Ameren Illinois (AILIN) 10-K 22 Feb 21 2020 FY Annual report Financial data

Cover Page

Cover Page - USD ($)	12 Months Ended
	Dec. 31, 2020	Jan. 29, 2021	Jun. 30, 2020
Entity Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
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 Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
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
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 17,299,078,950
Entity Common Stock, Shares Outstanding		253,355,105
Documents Incorporated by Reference	Portions of the definitive proxy statement of Ameren Corporation and portions of the definitive information statements of Union Electric Company and Ameren Illinois Company for the 2021 annual meetings of shareholders are incorporated by reference into Part III of this Form 10-K.
Entity Central Index Key	0001002910
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
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
Title of 12(g) Security	Preferred Stock, cumulative, no par value, stated value $100 per share
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
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	2020
Document Fiscal Period Focus	FY
Amendment Flag	false
No Trading Symbol Flag	true
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
Title of 12(g) Security	Preferred Stock, cumulative, $100 par valueDepositary Shares, each representing 1/4 of a share of 6.625%Preferred Stock, cumulative, $100 par value
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
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	2020
Document Fiscal Period Focus	FY
Amendment Flag	false
No Trading Symbol Flag	true

Consolidated Statement Of Incom

Consolidated Statement Of Income (Loss) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Revenues:
Total operating revenues	$ 5,794	$ 5,910	$ 6,291
Operating Expenses:
Fuel	490	535	769
Purchased power	513	556	581
Natural gas purchased for resale	272	331	374
Other operations and maintenance	1,661	1,745	1,772
Depreciation and amortization	1,075	995	955
Taxes other than income taxes	483	481	483
Total operating expenses	4,494	4,643	4,934
Operating Income	1,300	1,267	1,357
Other Income, Net	151	130	102
Interest Charges	419	381	401
Income Before Income Taxes	1,032	1,016	1,058
Income Taxes	155	182	237
Net Income	877	834	821
Less: Net Income Attributable to Noncontrolling Interests	6	6	6
Net Income Attributable to Ameren Common Shareholders	871	828	815
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	16	5	(4)
Comprehensive Income	893	839	817
Less: Comprehensive Income Attributable to Noncontrolling Interests	6	6	6
Comprehensive Income Attributable to Ameren Common Shareholders	$ 887	$ 833	$ 811
Earnings per Common Share – Basic:
Earnings per Common Share – Basic	$ 3.53	$ 3.37	$ 3.34
Earnings per Common Share – Diluted:
Earnings per Common Share – Diluted	$ 3.50	$ 3.35	$ 3.32
Weighted-average Common Shares Outstanding – Basic	247	245.6	243.8
Weighted-average Common Shares Outstanding – Diluted	248.7	247.1	245.8
Union Electric Company
Operating Revenues:
Total operating revenues	$ 3,109	$ 3,243	$ 3,589
Operating Expenses:
Fuel	490	535	769
Purchased power	171	193	164
Natural gas purchased for resale	43	53	56
Other operations and maintenance	886	960	972
Depreciation and amortization	604	556	550
Taxes other than income taxes	328	329	329
Total operating expenses	2,522	2,626	2,840
Operating Income	587	617	749
Other Income, Net	76	58	56
Interest Charges	190	178	200
Income Before Income Taxes	473	497	605
Income Taxes	34	68	124
Net Income	439	429	481
Preferred Stock Dividends	3	3	3
Net Income Available to Common Shareholder	436	426	478
Ameren Illinois Company
Operating Revenues:
Total operating revenues	2,535	2,527	2,576
Operating Expenses:
Purchased power	355	368	429
Natural gas purchased for resale	229	278	318
Other operations and maintenance	775	782	799
Depreciation and amortization	434	406	374
Taxes other than income taxes	140	143	144
Total operating expenses	1,933	1,977	2,064
Operating Income	602	550	512
Other Income, Net	59	53	42
Interest Charges	155	147	149
Income Before Income Taxes	506	456	405
Income Taxes	124	110	98
Net Income	382	346	307
Preferred Stock Dividends	3	3	3
Net Income Available to Common Shareholder	379	343	304
Electric
Operating Revenues:
Total operating revenues	4,911	4,981	5,339
Electric | Union Electric Company
Operating Revenues:
Total operating revenues	2,984	3,109	3,451
Electric | Ameren Illinois Company
Operating Revenues:
Total operating revenues	1,775	1,730	1,761
Natural gas
Operating Revenues:
Total operating revenues	883	929	952
Natural gas | Union Electric Company
Operating Revenues:
Total operating revenues	125	134	138
Natural gas | Ameren Illinois Company
Operating Revenues:
Total operating revenues	$ 760	$ 797	$ 815

Consolidated Statement Of Inc_2

Consolidated Statement Of Income (Loss) (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Pension and other postretirement benefit plan activity, tax (benefit)	$ 5	$ 1	$ (1)

Consolidated Balance Sheet

Consolidated Balance Sheet - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Current Assets:
Cash and cash equivalents	$ 139	$ 16
Accounts receivable - trade (less allowance for doubtful accounts)	415	393
Unbilled revenue	269	278
Miscellaneous accounts receivable	65	63
Inventories	521	494
Current regulatory assets	109	69
Other current assets	135	118
Total current assets	1,653	1,431
Property, Plant, and Equipment, Net	26,807	24,376
Investments and Other Assets:
Nuclear decommissioning trust fund	982	847
Goodwill	411	411
Regulatory assets	1,100	992
Other assets	1,077	876
Total investments and other assets	3,570	3,126
TOTAL ASSETS	32,030	28,933
Current Liabilities:
Current maturities of long-term debt	8	442
Short-term debt	490	440
Accounts and wages payable	958	874
Interest accrued	114	94
Current Asset Retirement Obligation	60	53
Current regulatory liabilities	121	164
Other current liabilities	489	491
Total current liabilities	2,180	2,505
Long-term Debt, Net	11,078	8,915
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes and investment tax credits, net	3,211	2,919
Regulatory liabilities	5,282	4,887
Asset retirement obligations	696	638
Pension and other postretirement benefits	37	401
Other deferred credits and liabilities	466	467
Total deferred credits and other liabilities	9,692	9,312
Commitments and Contingencies (Notes 2, 9, and 14)
Shareholders' Equity:
Common stock	3	2
Other paid-in capital, principally premium on common stock	6,179	5,694
Retained earnings	2,757	2,380
Accumulated other comprehensive loss	(1)	(17)
Total shareholders' equity	8,938	8,059
Noncontrolling Interests	142	142
Total equity	9,080	8,201
TOTAL LIABILITIES AND EQUITY	32,030	28,933
Union Electric Company
Current Assets:
Cash and cash equivalents	136	9
Advances to money pool	139	0
Accounts receivable - trade (less allowance for doubtful accounts)	166	164
Accounts receivable – affiliates	57	30
Unbilled revenue	133	139
Miscellaneous accounts receivable	36	33
Inventories	386	373
Current regulatory assets	60	8
Other current assets	79	58
Total current assets	1,192	814
Property, Plant, and Equipment, Net	13,879	12,635
Investments and Other Assets:
Nuclear decommissioning trust fund	982	847
Regulatory assets	347	285
Other assets	383	356
Total investments and other assets	1,712	1,488
TOTAL ASSETS	16,783	14,937
Current Liabilities:
Current maturities of long-term debt	8	92
Short-term debt	0	234
Accounts and wages payable	501	465
Accounts payable – affiliates	46	52
Taxes Accrued	42	24
Interest accrued	53	48
Current Asset Retirement Obligation	60	53
Current regulatory liabilities	26	62
Other current liabilities	97	96
Total current liabilities	833	1,126
Long-term Debt, Net	5,096	4,098
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes and investment tax credits, net	1,742	1,612
Regulatory liabilities	3,110	2,937
Asset retirement obligations	691	634
Pension and other postretirement benefits	35	141
Other deferred credits and liabilities	66	40
Total deferred credits and other liabilities	5,644	5,364
Commitments and Contingencies (Notes 2, 9, and 14)
Shareholders' Equity:
Common stock	511	511
Other paid-in capital, principally premium on common stock	2,518	2,027
Preferred stock	80	80
Retained earnings	2,101	1,731
Total shareholders' equity	5,210	4,349
TOTAL LIABILITIES AND EQUITY	16,783	14,937
Ameren Illinois Company
Current Assets:
Cash and cash equivalents	0	0
Accounts receivable - trade (less allowance for doubtful accounts)	234	215
Accounts receivable – affiliates	64	28
Unbilled revenue	136	139
Miscellaneous accounts receivable	12	25
Inventories	135	121
Current regulatory assets	37	57
Other current assets	29	29
Total current assets	647	614
Property, Plant, and Equipment, Net	11,201	10,083
Investments and Other Assets:
Goodwill	411	411
Regulatory assets	742	694
Other assets	534	383
Total investments and other assets	1,687	1,488
TOTAL ASSETS	13,535	12,185
Current Liabilities:
Short-term debt	0	53
Accounts and wages payable	363	299
Accounts payable – affiliates	51	82
Customer deposits	74	77
Current regulatory liabilities	88	84
Other current liabilities	221	249
Total current liabilities	816	844
Long-term Debt, Net	3,946	3,575
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes and investment tax credits, net	1,367	1,224
Regulatory liabilities	2,063	1,849
Pension and other postretirement benefits	69	214
Environmental remediation	57	87
Other deferred credits and liabilities	251	260
Total deferred credits and other liabilities	3,807	3,634
Commitments and Contingencies (Notes 2, 9, and 14)
Shareholders' Equity:
Common stock	0	0
Other paid-in capital, principally premium on common stock	2,652	2,188
Preferred stock	62	62
Retained earnings	2,252	1,882
Total shareholders' equity	4,966	4,132
TOTAL LIABILITIES AND EQUITY	$ 13,535	$ 12,185

