(99)
Other revenues not from contracts with customers	6		—		—		6

Three Months 2020:
Revenues from alternative revenue programs	$	(110)	$	3	$	5	$	(102)
Other revenues not from contracts with customers	5		—		—		5

Nine Months 2021:
Revenues from alternative revenue programs	$	(2)	$	—	$	4	$	2
Other revenues not from contracts with customers	9		2		—		11

Nine Months 2020:
Revenues from alternative revenue programs	$	(59)	$	17	$	29	$	(13)
Other revenues not from contracts with customers	6		1		—		7

Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2019
Basis Of Presentation And Significant Accounting Policies [Line Items]
Percentage of accounts receivable balances that are 30 days past due or are part of a deferred payment arrangement	18.00%		13.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 97		$ 65
Noncontrolling Interest in Variable Interest Entity	53	$ 37
Cash Surrender Value of Life Insurance	273	272
Corporate owned life insurance, borrowings	$ 109	107
Union Electric Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Percentage of accounts receivable balances that are 30 days past due or are part of a deferred payment arrangement	12.00%		9.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 33		$ 23
Union Electric Company \| Final Rate Order
Basis Of Presentation And Significant Accounting Policies [Line Items]
COVID-19 pandemic costs	$ 9
Ameren Illinois Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Percentage of accounts receivable balances that are 30 days past due or are part of a deferred payment arrangement	25.00%		18.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 64		$ 42
Cash Surrender Value of Life Insurance	115	115
Corporate owned life insurance, borrowings	109	$ 107
Partnership Funding Commitment
Basis Of Presentation And Significant Accounting Policies [Line Items]
Unrecorded Unconditional Purchase Obligation	$ 27

Rate And Regulatory Matters (Narrative-Missouri) (Detail) $ in Millions	1 Months Ended	3 Months Ended			9 Months Ended
	Jan. 31, 2021 MWh	Dec. 31, 2021 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) MWh	Sep. 30, 2020 USD ($)	Feb. 28, 2021 USD ($)	Dec. 31, 2020 USD ($)
Rate And Regulatory Matters [Line Items]
Regulatory assets			$ 1,235		$ 1,235			$ 1,100
Revenues			1,811	$ 1,628	4,849	$ 4,466
Union Electric Company
Rate And Regulatory Matters [Line Items]
Atchison Renewable Energy Center purchase price					500
Regulatory assets			483		483			$ 347
Pending Rate Case \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 299
Public Utilities, Requested Return on Equity, Percentage					9.90%
Public Utilities, Requested Equity Capital Structure, Percentage					51.90%
Rate Base					$ 10,000
Percentage of High Prairie Costs Challenged by the OPC					25.00%
Percentage of High Prairie Return Challenged by the MIEC					20.00%
Pending Rate Case \| Electricity \| Union Electric Company \| Subsequent Event
Rate And Regulatory Matters [Line Items]
MoPSC Staff Recommended Rate Increase (Decrease)		$ 188
MoPSC Staff Recommended Return on Equity, Percentage		9.50%
MoPSC Staff Recommended Equity Capital Structure, Percentage		50.32%
MoPSC Staff Recommended Rate Base		$ 10,000
Pending Rate Case \| Natural Gas \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 9
Public Utilities, Requested Return on Equity, Percentage					9.80%
Public Utilities, Requested Equity Capital Structure, Percentage					51.90%
Rate Base					$ 310
Pending Rate Case \| Natural Gas \| Union Electric Company \| Subsequent Event
Rate And Regulatory Matters [Line Items]
MoPSC Staff Recommended Rate Increase (Decrease)		$ 4
MoPSC Staff Recommended Return on Equity, Percentage		9.50%
MoPSC Staff Recommended Equity Capital Structure, Percentage		50.32%
MoPSC Staff Recommended Rate Base		$ 301
Wind Generation Facility \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Amount of Megawatts \| MWh	300				700
Final Rate Order \| Union Electric Company
Rate And Regulatory Matters [Line Items]
COVID-19 pandemic costs					$ 9
Regulatory assets			$ 5		5
Revenues					4
Final Rate Order \| Electricity \| Union Electric Company \| MEEIA 2019 [Domain]
Rate And Regulatory Matters [Line Items]
Revenues					9	$ 6
Incentive Award if Energy Efficiency Goals Are Achieved					13
MEEIA energy efficiency investments for the 2023 program year					$ 75
Under Recovered PGA \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory Assets							$ 53
Under Recovered PGA \| Union Electric Company \| Subsequent Event \| Minimum
Rate And Regulatory Matters [Line Items]
Amortization Period		12 months
Under Recovered PGA \| Union Electric Company \| Subsequent Event \| Maximum
Rate And Regulatory Matters [Line Items]
Amortization Period		36 months

Rate And Regulatory Matters (Narrative-Illinois) (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) numberOfWindGenerationFacilities	Sep. 30, 2020 USD ($)
Rate And Regulatory Matters [Line Items]
Capital expenditures			$ 2,098	$ 1,884
Revenues	$ 1,811	$ 1,628	4,849	4,466
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Capital expenditures			1,026	1,031
Revenues	$ 645	$ 589	2,097	$ 1,879
Annual investment in energy-efficiency programs			$ 100
Deferred payment arrangement extension			18 months
Financial assistance program costs			$ 4
CEJA \| Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Energy Transition Assistance Fund Surcharge			50
CEJA \| Ameren Illinois Company \| Minimum
Rate And Regulatory Matters [Line Items]
Increase in the Annual Investment in Energy-Efficiency Programs			10
CEJA \| Ameren Illinois Company \| Maximum
Rate And Regulatory Matters [Line Items]
Increase in the Annual Investment in Energy-Efficiency Programs			$ 40
CEJA \| Ameren Illinois Company \| Electric Distribution
Rate And Regulatory Matters [Line Items]
Rate Case Filing Frequency			4 years
Public Utilities, Approved Equity Capital Structure, Percentage			50.00%
Multi-Year Rate Plan Reconciliation Cap			105.00%
Amortization Period			2 years
Amortization Start Date			1 year
CEJA \| Ameren Illinois Company \| Electric Distribution \| Minimum
Rate And Regulatory Matters [Line Items]
Return on Equity Adjustment			0.20%
Public Utilities, Requested Rate Increase (Decrease), Percentage			50.00%
CEJA \| Ameren Illinois Company \| Electric Distribution \| Maximum
Rate And Regulatory Matters [Line Items]
Return on Equity Adjustment			0.60%
CEJA \| Ameren Illinois Company \| Electricity, Generation
Rate And Regulatory Matters [Line Items]
Capital expenditures			$ 20
Number of Solar Generation Pilot Projects \| numberOfWindGenerationFacilities			2
IEIMA \| Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage			5.80%
Final Rate Order \| IEIMA \| Ameren Illinois Company \| Electric Distribution
Rate And Regulatory Matters [Line Items]
Amortization Period			2 years
Pending Rate Case \| Ameren Illinois Company \| Natural Gas
Rate And Regulatory Matters [Line Items]
Natural Gas Capital Investment Prudency Challenge			$ 70
Challenged Recoveries Under the QIP			3
Pending Rate Case \| IEIMA \| Ameren Illinois Company \| Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 59
Public Utilities, Requested Equity Capital Structure, Percentage			51.00%
Pending Rate Case \| FEJA energy-efficiency rider \| Ameren Illinois Company \| Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 10
Renewable Energy Credits \| Ameren Illinois Company \| Electricity, Generation
Rate And Regulatory Matters [Line Items]
Revenues			$ 100