Consolidated Balance Sheet (Par

Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Accounts receivable - trade, allowance for doubtful accounts	$ 50	$ 17
Common Stock, Par or Stated Value Per Share (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	400,000,000	400,000,000
Common Stock, Shares, Outstanding	253,300,000	246,200,000
Union Electric Company
Accounts receivable - trade, allowance for doubtful accounts	$ 16	$ 7
Common Stock, No Par Value (in dollars per share)	$ 5	$ 5
Common stock, shares authorized	150,000,000	150,000,000
Common Stock, Shares, Outstanding	102,100,000	102,100,000
Ameren Illinois Company
Accounts receivable - trade, allowance for doubtful accounts	$ 34	$ 10
Common Stock, No Par Value (in dollars per share)	$ 0	$ 0
Common stock, shares authorized	45,000,000	45,000,000
Common Stock, Shares, Outstanding	25,500,000	25,500,000

Consolidated Statement Of Cash

Consolidated Statement Of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash Flows From Operating Activities:
Net income	$ 877	$ 834	$ 821
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	1,085	1,002	938
Amortization of nuclear fuel	68	79	95
Amortization of debt issuance costs and premium/discounts	22	19	20
Deferred income taxes and investment tax credits, net	148	167	224
Allowance for equity funds used during construction	(32)	(28)	(36)
Stock-based compensation costs	21	20	20
Other	22	(14)	44
Changes in assets and liabilities:
Receivables	(47)	79	(24)
Inventories	(25)	(10)	39
Accounts and wages payable	40	(3)	(22)
Taxes accrued	34	(8)	(10)
Regulatory assets and liabilities	(254)	164	201
Assets, other	(83)	(59)	2
Liabilities, other	(111)	(33)	(117)
Pension and other postretirement benefits	(38)	(39)	(25)
Net cash provided by operating activities	1,727	2,170	2,170
Cash Flows From Investing Activities:
Capital expenditures	(2,669)	(2,411)	(2,286)
Wind generation expenditures	(564)	0	0
Nuclear fuel expenditures	(66)	(31)	(52)
Purchases of securities – nuclear decommissioning trust fund	(224)	(256)	(315)
Proceeds from sales and maturities	183	260	299
Purchase of bonds	0	(207)	0
Proceeds from sale of remarketed bonds	0	207	0
Other	11	3	18
Net cash used in investing activities	(3,329)	(2,435)	(2,336)
Cash Flows From Financing Activities:
Dividends on common stock	(494)	(472)	(451)
Dividends paid to noncontrolling interest holders	(6)	(6)	(6)
Short-term debt, net	50	(157)	112
Maturities of long-term debt	(442)	(580)	(841)
Issuances of long-term debt	2,183	1,527	1,352
Issuances of common stock	476	68	74
Employee payroll taxes related to stock-based compensation	(20)	(29)	(19)
Debt issuance costs	(20)	(17)	(14)
Other	0	0	(2)
Net cash provided by financing activities	1,727	334	205
Net change in cash, cash equivalents, and restricted cash	125	69	39
Cash, cash equivalents, and restricted cash at beginning of year	176	107	68
Cash, cash equivalents, and restricted cash at end of year	301	176	107
Cash Paid (Refunded) During the Year:
Interest (net of amounts capitalized, respectively)	383	367	387
Income taxes, net	13	13	21
Union Electric Company
Cash Flows From Operating Activities:
Net income	439	429	481
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	613	564	533
Amortization of nuclear fuel	68	79	95
Amortization of debt issuance costs and premium/discounts	6	5	6
Deferred income taxes and investment tax credits, net	17	(19)	(9)
Allowance for equity funds used during construction	(19)	(19)	(27)
Other	22	13	17
Changes in assets and liabilities:
Receivables	(8)	75	(32)
Inventories	(11)	(13)	30
Accounts and wages payable	26	16	(21)
Taxes accrued	9	(15)	(1)
Regulatory assets and liabilities	(166)	17	201
Assets, other	(2)	(28)	2
Liabilities, other	(80)	(32)	(13)
Pension and other postretirement benefits	(3)	(5)	(2)
Net cash provided by operating activities	911	1,067	1,260
Cash Flows From Investing Activities:
Capital expenditures	(1,102)	(1,076)	(914)
Wind generation expenditures	(564)	0	0
Nuclear fuel expenditures	(66)	(31)	(52)
Purchases of securities – nuclear decommissioning trust fund	(224)	(256)	(315)
Proceeds from sales and maturities	183	260	299
Purchase of bonds	0	(207)	0
Proceeds from sale of remarketed bonds	0	207	0
Money pool advances, net	(139)	0	0
Other	8	8	6
Net cash used in investing activities	(1,904)	(1,095)	(976)
Cash Flows From Financing Activities:
Dividends on common stock	(66)	(430)	(375)
Dividends on preferred stock	(3)	(3)	(3)
Short-term debt, net	(234)	179	16
Maturities of long-term debt	(92)	(580)	(384)
Issuances of long-term debt	1,012	778	423
Debt issuance costs	(9)	(9)	(5)
Capital contribution from parent	491	124	45
Net cash provided by financing activities	1,099	59	(283)
Net change in cash, cash equivalents, and restricted cash	106	31	1
Cash, cash equivalents, and restricted cash at beginning of year	39	8	7
Cash, cash equivalents, and restricted cash at end of year	145	39	8
Cash Paid (Refunded) During the Year:
Interest (net of amounts capitalized, respectively)	190	190	196
Income taxes, net	25	101	128
Ameren Illinois Company
Cash Flows From Operating Activities:
Net income	382	346	307
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	434	405	375
Amortization of debt issuance costs and premium/discounts	12	12	13
Deferred income taxes and investment tax credits, net	118	80	88
Allowance for equity funds used during construction	(13)	(9)	(9)
Other	8	7	11
Changes in assets and liabilities:
Receivables	(28)	11	0
Inventories	(15)	2	8
Accounts and wages payable	15	(19)	(13)
Taxes accrued	(23)	21	(13)
Regulatory assets and liabilities	(72)	155	1
Assets, other	(76)	(23)	(1)
Liabilities, other	(46)	(5)	(92)
Pension and other postretirement benefits	(30)	(30)	(25)
Net cash provided by operating activities	679	962	659
Cash Flows From Investing Activities:
Capital expenditures	(1,447)	(1,208)	(1,258)
Other	3	3	10
Net cash used in investing activities	(1,444)	(1,205)	(1,248)
Cash Flows From Financing Activities:
Dividends on common stock	(9)	0	0
Dividends on preferred stock	(3)	(3)	(3)
Short-term debt, net	(53)	(19)	10
Money pool borrowings, net	19	0	0
Maturities of long-term debt	0	0	(457)
Issuances of long-term debt	373	299	929
Debt issuance costs	(4)	(4)	(9)
Capital contribution from parent	464	15	160
Other	0	0	(2)
Net cash provided by financing activities	787	288	628
Net change in cash, cash equivalents, and restricted cash	22	45	39
Cash, cash equivalents, and restricted cash at beginning of year	125	80	41
Cash, cash equivalents, and restricted cash at end of year	147	125	80
Cash Paid (Refunded) During the Year:
Interest (net of amounts capitalized, respectively)	137	127	144
Income taxes, net	$ 41	$ 4	$ 28

Consolidated Statement Of Cas_2

Consolidated Statement Of Cash Flows (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Interest Paid, Capitalized, Investing Activities	$ 16	$ 20	$ 21
Union Electric Company
Interest Paid, Capitalized, Investing Activities	10	12	14
Ameren Illinois Company
Interest Paid, Capitalized, Investing Activities	$ 6	$ 8	$ 7

Consolidated Statement Of Stock

Consolidated Statement Of Stockholders' Equity - USD ($) $ in Millions	Total	Common Stock	Other Paid-In Capital	Retained Earnings	Deferred Retirement Benefit Costs	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interest	Total Ameren Corporation Stockholders' Equity	Union Electric Company	Union Electric CompanyCommon Stock	Union Electric CompanyOther Paid-In Capital	Union Electric CompanyPreferred Stock Not Subject To Mandatory Redemption	Union Electric CompanyRetained Earnings	Ameren Illinois Company	Ameren Illinois CompanyCommon Stock	Ameren Illinois CompanyOther Paid-In Capital	Ameren Illinois CompanyPreferred Stock Not Subject To Mandatory Redemption	Ameren Illinois CompanyRetained Earnings
Beginning of year at Dec. 31, 2017		$ 2	$ 5,540	$ 1,660	$ (18)		$ 142				$ 1,858		$ 1,632			$ 2,013		$ 1,235
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Settlement of forward sale agreement through common shares issuance		0	0
Stock Issued During Period, Value, New Issues			74
Stock-based compensation activity			13
Capital contribution from parent									$ 45		45			$ 160		160
Net income	$ 821								481				481	307				307
Net Income (Loss) Available to Common Stockholders, Basic	815			815
Common stock dividends				(451)									(375)					0
Preferred stock dividends													(3)					(3)
Change in deferred retirement benefit costs	4				(4)
Net income attributable to noncontrolling interest holders	$ 6						6
Dividends paid to noncontrolling interest holders							(6)
Beginning of year (shares) at Dec. 31, 2017	242,600,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Shares, New Issues	1,200,000
Stock Issued During Period, Shares, Other	700,000
End of year (shares) at Dec. 31, 2018	244,500,000
End of year at Dec. 31, 2018	$ 7,773	2	5,627	2,024	(22)	$ (22)	142			$ 511	1,903	$ 80	1,735		$ 0	2,173	$ 62	1,539
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Dividends, Per Share, Cash Paid	$ 1.8475
Stockholders' equity, end of year at Dec. 31, 2018								$ 7,631	4,229					3,774
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under forward sale agreement	0
Settlement of forward sale agreement through common shares issuance		0	0
Stock Issued During Period, Value, New Issues			68
Stock-based compensation activity			(1)
Capital contribution from parent									124		124			15		15
Net income	$ 834								$ 429				429	$ 346				346
Net Income (Loss) Available to Common Stockholders, Basic	828			828
Common stock dividends				(472)									(430)					0
Preferred stock dividends													(3)					(3)
Change in deferred retirement benefit costs	(5)				5
Net income attributable to noncontrolling interest holders	$ 6						6
Dividends paid to noncontrolling interest holders							(6)
Stock Issued During Period, Shares, New Issues	900,000
Stock Issued During Period, Shares, Other	800,000
End of year (shares) at Dec. 31, 2019	246,200,000								102,100,000					25,500,000
End of year at Dec. 31, 2019	$ 8,201	2	5,694	2,380	(17)	(17)	142			511	2,027	80	1,731		0	2,188	62	1,882
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Dividends, Per Share, Cash Paid	$ 1.9200
Stockholders' equity, end of year at Dec. 31, 2019	$ 8,059							8,059	$ 4,349					$ 4,132
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under forward sale agreement	0
Settlement of forward sale agreement through common shares issuance		1	424
Stock Issued During Period, Value, New Issues			51
Stock-based compensation activity			10
Capital contribution from parent									491		491			464		464
Net income	$ 877								$ 439				439	$ 382				382
Net Income (Loss) Available to Common Stockholders, Basic	871			871
Common stock dividends				(494)									(66)					(9)
Preferred stock dividends													(3)					(3)
Change in deferred retirement benefit costs	(16)				16
Net income attributable to noncontrolling interest holders	$ 6						6
Dividends paid to noncontrolling interest holders							(6)
Stock Issued During Period, Shares, New Issues	700,000
Stock Issued During Period, Shares, Other	500,000
End of year (shares) at Dec. 31, 2020	253,300,000								102,100,000					25,500,000
End of year at Dec. 31, 2020	$ 9,080	$ 3	$ 6,179	$ 2,757	$ (1)	$ (1)	$ 142			$ 511	$ 2,518	$ 80	$ 2,101		$ 0	$ 2,652	$ 62	$ 2,252
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Dividends, Per Share, Cash Paid	$ 2
Stockholders' equity, end of year at Dec. 31, 2020	$ 8,938							$ 8,938	$ 5,210					$ 4,966
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under forward sale agreement	5,900,000