Rate And Regulatory Matters (Narrative-Federal) (Detail) - USD ($) $ in Millions	1 Months Ended		9 Months Ended
	May 31, 2020	Nov. 30, 2019	Sep. 30, 2021	Dec. 31, 2020
Rate And Regulatory Matters [Line Items]
Regulatory liabilities			$ 5,425	$ 5,282
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory liabilities			2,193	$ 2,063
Final Rate Order \| Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory liabilities			$ 9
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	10.02%	9.88%	10.32%
Customer Requested Rate on Equity			9.15%
Incentive adder to FERC allowed base return on common equity			0.50%
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%

Short-Term Debt And Liquidity (Narrative) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020	Sep. 30, 2021 USD ($)	Sep. 30, 2020	Dec. 31, 2020 USD ($)
Short-term Debt [Line Items]
Short-term debt	$ 553		$ 553		$ 490
Credit Agreements
Short-term Debt [Line Items]
Net Liquidity Available	$ 1,800		$ 1,800
Actual debt-to-capital ratio	0.57		0.57
Utilities
Short-term Debt [Line Items]
Short-term Debt, Weighted Average Interest Rate, over Time	0.06%	0.10%	0.17%	0.81%
Union Electric Company \| Missouri Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.48		0.48
Ameren Illinois Company \| Illinois Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.45		0.45
Ameren (parent) \| Commercial Paper
Short-term Debt [Line Items]
Short-term debt	$ 553		$ 553		$ 490

Short-Term Debt and Liquidity (Short-Term Debt Activity) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Short-term Debt [Line Items]
Average daily amount outstanding	$ 665	$ 257
Weighted-average interest rate	0.22%	1.70%
Short-term Debt, Maximum Amount Outstanding During Period	$ 1,134	$ 908
Peak interest rate	0.33%	5.05%
Ameren (parent)
Short-term Debt [Line Items]
Average daily amount outstanding	$ 404	$ 62
Weighted-average interest rate	0.23%	2.04%
Short-term Debt, Maximum Amount Outstanding During Period	$ 650	$ 425
Peak interest rate	0.33%	3.30%
Union Electric Company
Short-term Debt [Line Items]
Average daily amount outstanding	$ 121	$ 142
Weighted-average interest rate	0.22%	1.76%
Short-term Debt, Maximum Amount Outstanding During Period	$ 546	$ 573
Peak interest rate	0.25%	5.05%
Ameren Illinois Company
Short-term Debt [Line Items]
Average daily amount outstanding	$ 140	$ 53
Weighted-average interest rate	0.22%	1.10%
Short-term Debt, Maximum Amount Outstanding During Period	$ 485	$ 243
Peak interest rate	0.25%	3.40%

Long-Term debt and Equity Financings (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	1 Months Ended	3 Months Ended			9 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 03, 2021	May 31, 2021	Feb. 11, 2021
Long-Term Debt And Equity Financings [Line Items]
Shares issued under the DRPlus and 401(k) plan		100		100	400	500
Stock Issued During Period, Shares, Other		0	500	0	500	500
Stock Issued During Period, Value, Other			$ 33
Shares issued under forward sale agreement (in shares)		0		0	1,600	0
Common Stock Value Issued Through Forward Sale Agreement									$ 113
Maximum Value Of Shares To Be Issued Under ATM Program								$ 750
Stock Issued During Period, Shares, New Issues		400		0	1,800	0
Payments of Stock Issuance Costs		$ 1			$ 2
Quantity of shares issued under ATM program		400			1,800
Value of shares issued under ATM program		$ 27			$ 148
Forward Contract Indexed to Issuer's Equity, Indexed Shares (in shares)							400
Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share	$ 87.87
Forward Contract Indexed to Issuer's Equity, Settlement Alternatives, Shares, at Fair Value	400	400			400
Forward Contract Indexed to Issuer's Equity, Settlement Alternatives, Cash, at Fair Value	$ 31	$ 31			$ 31
Other Paid-in Capital
Long-Term Debt And Equity Financings [Line Items]
Shares issued under the DRPlus and 401(k) plan		12		$ 10	36	$ 37
Shares issued under the ATM program		27		0	148	0
Ameren (parent) \| Unsecured Debt \| Senior Secured Notes 1.75% Due 2028 [Domain]
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 450	$ 450			$ 450
Debt Instrument, Interest Rate, Stated Percentage	1.75%	1.75%			1.75%
Proceeds from Issuance of Debt					$ 447
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Capital contributions from parent					183	0
Union Electric Company \| Other Paid-in Capital
Long-Term Debt And Equity Financings [Line Items]
Capital contributions from parent		$ 0		0	183	0
Union Electric Company \| Secured Debt \| Senior Secured Notes 2.15% Due 2032
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 525	$ 525			$ 525
Debt Instrument, Interest Rate, Stated Percentage	2.15%	2.15%			2.15%
Proceeds from Issuance of Secured Debt					$ 521
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Capital contributions from parent					145	350
Redemption of preferred stock					13	0
Ameren Illinois Company \| Other Paid-in Capital
Long-Term Debt And Equity Financings [Line Items]
Capital contributions from parent		$ 75		$ 0	$ 145	$ 350
Ameren Illinois Company \| Series6625
Long-Term Debt And Equity Financings [Line Items]
Preferred Stock, Dividend Rate, Percentage					6.625%
Redemption of preferred stock					$ 12
Ameren Illinois Company \| Series775
Long-Term Debt And Equity Financings [Line Items]
Preferred Stock, Dividend Rate, Percentage					7.75%
Redemption of preferred stock					$ 1
Ameren Illinois Company \| Secured Debt \| First Mortgage Bonds, 1.55%, 350 Million Due 2051
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 350	$ 350			$ 350
Debt Instrument, Interest Rate, Stated Percentage	2.90%	2.90%			2.90%
Proceeds from Issuance of Secured Debt					$ 345
Ameren Illinois Company \| Secured Debt \| First Mortgage Bonds, 0.375%, 100 Million Due 2023
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 100	$ 100			$ 100
Debt Instrument, Interest Rate, Stated Percentage	0.375%	0.375%			0.375%
Proceeds from Issuance of Secured Debt					$ 100

Other Income, Net (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
Other Income, Net (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 14	$ 12	$ 30	$ 25
Interest income on industrial development revenue bonds	7	7	19	19
Other interest income	0	0	1	2
Non-service cost components of net periodic benefit income	34	32	102	85
Miscellaneous income	3	1	14	10
Donations	(1)	0	(5)	(14)
Miscellaneous expense	(1)	(4)	(10)	(10)
Total Other Income, Net	56	48	151	117
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	7	7	17	15
Interest income on industrial development revenue bonds	7	7	19	19
Non-service cost components of net periodic benefit income	13	13	41	32
Miscellaneous income	2	1	3	3
Donations	0	0	(1)	(9)
Miscellaneous expense	(2)	(2)	(5)	(5)
Total Other Income, Net	27	26	74	55
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	(3)	(3)	(6)	(1)
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	7	5	13	10
Other interest income	0	0	1	2
Non-service cost components of net periodic benefit income	13	12	41	36
Miscellaneous income	0	1	3	6
Donations	(1)	0	(4)	(5)
Miscellaneous expense	0	(1)	(5)	(4)
Total Other Income, Net	$ 19	$ 17	$ 49	45
Final Rate Order \| Electricity \| Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Donations				$ (8)

Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) lb in Thousands, gal in Millions, MWh in Millions, MMBTU in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021 MMBTU MWh lb gal	Dec. 31, 2020 MWh MMBTU lb gal
Fuel Oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume \| gal	28	43
Natural Gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MMBTU	169	147
Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MWh	12	13
Uranium
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass \| lb	400	365
Union Electric Company \| Fuel Oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume \| gal	28	43
Union Electric Company \| Natural Gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MMBTU	33	33
Union Electric Company \| Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MWh	6	6
Union Electric Company \| Uranium
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass \| lb	400	365
Ameren Illinois Company \| Fuel Oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume \| gal	0	0
Ameren Illinois Company \| Natural Gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MMBTU	136	114
Ameren Illinois Company \| Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MWh	6	7
Ameren Illinois Company \| Uranium
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass \| lb	0	0

Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Derivative [Line Items]
Derivative assets	$ 145	$ 22
Derivative liabilities	249	221
Fuel Oils \| Other Current Assets
Derivative [Line Items]
Derivative assets	9	2
Fuel Oils \| Other Assets
Derivative [Line Items]
Derivative assets	5	0
Fuel Oils \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	7
Fuel Oils \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	2
Natural Gas \| Other Current Assets
Derivative [Line Items]
Derivative assets	68	9
Natural Gas \| Other Assets
Derivative [Line Items]
Derivative assets	28	4
Natural Gas \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	4	2
Natural Gas \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	2	1
Power \| Other Current Assets
Derivative [Line Items]
Derivative assets	32	7
Power \| Other Assets
Derivative [Line Items]
Derivative assets	1	0
Power \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	80	20
Power \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	162	189
Uranium \| Other Assets
Derivative [Line Items]
Derivative assets	2	0
Uranium \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	1	0
Union Electric Company
Derivative [Line Items]
Derivative assets	69	12
Derivative liabilities	98	21
Union Electric Company \| Fuel Oils \| Other Current Assets
Derivative [Line Items]
Derivative assets	9	2
Union Electric Company \| Fuel Oils \| Other Assets
Derivative [Line Items]
Derivative assets	5	0
Union Electric Company \| Fuel Oils \| Mark-to-market derivative liabilities
Derivative [Line Items]
Derivative liabilities	0	7
Union Electric Company \| Fuel Oils \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Union Electric Company \| Fuel Oils \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	2
Union Electric Company \| Natural Gas \| Other Current Assets
Derivative [Line Items]
Derivative assets	13	1
Union Electric Company \| Natural Gas \| Other Assets
Derivative [Line Items]
Derivative assets	8	2
Union Electric Company \| Natural Gas \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Union Electric Company \| Natural Gas \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Union Electric Company \| Power \| Other Current Assets
Derivative [Line Items]
Derivative assets	31	7
Union Electric Company \| Power \| Other Assets
Derivative [Line Items]
Derivative assets	1	0
Union Electric Company \| Power \| Mark-to-market derivative liabilities
Derivative [Line Items]
Derivative liabilities	75	3
Union Electric Company \| Power \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Union Electric Company \| Power \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	22	8
Union Electric Company \| Uranium \| Other Assets
Derivative [Line Items]
Derivative assets	2	0
Union Electric Company \| Uranium \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	1	0
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	76	10
Derivative liabilities	151	200
Ameren Illinois Company \| Fuel Oils \| Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company \| Fuel Oils \| Other Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company \| Fuel Oils \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company \| Fuel Oils \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company \| Natural Gas \| Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company \| Natural Gas \| Other Assets
Derivative [Line Items]
Derivative assets	20	2
Ameren Illinois Company \| Natural Gas \| Mark-to-market derivative assets
Derivative [Line Items]
Derivative assets	55	8
Ameren Illinois Company \| Natural Gas \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	4	1
Ameren Illinois Company \| Natural Gas \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	2	1
Ameren Illinois Company \| Power \| Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company \| Power \| Other Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company \| Power \| Mark-to-market derivative assets
Derivative [Line Items]
Derivative assets	1	0
Ameren Illinois Company \| Power \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	5	17
Ameren Illinois Company \| Power \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	140	181
Ameren Illinois Company \| Uranium \| Other Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company \| Uranium \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	$ 0	$ 0

Derivative Financial Instruments (Offsetting Assets and Liabilities) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	$ 145	$ 22
Gross derivative instruments not offset in the balance sheet	16
Gross cash collateral received not offset in the balance sheet	5
Net derivative asset	124
Gross derivative liability amount recognized on the balance sheet	249	221
Gross derivative instruments not offset in the balance sheet	16
Gross cash collateral posted not offset in the balance sheet	57
Net derivative liability	176
Union Electric Company
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	69	12
Gross derivative instruments not offset in the balance sheet	14
Gross cash collateral received not offset in the balance sheet	2
Net derivative asset	53
Gross derivative liability amount recognized on the balance sheet	98	21
Gross derivative instruments not offset in the balance sheet	14
Gross cash collateral posted not offset in the balance sheet	57
Net derivative liability	27
Ameren Illinois Company
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	76	10
Gross derivative instruments not offset in the balance sheet	2
Gross cash collateral received not offset in the balance sheet	3
Net derivative asset	71
Gross derivative liability amount recognized on the balance sheet	151	$ 200
Gross derivative instruments not offset in the balance sheet	2
Gross cash collateral posted not offset in the balance sheet	0
Net derivative liability	$ 149

Derivative Financial Instruments (Credit Risk) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($)
Credit Derivatives [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	$ 127
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure	114
Union Electric Company
Credit Derivatives [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	51
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure	39
Ameren Illinois Company
Credit Derivatives [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	76
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure	$ 75

Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features (Details) $ in Millions	Sep. 30, 2021 USD ($)
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	$ 51
Cash Collateral Posted	3
Potential Aggregate Amount of Additional Collateral Required	32
Union Electric Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	45
Cash Collateral Posted	3
Potential Aggregate Amount of Additional Collateral Required	28
Ameren Illinois Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	6
Cash Collateral Posted	0
Potential Aggregate Amount of Additional Collateral Required	$ 4

Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	$ 1,068	$ 977
Assets fair value	1,213	999
Excluded receivables, payables, and accrued income, net	8	5
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	743	680
Assets fair value	772	682
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	325	297
Assets fair value	407	306
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Assets fair value	34	11
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	145	22
Derivative liabilities	249	221
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	29	2
Derivative liabilities	54	14
Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	82	9
Derivative liabilities	2	2
Commodity Contract \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	34	11
Derivative liabilities	193	205
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	1,068	977
Assets fair value	1,137	989
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	743	680
Assets fair value	769	682
Union Electric Company \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	325	297
Assets fair value	345	300
Union Electric Company \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Assets fair value	23	7
Union Electric Company \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	69	12
Derivative liabilities	98	21
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	26	2
Derivative liabilities	54	14
Union Electric Company \| Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	20	3
Derivative liabilities	0	1
Union Electric Company \| Commodity Contract \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	23	7
Derivative liabilities	44	6
Union Electric Company \| Fuel Oils \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	14	2
Derivative liabilities	0	9
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	14	0
Derivative liabilities	0	6
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 \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	2
Derivative liabilities	0	3
Union Electric Company \| Natural Gas \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	21	3
Derivative liabilities	0	1
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 assets	0	0
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 assets	20	3
Derivative liabilities	0	1
Union Electric Company \| Natural Gas \| Commodity Contract \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	0
Derivative liabilities	0	0
Union Electric Company \| Power \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	32	7
Derivative liabilities	97	11
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	12	2
Derivative liabilities	54	8
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	0
Derivative liabilities	0	0
Union Electric Company \| Power \| Commodity Contract \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	20	5
Derivative liabilities	43	3
Union Electric Company \| Uranium \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	0
Derivative liabilities	1	0
Union Electric Company \| Uranium \| 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 \| Uranium \| 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 \| Uranium \| Commodity Contract \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	0
Derivative liabilities	1	0
Union Electric Company \| Equity Securities \| U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	743	680
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	743	680
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 \| 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 \| US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	141	115
Union Electric Company \| Debt Securities \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	128	115
Union Electric Company \| Debt Securities \| Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	56	67
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) \| 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) \| 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) \| US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	141	115
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	128	115
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	56	67
Union Electric Company \| Debt Securities \| 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 \| 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 \| Level 3 \| Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets fair value	76	10
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]
Assets fair value	3	0
Ameren Illinois Company \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets fair value	62	6
Ameren Illinois Company \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets fair value	11	4
Ameren Illinois Company \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	151	200
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	2	1
Ameren Illinois Company \| Commodity Contract \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	149	199
Ameren Illinois Company \| Natural Gas \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	75	10
Derivative liabilities	6	2
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	3	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	62	6
Derivative liabilities	2	1
Ameren Illinois Company \| Natural Gas \| Commodity Contract \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	10	4
Derivative liabilities	4	1
Ameren Illinois Company \| Power \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	0
Derivative liabilities	145	198
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 assets	0	0
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 assets	0	0
Derivative liabilities	0	0
Ameren Illinois Company \| Power \| Commodity Contract \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	0
Derivative liabilities	$ 145	$ 198

Fair Value Measurements (Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level Three In The Fair Value Hierarchy) (Detail) - Power - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ (171)	$ (213)	$ (196)	$ (211)
Included in regulatory assets/liabilities	3	10	21	16
Settlements, assets	1		8
Settlement, liabilities		0		(8)
Change in unrealized gains (losses) related to assets/liabilities held at period end	2	9	21	5
Ending balance	(167)	(203)	(167)	(203)
Union Electric Company
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(5)	16	2	13
Included in regulatory assets/liabilities	(16)	(2)	(22)	18
Settlement, liabilities	(2)	(4)	(3)	(21)
Change in unrealized gains (losses) related to assets/liabilities held at period end	(17)	(2)	(21)	6
Ending balance	(23)	10	(23)	10
Ameren Illinois Company
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(166)	(229)	(198)	(224)
Included in regulatory assets/liabilities	19	12	43	(2)
Settlements, assets	3	4	11	13
Change in unrealized gains (losses) related to assets/liabilities held at period end	19	11	42	(1)
Ending balance	$ (144)	$ (213)	$ (144)	$ (213)

Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) - Power $ in Millions	Sep. 30, 2021 USD ($) $ / MWh $ / MMBTU	Dec. 31, 2020 USD ($) $ / MWh $ / MMBTU
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets \| $	$ 21	$ 5
Derivative liabilities \| $	$ (188)	$ (201)
Commodity Forward Price \| Discounted Cash Flow \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	28	23
Commodity Forward Price \| Discounted Cash Flow \| Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	77	37
Commodity Forward Price \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	40	29
Nodal Basis \| Discounted Cash Flow \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	(4)	(6)
Nodal Basis \| Discounted Cash Flow \| Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0	0
Nodal Basis \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	(2)	(2)
Commodity Future Price \| Discounted Cash Flow \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input \| $ / MMBTU	(4)	2
Commodity Future Price \| Discounted Cash Flow \| Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input \| $ / MMBTU	(2)	6
Commodity Future Price \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input \| $ / MMBTU	(3)	3

Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Financial Assets and Liabilities) (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	$ 145	$ 301	$ 168	$ 176
Short-term debt	553	490
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	11	145	10	39
Advances to money pool	24	139
Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	122	147	$ 147	$ 125
Advances to money pool	41	0
Borrowings from money pool	0	19
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	145	301
Investments in industrial development revenue bonds	256	256
Short-term debt	553	490
Long-term debt (including current portion)	12,501	11,086
Debt Issuance Costs, Net	93	84
Carrying Amount \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	11	145
Advances to money pool	24	139
Investments in industrial development revenue bonds	256	256
Long-term debt (including current portion)	5,626	5,104
Debt Issuance Costs, Net	39	36
Carrying Amount \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	122	147
Advances to money pool	41
Long-term debt (including current portion)	4,391	3,946
Debt Issuance Costs, Net	40	36
Borrowings from money pool		19
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	145	301
Investments, Fair Value Disclosure	256	256
Short-term Debt, Fair Value	553	490
Long-term Debt, Fair Value	14,096	13,315
Fair Value \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	145	301
Investments, Fair Value Disclosure	0	0
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	0	0
Fair Value \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	0	0
Investments, Fair Value Disclosure	256	256
Short-term Debt, Fair Value	553	490
Long-term Debt, Fair Value	13,576	12,778
Fair Value \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	0	0
Investments, Fair Value Disclosure	0	0
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	520	537
Fair Value \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	11	145
Advances to money pool	24	139
Investments, Fair Value Disclosure	256	256
Long-term Debt, Fair Value	6,392	6,160
Fair Value \| Union Electric Company \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	11	145
Advances to money pool	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Fair Value \| Union Electric Company \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	0	0
Advances to money pool	24	139
Investments, Fair Value Disclosure	256	256
Long-term Debt, Fair Value	6,392	6,160
Fair Value \| Union Electric Company \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	0	0
Advances to money pool	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Fair Value \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	122	147
Advances to money pool	41
Long-term Debt, Fair Value	5,019	4,822
Borrowings from money pool		19
Fair Value \| Ameren Illinois Company \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	122	147
Advances to money pool	0
Long-term Debt, Fair Value	0	0
Borrowings from money pool		0
Fair Value \| Ameren Illinois Company \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	0	0
Advances to money pool	41
Long-term Debt, Fair Value	5,019	4,822
Borrowings from money pool		19
Fair Value \| Ameren Illinois Company \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	0	0
Advances to money pool	0
Long-term Debt, Fair Value	$ 0	0
Borrowings from money pool		$ 0