Summary Of Significant Accounti

Summary Of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
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 also 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 Missouri was incorporated in Missouri in 1922 and is successor to a number of companies, the oldest of which was organized in 1881. It is the largest electric utility in the state of Missouri. It supplies electric and natural gas service to a 24,000-square-mile area in central and eastern Missouri, which includes the Greater St. Louis area. Ameren Missouri supplies electric service to 1.2 million customers and natural gas service to 0.1 million customers. • Ameren Illinois Company, doing business as Ameren Illinois, operates rate-regulated electric transmission, electric distribution, and natural gas distribution businesses in Illinois. Ameren Illinois was incorporated in Illinois in 1923 and is the successor to a number of companies, the oldest of which was organized in 1902. Ameren Illinois supplies electric and natural gas utility service to a 43,700 square mile area in central and southern Illinois. Ameren Illinois supplies electric service to 1.2 million customers and natural gas service to 0.8 million customers. • Ameren Transmission Company of Illinois, doing business as ATXI, operates a FERC rate-regulated electric transmission business in the MISO. ATXI was incorporated in Illinois in 2006. In December 2020, ATXI completed construction of the ninth and final line segment of the Illinois Rivers transmission line, a MISO-approved electric transmission line. ATXI also operates the Spoon River and Mark Twain transmission lines, which were placed in service in February 2018 and December 2019, respectively. The COVID-19 pandemic continues to be a constantly evolving situation. In 2020, we experienced a net decrease in our sales volumes, an increase in our accounts receivable balances that were past due or that were a part of a deferred payment arrangement, and a decline in our cash collections from customers. 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. Shelter-in-place orders began taking effect in our service territories in mid-March 2020. These orders generally required individuals to remain at home and precluded or limited the operation of businesses that were deemed nonessential. While our business operations were deemed essential and were not directly impacted by the shelter-in-place orders, approximately 65% of our workforce transitioned to remote working arrangements in mid-March 2020. In order to work more effectively in certain areas, a portion of our workforce returned to our work locations in early June 2020 under a phased approach, and, as of the date of this filing, approximately 50% of our workforce continues to work remotely. In mid-May 2020, shelter-in-place orders effective in our service territories began to be relaxed, with fewer restrictions on social activities and nonessential businesses beginning to reopen. However, certain restrictions remain in place that limit individual activities and the operation of nonessential businesses. Additional restrictions may be imposed in the future. We continue to assess the impacts the 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. 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, changes in sales volumes at Ameren Missouri and those associated with Ameren Illinois’ large nonresidential natural gas customers may affect net income. With respect to 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. Pursuant to a June 2020 ICC order, Ameren Illinois’ electric bad debt rider provided for the recovery of bad debt expense in 2020, which reverted to the recovery of bad debt write-offs, net of any subsequent recoveries, in 2021. 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 does not expect a material impact to earnings from increases in bad debt expense. In October 2020, Ameren Missouri filed requests with the MoPSC for accounting authority orders related to certain impacts resulting from the COVID-19 pandemic. If issued as requested, the orders would allow Ameren Missouri to accumulate certain costs incurred related to the COVID-19 pandemic, including bad debt write-offs, net of cost savings, as well as forgone customer late fee and reconnection fee revenues, for a specified time period, for potential recovery in future electric and natural gas service regulatory rate reviews. Our customers’ payment for our services has been adversely affected by the COVID-19 pandemic, resulting in a decrease to our cash flow from operations. As of December 31, 2020, accounts receivable balances that were 30 days or greater past due or that were a part of a deferred payment arrangement represented 29%, 22%, and 35%, or $133 million, $40 million, and $93 million, of Ameren’s, Ameren Missouri’s, and Ameren Illinois’ customer trade receivables before allowance for doubtful accounts, respectively. As of December 31, 2019, these percentages were 18%, 18%, and 20%, or $75 million, $30 million, and $45 million, for Ameren, Ameren Missouri, and Ameren Illinois, respectively. For information regarding Ameren Missouri’s and Ameren Illinois’ suspension and subsequent reinstatement of customer disconnections and late fee charges for nonpayment and Ameren Missouri’s requests for accounting authority orders related to the COVID-19 pandemic, see Note 2 – Rate and Regulatory Matters below. The Coronavirus Aid, Relief, and Economic Security Act is a federal law enacted in March 2020. Provisions in the act include temporary changes to the utilization of net operating losses, deferral of the payment of the employer portion of Social Security taxes, and additional funding for customer energy assistance, among other things. As of December 31, 2020, the implementation of the act by the Ameren Companies had no material impact to their financial statements. Ameren’s financial statements are prepared on a consolidated basis and therefore include the accounts of its majority-owned subsidiaries. All intercompany transactions have been eliminated, except as disclosed in Note 13 – Related-party Transactions. Ameren Missouri and Ameren Illinois have no subsidiaries. All tabular dollar amounts are in millions, unless otherwise indicated. Our accounting policies conform to GAAP. Our financial statements reflect all adjustments (which include normal, recurring adjustments) that are necessary, in our opinion, for a fair presentation 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. Regulation Our customer rates are regulated by the MoPSC, the ICC, and the FERC. We defer certain costs as assets pursuant to actions of rate regulators or because of expectations that we will be able to recover such costs in future rates charged to customers. We also defer certain amounts as liabilities pursuant to actions of rate regulators or based on the expectation that such amounts will be returned to customers in future rates. Regulatory assets and liabilities are amortized consistent with the period of expected regulatory treatment. See Note 2 – Rate and Regulatory Matters for additional information on our regulatory frameworks, regulatory recovery mechanisms, and regulatory assets and liabilities recorded at December 31, 2020 and 2019. We continually assess the recoverability of our respective regulatory assets. Regulatory assets are charged to earnings when it is no longer probable that such amounts will be recovered through future revenues. To the extent that reductions in customers’ rates or refunds to customers related to regulatory liabilities are no longer probable, the amounts are credited to earnings. Cash, Cash Equivalents, and Restricted Cash Cash and cash equivalents include short-term, highly liquid investments purchased with an original maturity of three months or less. Cash and cash equivalents subject to legal or contractual restrictions and not readily available for use for general corporate purposes are classified as restricted cash. See Note 15 – Supplemental Information for a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets and the statements of cash flows. Allowance for Doubtful Accounts Receivable The allowance for doubtful accounts represents our estimate of existing accounts receivable that will ultimately be uncollectible. The allowance is calculated by applying estimated loss factors to various classes of outstanding receivables, including unbilled revenue. The loss factors used to estimate uncollectible accounts are based upon both historical collections experience and management’s estimate of future collections success given the existing and anticipated future collections environment. Ameren Illinois has bad debt riders that adjust rates for net write-offs of customer accounts receivable above or below those being collected in rates. In 2020, the rider for electric distribution allowed for recovery of bad debt expense recognized under GAAP. See Note 2 – Rate and Regulatory Matters for additional information. Inventories Inventories are recorded at the lower of weighted-average cost or net realizable value. Inventories are capitalized when purchased and then expensed