Related Party Transactions (Narrative) (Details) - Ameren Illinois Company $ in Millions	1 Months Ended	9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2021 MWh $ / MWh
April 2021 Procurement \| Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Related Party Long Term Contract For Purchase of Electric Power \| MWh		33,600
Related Party Long Term Contract For Purchase of Electric Power Rate \| $ / MWh		34
September 2021 Procurement Event \| Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Related Party Long Term Contract For Purchase of Electric Power \| MWh		136,000
Related Party Long Term Contract For Purchase of Electric Power Rate \| $ / MWh		37
September 2021 Procurement \| Ameren Illinois Capacity Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Energy Supply Agreements Amount \| $	$ 2

Related Party Transactions (Schedule of Affiliate Receivables and Payables) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Union Electric Company
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	$ 47	$ 46
Accounts Receivable, Related Parties, Current	51	57
Union Electric Company \| Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	0	0
Union Electric Company \| Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	0	9
Ameren Illinois Company
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	59	51
Accounts Receivable, Related Parties, Current	94	64
Ameren Illinois Company \| Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	12	6
Ameren Illinois Company \| Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	$ 44	$ 15

Related Party Transactions (Effects of Related-party Transactions on the Statement of Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Union Electric Company \| Ameren Missouri Power Supply Agreements with Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues	$ 5	$ 5	$ 10	$ 11
Union Electric Company \| Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	6	6	20	19
Operating Expenses	1	1	1	1
Union Electric Company \| Ameren Missouri and Ameren Illinois Miscellaneous Support Services
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	1
Union Electric Company \| Total Related Party Operating Revenues
Related Party Transaction [Line Items]
Operating Revenues	11	11	30	31
Union Electric Company \| Ameren Missouri and Ameren Illinois Transmission Services from ATXI
Related Party Transaction [Line Items]
Operating Expenses	1		3
Union Electric Company \| Purchased Power
Related Party Transaction [Line Items]
Operating Expenses	1		3
Union Electric Company \| Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	41	36	110	103
Union Electric Company \| Total Related Party Other Operations and Maintenance
Related Party Transaction [Line Items]
Operating Expenses	41	36	110	103
Union Electric Company \| Money pool borrowings (advances)
Related Party Transaction [Line Items]
Interest Charges (Income)	1	1	1	1
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	1
Operating Expenses	1	1	3	3
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois Miscellaneous Support Services
Related Party Transaction [Line Items]
Operating Revenues	3	1	5	1
Ameren Illinois Company \| Total Related Party Operating Revenues
Related Party Transaction [Line Items]
Operating Revenues	3	1	5	2
Ameren Illinois Company \| Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Operating Expenses	5	5	10	11
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois Transmission Services from ATXI
Related Party Transaction [Line Items]
Operating Expenses	1	1	1	1
Ameren Illinois Company \| Purchased Power
Related Party Transaction [Line Items]
Operating Expenses	5	5	11	12
Ameren Illinois Company \| Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	37	34	101	98
Ameren Illinois Company \| Total Related Party Other Operations and Maintenance
Related Party Transaction [Line Items]
Operating Expenses	38	35	104	101
Ameren Illinois Company \| Money pool borrowings (advances)
Related Party Transaction [Line Items]
Interest Charges (Income)	$ 1	$ 1	$ 1	$ 1

Commitments And Contingencies (Environmental Matters) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($) numberOfWindGenerationFacilities center scrubber site	Dec. 31, 2020 USD ($)
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 764	$ 756
CEJA \| Electricity, Generation
Loss Contingencies [Line Items]
Number of Ameren Missouri Natural Gas Energy Centers Subject to CEJA \| numberOfWindGenerationFacilities	5
Number of Early Energy Center Closures \| numberOfWindGenerationFacilities	1
Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 175
Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 225
Union Electric Company
Loss Contingencies [Line Items]
Number of Energy Center Scrubbers \| scrubber	2
Number of Energy Centers Constructing Wastewater Treatment Facilities \| center	3
Number of energy centers \| center	4
Number of Energy Centers Closing Surface Impoundments \| center	3
Asset Retirement Obligation	$ 759	751
Union Electric Company \| Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	175
Union Electric Company \| Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	225
Ameren Illinois Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	5	$ 5
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	75
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	100
Manufactured Gas Plant
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 80
Manufactured Gas Plant \| Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites \| site	44
Accrual for environmental loss contingencies	$ 80
Manufactured Gas Plant \| Ameren Illinois Company \| Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	80
Manufactured Gas Plant \| Ameren Illinois Company \| Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	145
Rush Island Energy Center \| Union Electric Company
Loss Contingencies [Line Items]
Estimated capital costs to comply with preliminary court order	1,000
Rate Base	400
Rush Island Energy Center \| Union Electric Company \| Minimum
Loss Contingencies [Line Items]
Estimated operations and maintenance costs to comply with preliminary court order	30
Rush Island Energy Center \| Union Electric Company \| Maximum
Loss Contingencies [Line Items]
Estimated operations and maintenance costs to comply with preliminary court order	50
New CCR Rules Estimate
Loss Contingencies [Line Items]
Asset Retirement Obligation	88
New CCR Rules Estimate \| Union Electric Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 88

Callaway Energy Center (Insurance Disclosure) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Nuclear Waste Matters [Line Items]
Decommissioning Cost	$ 7.0
Frequency of Decommissioning Cost Study	3 years
Miscellaneous accounts receivable	$ 111.0	$ 65.0
Number Of Years The Limit Of Liability And The Maximum Potential Annual Payments Are Adjusted	5 years
Number Of Weeks Of Coverage After The First Twelve Weeks Of An Outage	1
Number Of Additional Weeks After Initial Indemnity Coverage For Power Outage	1.365
Union Electric Company
Nuclear Waste Matters [Line Items]
Miscellaneous accounts receivable	$ 93.0	$ 36.0
Nuclear Plant \| Union Electric Company
Nuclear Waste Matters [Line Items]
Estimated Nuclear Generator Repairs	60.0
Accidental Outage - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Amount Of Weekly Indemnity Coverage Commencing Twelve Weeks After Power Outage	4.5
Insurance Aggregate Maximum Coverage	490.0
Insurance Maximum Coverage per Incident	7.0
Amount Of Additional Weekly Indemnity Coverage Commencing After Initial Indemnity Coverage	3.6
Amount Of Weekly Indemnity Coverage Thereafter Not Exceeding Policy Limit	490.0
Sub-Limit Of Amount Of Weekly Indemnity Coverage Thereafter Not Exceeding Policy Limit For Non-Nuclear Events	328.0
Public Liability And Nuclear Worker Liability - American Nuclear Insurers
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	450.0
Insurance Maximum Coverage per Incident	0.0
Public Liability And Nuclear Worker Liability - Pool Participation
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	13,073.0
Insurance Maximum Coverage per Incident	138.0
Threshold Amount For Retrospective Insurance Assessment For Covered Loss Under Public Liability And Nuclear Worker Liability Insurance Policy	450.0
Maximum Annual Payment Per Incident At Licensed Commercial Nuclear Reactor	21.0
Public Liability
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	13,523.0
Insurance Maximum Coverage per Incident	138.0
Property Damage - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	3,200.0
Insurance Maximum Coverage per Incident	25.0
Radiation Event
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	2,700.0
Aggregate Nuclear Power Industry Insurance Policy Limit For Losses From Terrorist Attacks Within Twelve Month Period	3,200.0
Non-Radiation Event
Nuclear Waste Matters [Line Items]
Miscellaneous accounts receivable	35.0
Insurance Aggregate Maximum Coverage	2,300.0
Aggregate Nuclear Power Industry Insurance Policy Limit For Losses From Terrorist Attacks Within Twelve Month Period	1,800.0
Property Damage European Mutual Association for Nuclear Insurance
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	$ 490.0

Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	$ (34)	$ (32)	$ (102)	$ (85)
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	33	28	100	83
Interest cost	38	43	114	130
Expected return on plan assets	(74)	(73)	(223)	(218)
Prior service benefit	0	0	0	(1)
Actuarial loss (gain)	18	15	55	45
Total non-service cost components	(18)	(15)	(54)	(44)
Net periodic benefit cost (income)	15	13	46	39
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	5	4	17	14
Interest cost	9	10	25	29
Expected return on plan assets	(20)	(20)	(60)	(60)
Prior service benefit	(1)	(1)	(3)	(3)
Actuarial loss (gain)	(1)	(3)	(4)	(7)
Total non-service cost components	(13)	(14)	(42)	(41)
Net periodic benefit cost (income)	$ (8)	$ (10)	$ (25)	$ (27)

Retirement Benefits (Summary of Benefit Plan Costs Incurred) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 15	$ 13	$ 46	$ 39
Pension Plan \| Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	7	5	22	16
Pension Plan \| Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	8	8	25	24
Pension Plan \| Other Affiliated Entities And Intercompany Eliminations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	0	0	(1)	(1)
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(8)	(10)	(25)	(27)
Other Postretirement Benefit Plan, Defined Benefit \| Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(1)	(2)	(3)	(4)
Other Postretirement Benefit Plan, Defined Benefit \| Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(8)	(8)	(23)	(24)
Other Postretirement Benefit Plan, Defined Benefit \| Other Affiliated Entities And Intercompany Eliminations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 1	$ 0	$ 1	$ 1

Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Taxes [Line Items]
Federal statutory corporate income tax rate	21.00%	21.00%	21.00%	21.00%
Increases (decreases) from:
Amortization of deferred investment tax credit	(1.00%)	(1.00%)	0.00%	0.00%
Amortization of excess deferred taxes	(8.00%)	(9.00%)	(9.00%)	(9.00%)
Depreciation differences	0.00%	0.00%
Renewable and other tax credits	(2.00%)	(1.00%)	(3.00%)	(1.00%)
State tax	4.00%	4.00%	5.00%	5.00%
Stock-based compensation	0.00%	1.00%	(1.00%)	(1.00%)
Effective income tax rate	14.00%	15.00%	13.00%	15.00%
Union Electric Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate	21.00%	21.00%	21.00%	21.00%
Increases (decreases) from:
Amortization of deferred investment tax credit	(1.00%)	(1.00%)	0.00%	(1.00%)
Amortization of excess deferred taxes	(17.00%)	(16.00%)	(17.00%)	(16.00%)
Depreciation differences	0.00%	0.00%
Renewable and other tax credits	(8.00%)	(2.00%)	(8.00%)	(1.00%)
State tax	4.00%	4.00%	3.00%	3.00%
Stock-based compensation	0.00%	0.00%	0.00%	0.00%
Effective income tax rate	(1.00%)	6.00%	(1.00%)	6.00%
Ameren Illinois Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate	21.00%	21.00%	21.00%	21.00%
Increases (decreases) from:
Amortization of deferred investment tax credit	0.00%	0.00%	0.00%	0.00%
Amortization of excess deferred taxes	(2.00%)	(3.00%)	(3.00%)	(3.00%)
Depreciation differences	(1.00%)	(1.00%)
Renewable and other tax credits	0.00%	0.00%	0.00%	0.00%
State tax	7.00%	7.00%	7.00%	7.00%
Stock-based compensation	0.00%	0.00%	0.00%	0.00%
Effective income tax rate	25.00%	24.00%	25.00%	25.00%

Supplemental Information (Cash and Cash Equivalents) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	$ 7	$ 139
Restricted cash included in “Other current assets”	16	17
Restricted cash included in “Other assets”	116	141
Restricted cash included in “Nuclear decommissioning trust fund”	6	4
Total cash, cash equivalents, and restricted cash	145	301	$ 168	$ 176
Union Electric Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	0	136
Restricted cash included in “Other current assets”	5	5
Restricted cash included in “Other assets”	0	0
Restricted cash included in “Nuclear decommissioning trust fund”	6	4
Total cash, cash equivalents, and restricted cash	11	145	10	39
Ameren Illinois Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	0	0
Restricted cash included in “Other current assets”	6	6
Restricted cash included in “Other assets”	116	141
Restricted cash included in “Nuclear decommissioning trust fund”	0	0
Total cash, cash equivalents, and restricted cash	$ 122	$ 147	$ 147	$ 125

Supplemental Information (Allowance for Doubtful Accounts) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning of period	$ 42	$ 25	$ 50	$ 17
Bad debt expense	7	21	8	31
Net write-offs	(13)	(2)	(22)	(4)
End of period	36	44	36	44
Payables for purchased receivables	35		35		$ 28
Union Electric Company
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning of period	16	9	16	7
Bad debt expense	2	5	5	9
Net write-offs	(4)	0	(7)	(2)
End of period	14	14	14	14
Ameren Illinois Company
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning of period	26	16	34	10
Bad debt expense	5	16	3	22
Net write-offs	(9)	(2)	(15)	(2)
End of period	22	$ 30	22	$ 30
Payables for purchased receivables	$ 35		$ 35		$ 28

Supplemental Information (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures, including wind generation expenditures	$ 396	$ 311
Net realized and unrealized gain – nuclear decommissioning trust fund	85	43
Issuance of common stock for stock-based compensation	33	38
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures, including wind generation expenditures	209	115
Net realized and unrealized gain – nuclear decommissioning trust fund	85	43
Issuance of common stock for stock-based compensation	0	0
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures, including wind generation expenditures	182	191
Net realized and unrealized gain – nuclear decommissioning trust fund	0	0
Issuance of common stock for stock-based compensation	$ 0	$ 0

Supplemental Information (Schedule of Asset Retirement Obligations) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	$ 756
Liabilities incurred	18
Liabilities settled	(23)
Accretion	23
Change in estimates	(10)
Asset Retirement Obligation, Ending Balance	764
Other current liabilities	464	$ 407
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	751
Liabilities incurred	18
Liabilities settled	(23)
Accretion	23
Change in estimates	(10)
Asset Retirement Obligation, Ending Balance	759
Other current liabilities	111	87
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	5
Liabilities incurred	0
Liabilities settled	0
Accretion	0
Change in estimates	0
Asset Retirement Obligation, Ending Balance	5
Other current liabilities	170	184
Asset Retirement Obligation Balance
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Other current liabilities	$ 59	$ 60

Supplemental Information (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Amount	$ 5	$ 8
Deferred Compensation Liability, Classified, Noncurrent	$ 91		$ 90
January 1, 2021 Issuance
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	38 months
Performance Shares \| January 1, 2021 Issuance
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	293,058
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Granted in Period, Fair Value	$ 25
Stock Issued During Period Percentage Conversion Of Units, Low End	0.00%
Stock Issued During Period Percentage Conversion Of Units, High End	200.00%
Performance Shares \| Market performance measures achievement \| January 1, 2021 Issuance
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	251,177
Performance Shares \| Renewable generation and energy storage installation targets \| January 1, 2021 Issuance
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	41,881
Restricted Stock Units (RSUs) \| January 1, 2021 Issuance
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	125,562
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Granted in Period, Fair Value	$ 10

Supplemental Information (Schedule Of Excise Taxes) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accounting Policies [Line Items]
Excise tax expense	$ 81	$ 71	$ 213	$ 198
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	54	45	120	111
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 27	$ 26	$ 93	$ 87

Supplemental Information (Earnings Per Share) (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share Reconciliation [Abstract]
Weighted-average Common Shares Outstanding – Basic	257,300,000	247,100,000	255,900,000	246,800,000
Assumed settlement of performance share units and restricted stock units	1,300,000	1,500,000	1,300,000	1,200,000
Dilutive effect of forward sale agreements	0	600,000	0	400,000
Weighted-average Common Shares Outstanding – Diluted	258,600,000	249,200,000	257,200,000	248,400,000
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount (in shares)	0	0	0	0

Segment Information (Schedule Of Segment Reporting Information By Segment) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Revenues	$ 1,811	$ 1,628	$ 4,849	$ 4,466
Net income (loss) attributable to common shareholders	425	367	865	756
Capital expenditures	850	656	2,613	1,884
Wind generation expenditures	98		515	0
Union Electric Company
Segment Reporting Information [Line Items]
Revenues	1,129	1,001	2,642	2,473
Net income (loss) attributable to common shareholders	375	297	533	439
Wind generation expenditures	98		515	0
Ameren Illinois Company
Segment Reporting Information [Line Items]
Revenues	645	589	2,097	1,879
Net income (loss) attributable to common shareholders	79	77	314	280
Capital expenditures	380	370	1,026	1,031
Operating Segments \| Union Electric Company
Segment Reporting Information [Line Items]
Revenues	1,118	990	2,612	2,442
Net income (loss) attributable to common shareholders	375	297	533	439
Capital expenditures	466	262	1,567	778
Operating Segments \| Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Revenues	425	390	1,222	1,131
Net income (loss) attributable to common shareholders	36	34	123	107
Capital expenditures	143	129	429	391
Operating Segments \| Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
Revenues	127	122	642	533
Net income (loss) attributable to common shareholders	(8)	2	75	66
Capital expenditures	93	81	202	221
Operating Segments \| Ameren Transmission
Segment Reporting Information [Line Items]
Revenues	141	126	373	360
Net income (loss) attributable to common shareholders	73	62	175	168
Capital expenditures	154	185	426	490
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Revenues	428	391	1,227	1,133
Net income (loss) attributable to common shareholders	36	34	123	107
Capital expenditures	143	129	429	391
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
Revenues	127	122	642	533
Net income (loss) attributable to common shareholders	(8)	2	75	66
Capital expenditures	93	81	202	221
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Transmission
Segment Reporting Information [Line Items]
Revenues	90	76	228	213
Net income (loss) attributable to common shareholders	51	41	116	107
Capital expenditures	144	160	395	419
Intersegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	(33)	(27)	(88)	(73)
Capital expenditures	(8)	(3)	(14)	0
Intersegment Eliminations \| Union Electric Company
Segment Reporting Information [Line Items]
Intersegment revenues	(11)	(11)	(30)	(31)
Intersegment Eliminations \| Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Intersegment revenues	(3)	(1)	(5)	(2)
Intersegment Eliminations \| Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0	0
Intersegment Eliminations \| Ameren Transmission
Segment Reporting Information [Line Items]
Intersegment revenues	(19)	(15)	(53)	(40)
Intersegment Eliminations \| Ameren Illinois Company
Segment Reporting Information [Line Items]
Intersegment revenues	(18)	(15)	(49)	(39)
Intersegment Eliminations \| Ameren Illinois Company \| Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0	0
Intersegment Eliminations \| Ameren Illinois Company \| Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0	0
Intersegment Eliminations \| Ameren Illinois Company \| Ameren Illinois Transmission
Segment Reporting Information [Line Items]
Intersegment revenues	(18)	(15)	(49)	(39)
Other
Segment Reporting Information [Line Items]
Net income (loss) attributable to common shareholders	(51)	(28)	(41)	(24)
Capital expenditures	$ 2	$ 2	$ 3	$ 4