as consumed or capitalized as property, plant, and equipment when installed, as appropriate. See Note 15 – Supplemental Information for the components of inventories. Property, Plant, and Equipment, Net We capitalize the cost of additions to, and betterments of, units of property, plant, and equipment. The cost includes labor, material, applicable taxes, and overhead. An allowance for funds used during construction, as discussed below, is also capitalized as a cost of our rate-regulated assets. Maintenance expenditures are expensed as incurred. Beginning in 2020, maintenance expenses related to scheduled Callaway nuclear refueling and maintenance outages, which were previously expensed as incurred, are deferred and amortized over the number of expected months until the completion of the next refueling outage, which historically has been approximately 18 months . When units of depreciable property are retired, the original costs, and the associated removal cost, net of salvage, are charged to accumulated depreciation. If environmental expenditures are related to assets currently in use, as in the case of the installation of pollution control equipment, the cost is capitalized and depreciated over the expected life of the asset. See Asset Retirement Obligations section below and Note 3 – Property, Plant, and Equipment, Net for additional information. Ameren Missouri’s cost of nuclear fuel is capitalized as a part of “Property, Plant, and Equipment, Net” on the balance sheet and then amortized to “Operating Expenses – Fuel” in the statement of income on a unit-of-production basis. Depreciation Depreciation is provided over the estimated lives of the various classes of depreciable property by applying composite rates on a straight-line basis to the cost basis of such property. The composite rates include a provision for the estimated removal cost of property, plant, and equipment retired from service, net of salvage. The provision for depreciation for the Ameren Companies in 2020, 2019, and 2018 ranged from 3% to 4% of the average depreciable cost. See Note 3 – Property, Plant, and Equipment, Net for additional information on estimated depreciable lives. Allowance for Funds Used During Construction As a part of “Property, Plant, and Equipment, Net” on the balance sheet, we capitalize allowance for funds used during construction, which is the cost of borrowed funds and the cost of equity funds (preferred and common shareholders’ equity) applicable to eligible rate-regulated construction work in progress, in accordance with the utility industry’s accounting practice and GAAP. The amount of allowance for funds used during construction is calculated using a FERC-prescribed formula based on a rate, which incorporates the average cost of short-term debt, the average cost of long-term debt, and the cost of equity funds. The portion attributable to borrowed funds is recorded as a reduction of “Interest Charges” on the statements of income. The portion attributable to equity funds is recorded within “Other Income, Net” on the statements of income. This accounting practice offsets the effect on earnings of the cost of financing during construction. See Note 15 – Supplemental Information for the amount of allowance for funds used during construction capitalized and the average rate applied to eligible construction work in progress. Allowance for funds used during construction does not represent a current source of cash funds. Under accepted ratemaking practice, cash recovery of allowance for funds used during construction and other construction costs occurs when completed projects are placed in service and reflected in customer rates. Goodwill Goodwill represents the excess of the purchase price of an acquisition over the fair value of the net assets acquired. Ameren and Ameren Illinois had goodwill of $411 million at December 31, 2020 and 2019. Ameren has four reporting units: Ameren Missouri, Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Transmission. Ameren Illinois has three reporting units: Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Illinois Transmission. Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Illinois Transmission had goodwill of $238 million, $80 million, and $93 million, respectively, at December 31, 2020 and 2019. The Ameren Transmission reporting unit had the same $93 million of goodwill as the Ameren Illinois Transmission reporting unit at December 31, 2020 and 2019. Ameren and Ameren Illinois evaluate goodwill for impairment in each of their reporting units as of October 31 each year, or more frequently if events occur or circumstances change that would more likely than not reduce the fair value of their reporting units below their carrying amounts. To determine whether the fair value of a reporting unit is more likely than not greater than its carrying amount, Ameren and Ameren Illinois elect to perform either a qualitative assessment or to bypass the qualitative assessment and perform a quantitative test. Ameren and Ameren Illinois elected to perform a qualitative assessment for their annual goodwill impairment test conducted as of October 31, 2020. As part of this qualitative assessment, Ameren and Ameren Illinois evaluated, among other things, macroeconomic conditions, industry and market considerations such as observable industry market multiples, regulatory frameworks, cost factors, overall financial performance, and entity-specific events. The results of Ameren’s and Ameren Illinois’ qualitative assessment indicated that it was more likely than not that the fair value of each reporting unit exceeded its carrying value as of October 31, 2020, resulting in no impairment of Ameren’s or Ameren Illinois’ goodwill. Impairment of Long-lived Assets We evaluate long-lived assets classified as held and used for impairment when events or changes in circumstances indicate that the carrying value of such assets may not be recoverable. Whether an impairment has occurred is determined by comparing the estimated undiscounted cash flows attributable to the assets to the carrying value of the assets. If the carrying value exceeds the undiscounted cash flows, we recognize an impairment charge equal to the amount by which the carrying value exceeds the estimated fair value of the assets. In the period in which we determine that an asset meets held for sale criteria, we record an impairment charge to the extent the book value exceeds its estimated fair value less cost to sell. We did not identify any events or changes in circumstances that indicated that the carrying value of long-lived assets may not be recoverable in 2020 or 2019. Variable Interest Entities As of December 31, 2020 and 2019, Ameren had unconsolidated variable interests as a limited partner in various equity method investments, totaling $37 million and $28 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 December 31, 2020, the maximum exposure to loss related to these variable interest entities is limited to the investment in these partnerships of $37 million plus associated outstanding funding commitments of $35 million . Environmental Costs Liabilities for environmental costs are recorded on an undiscounted basis when it is probable that a liability has been incurred and the amount of the liability can be reasonably estimated. Costs are expensed or deferred as a regulatory asset when it is expected that the costs will be recovered from customers in future rates. See Note 14 – Commitments and Contingencies for additional information on liabilities for environmental costs. Asset Retirement Obligations We record the estimated fair value of legal obligations associated with the retirement of tangible long-lived assets in the period in which the liabilities are incurred and capitalize a corresponding amount as part of the book value of the related long-lived asset. In subsequent periods, we adjust AROs for accretion and changes in the estimated fair values of the obligations, with a corresponding increase or decrease in the asset book value for the fair value changes. Asset book values, reflected within “Property, Plant, and Equipment, Net” on the balance sheet, are depreciated over the remaining useful life of the related asset. Due to regulatory recovery, that depreciation is deferred as a regulatory balance. The depreciation of the asset book values at Ameren Missouri was $28 million , $18 million, and $14 million for the years ended December 31, 2020, 2019, and 2018, respectively, which was deferred as a reduction to the net regulatory liability. The net regulatory liability also reflects a deferral for the nuclear decommissioning trust fund balance for the Callaway Energy Center. The depreciation deferred to the regulatory asset at Ameren Illinois was immaterial in each respective period. Uncertainties as to the probability, timing, or amount of cash expenditures associated with AROs affect our estimates of fair value. Ameren and Ameren Missouri have recorded AROs for retirement costs associated with decommissioning of Ameren Missouri’s Callaway and High Prairie Renewable energy centers, CCR facilities, and river structures. Additionally, Ameren, Ameren Missouri, and Ameren Illinois have recorded AROs for retirement costs associated with asbestos removal and the disposal of certain transformers. See Note 15 – Supplemental Information for a reconciliation of the beginning and ending carrying amounts of AROs. Estimated funds collected from customers to pay for the future removal cost of property, plant, and equipment retired from service, net of salvage, represent a cost of removal regulatory liability. See the cost of removal regulatory liability balance in Note 2 – Rate and Regulatory Matters. 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 December 31, 2020, the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $272 million (December 31, 2019 – $264 million) and $115 million (December 31, 2019 – $123 million), respectively, while total borrowings against the policies were $107 million (December 31, 2019 – $114 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. The net cash surrender value of Ameren’s company-owned life insurance is affected by the investment performance of a separate account in which Ameren holds a beneficial interest. Operating Revenues We record revenues from contracts with customers for various electric and natural gas services, which primarily consist of retail distribution, electric transmission, and off-system arrangements. When more than one performance obligation exists in a contract, the consideration under the contract is allocated to the performance obligations based on the relative standalone selling price. Electric and natural gas retail distribution revenues are earned when the commodity is delivered to our customers. We accrue an estimate of electric and natural gas retail distribution revenues for service provided but unbilled at the end of each accounting period. Electric transmission revenues are earned as electric transmission services are provided. Off-system revenues are primarily comprised of MISO revenues and wholesale bilateral revenues. MISO revenues include the sale of electricity, capacity, and ancillary services. Wholesale bilateral revenues include the sale of electricity and capacity. MISO-related electricity and wholesale bilateral electricity revenues are earned as electricity is delivered. Capacity and ancillary service revenues are earned as services are provided. Retail distribution, electric transmission, and off-system revenues, including the underlying components described above, represent a series of goods or services that are substantially the same and have the same pattern of transfer over time to our customers. Revenues from contracts with customers are equal to the amounts billed and our estimate of electric and natural gas retail distribution services provided but unbilled at the end of each accounting period. Customers are billed at least monthly, and payments are due less than one month after goods and/or services are provided. See Note 16 – Segment Information for disaggregated revenue information. For certain regulatory recovery mechanisms that are alternative revenue programs rather than revenues from contracts with customers, we recognize revenues that have been authorized for rate recovery, are objectively determinable and probable of recovery, and are expected to be collected from customers within two years from the end of the year. Our alternative revenue programs include revenue requirement reconciliations, the MEEIA, the VBA, and the DCA. These revenues are subsequently recognized as revenues from contracts with customers when billed, with an offset to alternative revenue program revenues. As of December 31, 2020 and 2019, our remaining performance obligations 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. Accounting for MISO Transactions MISO-related purchase and sale transactions are recorded by Ameren, Ameren Missouri, and Ameren Illinois using settlement information provided by the MISO. Ameren Missouri records these purchase and sale transactions on a net hourly position. Ameren Missouri records net purchases in a single hour in “Operating Expenses – Purchased power” and net sales in a single hour in “Operating Revenues – Electric” in its statement of income. Ameren Illinois records net purchases in “Operating Expenses – Purchased power” in its statement of income to reflect all of its MISO transactions relating to the procurement of power for its customers. On occasion, Ameren Missouri’s and Ameren Illinois’ prior-period transactions will be resettled outside the routine settlement process because of a change in the MISO’s tariff or a material interpretation thereof. In these cases, Ameren Missouri and Ameren Illinois recognize revenues and expenses associated with resettlements once the resettlement is probable and the resettlement amount can be estimated. There were no material MISO resettlements in 2020, 2019, or 2018. Stock-based Compensation Stock-based compensation cost is measured at the grant date based on the fair value of the award, net of an assumed forfeiture rate. Ameren recognizes as compensation expense the estimated fair value of stock-based compensation on a straight-line basis over the requisite vesting period. See Note 11 – Stock-based Compensation for additional information. Unamortized Debt Discounts, Premiums, and Issuance Costs Long-term debt discounts, premiums, and issuance costs are amortized over the lives of the related issuances. Credit agreement fees are amortized over the term of the agreement. Income Taxes Ameren uses an asset and liability approach for its financial accounting and reporting of income taxes. Deferred tax assets and liabilities are recognized for transactions that are treated differently for financial reporting and income tax return purposes. These deferred tax assets and liabilities are based on statutory tax rates. We expect that regulators will reduce future revenues for deferred tax liabilities that were initially recorded at rates in excess of the current statutory rate. Therefore, reductions in certain deferred tax liabilities that were recorded because of decreases in the statutory rate have been credited to a regulatory liability. A regulatory asset has been established to recognize the probable recovery through future customer rates of tax benefits related to the equity component of allowance for funds used during construction, as well as the effects of tax rate increases. To the extent deferred tax balances are included in rate base, the revaluation of deferred taxes is recorded as a regulatory asset or liability on the balance sheet and will be collected from, or refunded to, customers. For deferred tax balances not included in rate base, the revaluation of deferred taxes is recorded as an adjustment to income tax expense on the income statement. See Note 12 – Income Taxes for further information regarding the revaluation of deferred taxes related to Missouri state corporate income tax rate changes. Ameren Missouri, Ameren Illinois, and all the other Ameren subsidiary companies are parties to a tax allocation agreement with Ameren (parent) that provides for the allocation of consolidated tax liabilities. The tax allocation agreement specifies that each party be allocated an amount of tax using a stand-alone calculation, which is similar to what would be owed or refunded had the party been separately subject to tax without considering the impact of consolidation. Any net benefit attributable to Ameren (parent) is reallocated to the other parties. This reallocation is treated as a capital contribution to the party receiving the benefit. See Note 13 – Related-party Transactions for information regarding capital contributions under the tax allocation agreement. Accounting Changes and Other Matters In the first quarter of 2020, the Ameren Companies adopted authoritative accounting guidance that requires credit losses on most financial assets carried at amortized cost and off-balance sheet credit exposures, such as financial guarantees or loan commitments, to be measured using a current expected credit loss (CECL) model. The guidance requires an entity to measure expected credit losses using relevant information about past events, current conditions, and reasonable and supportable forecasts that affect the collectibility of the reported amount. In addition, the guidance made certain changes to the impairment model applicable to available-for-sale debt securities, such as requiring credit losses to be presented as an allowance rather than a write-down on impaired debt securities for which there is neither an intent nor a more-likely-than-not requirement to sell. Our adoption of this guidance did not have a material impact on the Ameren Companies’ financial statements and did not result in a cumulative effect adjustment to retained earnings as of the adoption date. See Note 15 – Supplemental Information for additional information regarding credit losses on accounts receivable. In the first quarter of 2020, the Ameren Companies adopted authoritative accounting guidance that altered certain disclosure requirements in relation to fair value measurements. See Note 8 – Fair Value Measurements for our fair value measurement disclosures.