Segment Information (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 1,811	$ 1,628	$ 4,849	$ 4,466
Revenues	1,811	1,628	4,849	4,466
Revenues from alternative revenue programs	(95)	(102)	(6)	(16)
Other revenues not from contracts with customers	13	11	82	28
Union Electric Company
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	6	1	(9)	(8)
Other revenues not from contracts with customers	7	6	71	21
Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(97)	(110)	(2)	(59)
Other revenues not from contracts with customers	6	5	9	6
Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(5)	3	0	17
Other revenues not from contracts with customers	0	0	2	1
Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	1	4	5	34
Other revenues not from contracts with customers	0	0	0	0
Electricity
Disaggregation of Revenue [Line Items]
Revenues	1,668	1,489	4,108	3,846
Revenues	1,668	1,489	4,108	3,846
Electricity \| Residential
Disaggregation of Revenue [Line Items]
Revenues	795	689	1,882	1,775
Electricity \| Commercial
Disaggregation of Revenue [Line Items]
Revenues	555	470	1,301	1,193
Electricity \| Industrial
Disaggregation of Revenue [Line Items]
Revenues	124	113	315	298
Electricity \| Other
Disaggregation of Revenue [Line Items]
Revenues	194	217	610	580
Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	143	139	741	620
Revenues	143	139	741	620
Natural Gas \| Residential
Disaggregation of Revenue [Line Items]
Revenues	84	76	492	427
Natural Gas \| Commercial
Disaggregation of Revenue [Line Items]
Revenues	27	21	139	114
Natural Gas \| Industrial
Disaggregation of Revenue [Line Items]
Revenues	3	6	22	14
Natural Gas \| Other
Disaggregation of Revenue [Line Items]
Revenues	29	36	88	65
Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenues	645	589	2,097	1,879
Revenues	645	589	2,097	1,879
Revenues from alternative revenue programs	(99)	(102)	2	(13)
Other revenues not from contracts with customers	6	5	11	7
Ameren Illinois Company \| Residential
Disaggregation of Revenue [Line Items]
Revenues	333	302	1,143	1,039
Ameren Illinois Company \| Commercial
Disaggregation of Revenue [Line Items]
Revenues	166	145	518	459
Ameren Illinois Company \| Industrial
Disaggregation of Revenue [Line Items]
Revenues	29	31	114	102
Ameren Illinois Company \| Other
Disaggregation of Revenue [Line Items]
Revenues	117	111	322	279
Ameren Illinois Company \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(97)	(110)	(2)	(59)
Other revenues not from contracts with customers	6	5	9	6
Ameren Illinois Company \| Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(5)	3	0	17
Other revenues not from contracts with customers	0	0	2	1
Ameren Illinois Company \| Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	3	5	4	29
Other revenues not from contracts with customers	0	0	0	0
Ameren Illinois Company \| Electricity
Disaggregation of Revenue [Line Items]
Revenues	518	467	1,455	1,346
Ameren Illinois Company \| Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	127	122	642	533
Operating Segments \| Union Electric Company
Disaggregation of Revenue [Line Items]
Revenues	1,118	990	2,612	2,442
Revenues	1,129	1,001	2,642	2,473
Operating Segments \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	425	390	1,222	1,131
Revenues	428	391	1,227	1,133
Operating Segments \| Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	127	122	642	533
Revenues	127	122	642	533
Operating Segments \| Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenues	141	126	373	360
Revenues	160	141	426	400
Operating Segments \| Electricity \| Union Electric Company
Disaggregation of Revenue [Line Items]
Revenues	1,113	984	2,543	2,386
Operating Segments \| Electricity \| Union Electric Company \| Residential
Disaggregation of Revenue [Line Items]
Revenues	537	455	1,177	1,111
Operating Segments \| Electricity \| Union Electric Company \| Commercial
Disaggregation of Revenue [Line Items]
Revenues	412	343	899	828
Operating Segments \| Electricity \| Union Electric Company \| Industrial
Disaggregation of Revenue [Line Items]
Revenues	98	87	221	207
Operating Segments \| Electricity \| Union Electric Company \| Other
Disaggregation of Revenue [Line Items]
Revenues	66	99	246	240
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	428	391	1,227	1,133
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution \| Residential
Disaggregation of Revenue [Line Items]
Revenues	258	234	705	664
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution \| Commercial
Disaggregation of Revenue [Line Items]
Revenues	143	127	402	365
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution \| Industrial
Disaggregation of Revenue [Line Items]
Revenues	26	26	94	91
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution \| Other
Disaggregation of Revenue [Line Items]
Revenues	1	4	26	13
Operating Segments \| Electricity \| Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments \| Electricity \| Ameren Illinois Natural Gas \| Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments \| Electricity \| Ameren Illinois Natural Gas \| Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments \| Electricity \| Ameren Illinois Natural Gas \| Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments \| Electricity \| Ameren Illinois Natural Gas \| Other
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments \| Electricity \| Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenues	160	141	426	400
Operating Segments \| Electricity \| Ameren Transmission \| Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments \| Electricity \| Ameren Transmission \| Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments \| Electricity \| Ameren Transmission \| Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments \| Electricity \| Ameren Transmission \| Other
Disaggregation of Revenue [Line Items]
Revenues	160	141	426	400
Operating Segments \| Natural Gas \| Union Electric Company
Disaggregation of Revenue [Line Items]
Revenues	16	17	99	87
Operating Segments \| Natural Gas \| Union Electric Company \| Residential
Disaggregation of Revenue [Line Items]
Revenues	9	8	54	52
Operating Segments \| Natural Gas \| Union Electric Company \| Commercial
Disaggregation of Revenue [Line Items]
Revenues	4	3	23	20
Operating Segments \| Natural Gas \| Union Electric Company \| Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	1	2	3
Operating Segments \| Natural Gas \| Union Electric Company \| Other
Disaggregation of Revenue [Line Items]
Revenues	3	5	20	12
Operating Segments \| Natural Gas \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Illinois Electric Distribution \| Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Illinois Electric Distribution \| Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Illinois Electric Distribution \| Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Illinois Electric Distribution \| Other
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	127	122	642	533
Operating Segments \| Natural Gas \| Ameren Illinois Natural Gas \| Residential
Disaggregation of Revenue [Line Items]
Revenues	75	68	438	375
Operating Segments \| Natural Gas \| Ameren Illinois Natural Gas \| Commercial
Disaggregation of Revenue [Line Items]
Revenues	23	18	116	94
Operating Segments \| Natural Gas \| Ameren Illinois Natural Gas \| Industrial
Disaggregation of Revenue [Line Items]
Revenues	3	5	20	11
Operating Segments \| Natural Gas \| Ameren Illinois Natural Gas \| Other
Disaggregation of Revenue [Line Items]
Revenues	26	31	68	53
Operating Segments \| Natural Gas \| Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Transmission \| Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Transmission \| Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Transmission \| Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Transmission \| Other
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	428	391	1,227	1,133
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	127	122	642	533
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues	90	76	228	213
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	428	391	1,227	1,133
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution \| Residential
Disaggregation of Revenue [Line Items]
Revenues	258	234	705	664
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution \| Commercial
Disaggregation of Revenue [Line Items]
Revenues	143	127	402	365
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution \| Industrial
Disaggregation of Revenue [Line Items]
Revenues	26	26	94	91
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution \| Other
Disaggregation of Revenue [Line Items]
Revenues	1	4	26	13
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues	108	91	277	252
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Transmission \| Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Transmission \| Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Transmission \| Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Transmission \| Other
Disaggregation of Revenue [Line Items]
Revenues	108	91	277	252
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	127	122	642	533
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Natural Gas \| Residential
Disaggregation of Revenue [Line Items]
Revenues	75	68	438	375
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Natural Gas \| Commercial
Disaggregation of Revenue [Line Items]
Revenues	23	18	116	94
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Natural Gas \| Industrial
Disaggregation of Revenue [Line Items]
Revenues	3	5	20	11
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Natural Gas \| Other
Disaggregation of Revenue [Line Items]
Revenues	26	31	68	53
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenues	(33)	(27)	(88)	(73)
Revenues	(33)	(27)	(88)	(73)
Intersegment Eliminations \| Union Electric Company
Disaggregation of Revenue [Line Items]
Revenues	(11)	(11)	(30)	(31)
Intersegment Eliminations \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	(3)	(1)	(5)	(2)
Intersegment Eliminations \| Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations \| Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenues	(19)	(15)	(53)	(40)
Intersegment Eliminations \| Electricity
Disaggregation of Revenue [Line Items]
Revenues	(33)	(27)	(88)	(73)
Intersegment Eliminations \| Electricity \| Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations \| Electricity \| Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations \| Electricity \| Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations \| Electricity \| Other
Disaggregation of Revenue [Line Items]
Revenues	(33)	(27)	(88)	(73)
Intersegment Eliminations \| Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations \| Natural Gas \| Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations \| Natural Gas \| Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations \| Natural Gas \| Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations \| Natural Gas \| Other
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations \| Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenues	(18)	(15)	(49)	(39)
Revenues	(18)	(15)	(49)	(39)
Intersegment Eliminations \| Ameren Illinois Company \| Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations \| Ameren Illinois Company \| Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations \| Ameren Illinois Company \| Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations \| Ameren Illinois Company \| Other
Disaggregation of Revenue [Line Items]
Revenues	(18)	(15)	(49)	(39)
Intersegment Eliminations \| Ameren Illinois Company \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations \| Ameren Illinois Company \| Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations \| Ameren Illinois Company \| Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues	$ (18)	$ (15)	$ (49)	$ (39)