Rate And Regulatory Matters

Rate And Regulatory Matters	12 Months Ended
	Dec. 31, 2020
Public Utilities, General Disclosures [Abstract]
RATE AND REGULATORY MATTERS	RATE AND REGULATORY MATTERS Below is a summary of our regulatory frameworks and significant regulatory proceedings and related lawsuits. We are unable to predict the ultimate outcome of these matters, the timing of final decisions of the various agencies and courts, or the effect on our results of operations, financial position, or liquidity. Regulatory Frameworks Missouri The MoPSC regulates rates and other matters for Ameren Missouri’s electric service and natural gas distribution businesses. The rates Ameren Missouri charges customers for these services are established in a traditional regulatory rate review, which takes up to 11 months to complete, based on a historical test year and the allowed ROE established in the review. Ameren Missouri has recovery mechanisms, including the RESRAM, FAC, MEEIA, PGA, DCA, and ISRS, that allow customer rates to be adjusted without a traditional regulatory rate review. These riders, along with the PISA, each described in more detail below, partially mitigate the effects of regulatory lag. Ameren Missouri also employs other recovery mechanisms, including a pension and postretirement benefit cost tracker, an uncertain income tax position tracker, a tracker on certain excess deferred income taxes, a renewable energy standard cost tracker, and a solar rebate program cost tracker. Each of these trackers allows Ameren Missouri to defer the difference between actual costs incurred and costs included in customer rates as a regulatory asset or regulatory liability. The difference will be reflected in base rates in a subsequent MoPSC rate order. Ameren Missouri’s cost recovery under any of its recovery mechanisms is subject to MoPSC prudence reviews. The PISA permits Ameren Missouri to defer and recover 85% of the depreciation expense and earn a return at the applicable WACC on investments in certain property, plant, and equipment placed in service after September 1, 2018, and not included in base rates. The regulatory asset for accumulated PISA deferrals also earns a return at the applicable WACC, with all approved PISA deferrals added to rate base prospectively and recovered over a period of 20 years following a regulatory rate review. Additionally, under the RESRAM, Ameren Missouri is permitted to recover the 15% of depreciation expense not recovered under the PISA, and earn a return at the applicable WACC for investments in renewable generation plant placed in service. The deferrals are a regulatory asset until they are included in customer rates and collected in a subsequent period. Those investments not eligible for recovery under the PISA and the remaining 15% of certain property, plant, and equipment placed in service, unless eligible for recovery under the RESRAM, remain subject to regulatory lag. Ameren Missouri recognizes the cost of debt on PISA deferrals in revenue, instead of using the applicable WACC, with the difference recognized in revenues when recovery of such deferrals is reflected in customer rates. Under Missouri law, as a result of the PISA election, additional provisions apply to Ameren Missouri, including limitations on electric customer rate increases. If rate changes from the FAC or the RESRAM riders would cause rates to temporarily exceed the 2.85% rate cap, the overage would be deferred for future recovery in the next regulatory rate review; however, rates established in such regulatory rate review would be subject to the rate cap. Any deferred overages approved for recovery would be recovered in a manner consistent with costs recovered under the PISA. Excluding customer rates under the MEEIA rider, which are not subject to the rate cap, Ameren Missouri would incur a penalty equal to the amount of deferred overage that would cause customer rates to exceed the 2.85% rate cap. Ameren Missouri did not incur a penalty related to the rate cap in 2020. Both the rate increase limitation and the PISA are effective through December 2023. Missouri law provides for the ability to use the PISA, if Ameren Missouri requests and receives MoPSC approval for extension, through December 2028. The RESRAM permits Ameren Missouri to recover or refund, through customer rates, the difference between the cost of compliance with Missouri’s renewable energy standard and the amount set in base rates. Customer rates are adjusted for the RESRAM on an annual basis without a traditional regulatory rate review, subject to MoPSC prudence reviews. The difference between actual compliance costs and costs billed to customers in a given period is deferred as a regulatory asset or liability. The deferred amount is either billed or refunded to customers in a subsequent period. RESRAM regulatory assets earn carrying costs at short-term interest rates. The RESRAM permits Ameren Missouri to recover investments in wind generation and other renewables related to compliance with Missouri’s renewable energy standard, and earn a return at the applicable WACC on those investments not already provided for in customer rates or any other recovery mechanism. The FAC permits Ameren Missouri to recover or refund, through customer rates, 95% of the variance in net energy costs from the amount set in base rates without a traditional regulatory rate review, subject to MoPSC prudence reviews, with the remaining 5% of changes retained by Ameren Missouri. Net recovery of these costs through customer rates does not affect Ameren Missouri’s electric margins, as any change in revenue is offset by a corresponding change in fuel expense. The difference between actual net energy costs and costs billed to customers in a given period is deferred as a regulatory asset or liability. The deferred amount is either billed or refunded to customers in a subsequent period. FAC regulatory assets earn carrying costs at short-term interest rates. Ameren Missouri’s base rates for electric service are required to be reset at least every four years to allow for continued use of the FAC. The MEEIA permits Ameren Missouri to recover customer energy-efficiency program costs, the related lost electric margins, and any performance incentive through the MEEIA without a traditional regulatory rate review. MEEIA assets earn carrying costs at short-term interest rates. Ameren Missouri is a member of the MISO, and its transmission rate is calculated in accordance with the MISO Open Access Transmission, Energy, and Operating Reserve Markets Tariff. The FERC regulates the rates charged and the terms and conditions for wholesale electric transmission service. The transmission rate update each June is based on Ameren Missouri’s actual historical cost from the prior calendar year. This rate is not directly charged to Missouri retail customers because, in Missouri, bundled retail rates include an amount for transmission-related costs and revenues. The PGA allows Ameren Missouri to recover prudently incurred costs of natural gas purchased on behalf of its customers without a traditional regulatory rate review. These pass-through purchased gas costs do not affect Ameren Missouri’s natural gas margins, as any change in costs is offset by a corresponding change in revenues. The difference between actual natural gas costs and costs billed to customers in a given period is deferred as a regulatory asset or liability. The deferred amount is either billed or refunded to customers in a subsequent period. PGA regulatory assets earn carrying costs at short-term interest rates. The DCA ensures recoverability of the natural gas delivery service revenue requirement that is dependent on sales volumes for nearly all customers. The DCA allows Ameren Missouri to adjust natural gas delivery service rates without a traditional regulatory rate review when changes occur in sales volumes from those volumes approved by the MoPSC in the previous regulatory rate review. The difference between actual gas delivery service revenues billed to customers and revenues approved by the MoPSC in a given period is deferred as a regulatory asset or liability. DCA regulatory assets earn carrying costs at short-term interest rates. The deferred amount is either billed or refunded to customers in a subsequent period. In addition, the ISRS permits certain prudently incurred natural gas infrastructure replacement costs to be recovered from customers on a more timely basis between regulatory rate reviews. The ROE currently used by Ameren Missouri for purposes of the ISRS tariff is 9.725%. Illinois The ICC regulates rates and other matters for Ameren Illinois’ electric distribution service and natural gas distribution businesses. The rates Ameren Illinois charges customers for electric distribution service are calculated under a performance-based formula ratemaking framework. The rates Ameren Illinois charges customers for natural gas distribution service are established in a traditional regulatory rate review, which takes up to 11 months to complete, based on a future test year and an allowed ROE established in the review. Ameren Illinois’ election to use the electric distribution service performance-based formula ratemaking framework allowed by state law, described below, permits Ameren Illinois to adjust customer rates to recover the cost of electric distribution service on an annual basis. Ameren Illinois’ electric distribution service also has other cost recovery mechanisms in place that allow customer rates to be adjusted without a traditional regulatory rate review. Ameren Illinois’ electric distribution service business has riders for power procurement and transmission services incurred on behalf of its customers, renewable energy credit compliance, zero emission credits, and certain environmental costs, as well as bad debt write-offs and the costs of certain asbestos-related claims not recovered in base rates. These pass-through costs do not affect Ameren Illinois’ net income, as any change in costs is offset by a corresponding change in revenues. Ameren Illinois’ cost recovery under any of its recovery mechanisms is subject to ICC prudence reviews. Ameren Illinois’ electric distribution service performance-based formula ratemaking framework allows Ameren Illinois to reconcile electric distribution service rates to its actual revenue requirement on an annual basis. If a given year’s revenue requirement varies from the amount collected from customers, an adjustment is 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 is then collected from, or refunded to, customers within two years from the end of the year. In addition, Ameren Illinois’ electric customer energy-efficiency rider provides Ameren Illinois’ electric distribution service business with recovery of, and return on, energy-efficiency investments. Under formula ratemaking for both its electric distribution service and its electric energy-efficiency investments, the revenue requirements are based on recoverable costs, year-end rate base, and a year-end ratemaking capital structure, and earn a return at the applicable WACC. The ROE component of the applicable WACC is based on the annual average of the monthly yields of the 30-year United States Treasury bonds plus 580 basis points and any performance-related basis point adjustments, described in more detail below. Therefore, Ameren Illinois’ annual ROE for its electric distribution business is directly correlated to the yields on such bonds. In addition, regulatory assets applicable to formula ratemaking for both electric distribution service and electric energy-efficiency investments earn a return at the applicable WACC. However, Ameren Illinois recognizes the cost of debt on these regulatory assets in revenue, instead of the applicable WACC, with the difference recognized in revenues when recovery of such regulatory assets is reflected in customer rates. Unless extended, the performance-based formula ratemaking framework expires at the end of 2022. If not extended, Ameren Illinois would be required to establish future rates through a traditional regulatory rate review, which would allow the use of a future test year, with the ICC. The decoupling provisions extend beyond the end of the formula ratemaking by law, which ensures that Ameren Illinois’ electric distribution revenues authorized in a regulatory rate review are not affected by changes in sales volumes. See below for information regarding a request filed by Ameren Illinois to continue reconciling electric distribution service rates to the last annual revenue requirement approved by the ICC under the formula ratemaking framework after the framework expires or is no longer elected. Ameren Illinois electric distribution service business is also subject to performance standards. Failure to achieve the standards would result in a reduction in the company’s allowed ROE calculated under the formulas. The performance standards applicable to electric distribution service include improvements in service reliability to reduce both the frequency and duration of outages, a reduction in the number of estimated bills, a reduction of consumption from inactive meters, and a reduction in bad debt expense. The electric distribution service regulatory framework provides for ROE penalties up to 38 basis points annually in 2021 and 2022 if these performance standards are not met. The allowed ROE on energy-efficiency investments can be increased or decreased up to 200 basis points, depending on the achievement of annual energy savings goals. Any adjustments to the allowed ROE for energy-efficiency investments will depend on annual performance for a historical period relative to energy savings goals. In 2020, 2019, and 2018, there were no performance-related basis point adjustments that materially affected financial results. Ameren Illinois’ natural gas distribution business has recovery mechanisms, including the QIP, PGA, and VBA, that allow customer rates to be adjusted without a traditional regulatory rate review. These riders, described in more detail below, mitigate the effects of regulatory lag. Ameren Illinois employs other riders for natural gas customer energy-efficiency program costs and certain environmental costs, as well as bad debt expenses and invested capital taxes not recovered in base rates. Pass-through costs under the riders do not affect Ameren Illinois’ net income, as any change in costs is offset by a corresponding change in revenues. Ameren Illinois’ cost recovery under any of its recovery mechanisms is subject to ICC prudence reviews. The QIP provides Ameren Illinois with recovery of, and a return on, qualifying natural gas infrastructure investments that are placed in service between regulatory rate reviews. Infrastructure investments under the QIP earn a return at the applicable WACC. Eligible natural gas investments include projects to improve safety and reliability and modernization investments, such as smart meters. The deferrals are a regulatory asset until they are included in customer rates in a subsequent period. Recovery of the regulatory asset begins two months after the qualifying natural gas plant is placed in service and continues until such plant is included in base rates in a natural gas delivery service rate order. Ameren Illinois’ QIP is subject to a rate impact limitation of a cumulative 4% per year since the most recent delivery service rate order, with no single year exceeding 5.5%. If the rate impact limitation was met in a particular year, the amount of rate base causing the QIP rate to exceed the limitation would be exposed to regulatory lag until a year when that amount could be recovered under QIP or is added to rate base as a part of a regulatory rate review. Upon issuance of a natural gas delivery service rate order, QIP rate base is transferred to base rates and the QIP is reset to zero, which mitigates the risk that the QIP will exceed its statutory limitations in future years and ensures timely recovery of capital investments. Without legislative action, the QIP will sunset after December 2023. The PGA allows Ameren Illinois to recover prudently incurred costs of natural gas purchased on behalf of its customers without a traditional regulatory rate review. These pass-through purchased gas costs do not affect Ameren Illinois natural gas margins, as any change in costs is offset by a corresponding change in revenues. The difference between actual natural gas costs and costs billed to customers in a given period is deferred as a regulatory asset or liability. The deferred amount is either billed or refunded to customers in a subsequent period. PGA regulatory assets earn carrying costs at short-term interest rates. The VBA ensures recoverability of the natural gas distribution service revenue requirement that is dependent on sales volumes for residential and small nonresidential customers. For these rate classes, the VBA allows Ameren Illinois to adjust natural gas distribution service rates without a traditional regulatory rate review when changes occur in sales volumes from those volumes approved by the ICC in a previous regulatory rate review. The difference between allowed sales revenues and amounts billed to customers in a given period is deferred as a regulatory asset or liability. The deferred amount is collected from, or refunded to, customers in a subsequent period. VBA regulatory assets for a given year that are not fully collected by the end of the following year begin earning carrying costs at short-term interest rates. Federal The FERC regulates rates and other matters for Ameren Illinois’ transmission business and ATXI, as well as for Ameren Missouri. See discussion above related to Ameren Missouri. Both Ameren Illinois and ATXI are members of the MISO, and their transmission rates are calculated in accordance with the MISO Open Access Transmission, Energy, and Operating Reserve Markets Tariff. Ameren Illinois and ATXI have received FERC approval to use a company-specific, forward-looking formula ratemaking framework in setting their transmission rates. These forward-looking rates are updated annually and become effective each January with forecasted information. The formula rate framework provides for an annual reconciliation of the electric transmission service revenue requirement, which reflects the actual recoverable costs incurred and the 13-month average rate base for a given year, with the revenue requirement in customer rates, including an allowed ROE. If a given year’s revenue requirement varies from the amount collected from customers, an adjustment is 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 is collected from, or refunded to, customers within two years from the end of the year. FERC revenue requirement reconciliation adjustment regulatory assets earn carrying costs at each company’s short-term interest rates, while each company incurs interest at a FERC-prescribed rate on related regulatory liabilities. In addition, the FERC has approved transmission rate incentives, including a 50 basis point incentive adder to the allowed base ROE for Ameren Illinois and ATXI for participation in an RTO. Proceedings and Updates Missouri March 2020 MoPSC Electric Rate Order In March 2020, the MoPSC issued an order in Ameren Missouri’s July 2019 electric service regulatory rate review, approving nonunanimous stipulation and agreements. The order resulted in a decrease of $32 million to Ameren Missouri’s annual revenue requirement for electric retail service, which reflected infrastructure investments as of December 31, 2019. The order also provided for 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. The order reduced the annualized base level of net energy costs pursuant to the FAC by approximately $115 million from the base level established in the MoPSC’s March 2017 electric rate order. The order also changed the annualized regulatory asset and liability amortization amounts and the base level of expenses for trackers. On an annualized basis, these changes reflect approximately $20 million of increased revenues and approximate decreases in purchased power expenses of $15 million, other operating and maintenance expenses of $60 million, and income tax expenses of $20 million. Additionally, the annual revenue requirement incorporated increases of approximately $50 million for the reduction in sales volumes resulting from MEEIA programs and approximately $50 million of depreciation and amortization expense for amounts previously deferred under PISA. The increase in the annual revenue requirement related to the MEEIA programs is seasonally weighted to the summer. One of the stipulation and agreements approved by the MoPSC’s March 2020 order states that the net impact of the revenue and expense changes noted above reflects a 9.4% to 9.8% ROE on an unspecified percent of common equity applicable to rate base. In addition, the order required Ameren Missouri to donate $8 million to low-income assistance programs, which was reflected in results of operations in the first quarter of 2020. The new rates, base level of expenses, and amortizations became effective on April 1, 2020. In April 2020, the MoPSC issued another order in Ameren Missouri’s July 2019 electric service regulatory rate review, reaffirming the existing percentage of net energy cost variances allowed to be recovered or refunded under the FAC. Wind Generation Facilities In December 2020, Ameren Missouri acquired a 400-MW wind generation project located in northeastern Missouri for approximately $615 million, and placed the assets in service as the High Prairie Renewable Energy Center. The purchase price included $564 million of cash, a deferred purchase price obligation withheld as credit support in relation to certain potential claims, contingent consideration, and transaction costs. In January 2021, Ameren Missouri acquired an up-to 300-MW wind generation project located in northwestern Missouri. At the date of this filing, Ameren Missouri placed 120 MWs in service as the Atchison Renewable Energy Center, with a purchase price of approximately $200 million, including transaction costs. There have been changes to the schedule for this project, particularly as a result of component delivery delays. Ameren Missouri expects approximately 150 MWs of the up-to 300-MW project to be in service by the end of the first quarter of 2021, and the remaining portion to be in service later in 2021. Both renewable energy centers are expected to support Ameren Missouri’s compliance with the Missouri renewable energy standard. MEEIA In August 2020, the MoPSC issued an order approving a unanimous stipulation and agreement with respect to the 2022 program year of Ameren Missouri’s six-year MEEIA 2019 program. The order established performance incentives that would provide Ameren Missouri an opportunity to earn additional revenues, including $11 million if Ameren Missouri achieves certain energy-efficiency goals during the 2022 program year and an additional $1 million if Ameren Missouri exceeds its targeted energy-efficiency goals. Ameren Missouri intends to invest $70 million in energy-efficiency programs during the 2022 program year. The August 2020 order also approved Ameren Missouri’s energy savings results for the first year of the MEEIA 2019 program. As a result of this order and in accordance with revenue recognition guidance, Ameren Missouri recognized revenues of $6 million during 2020. As a result of MoPSC orders issued in September 2017, October 2018, January 2019, and September 2019 related to performance incentives for the MEEIA 2013 and MEEIA 2016 programs, and in accordance with revenue recognition guidance, Ameren Missouri recognized revenues of $37 million and $11 million during 2019 and 2018, respectively. Requests for Accounting Authority Orders Related to COVID-19 Pandemic Costs Ameren Missouri suspended disconnections for customer nonpayment and late fees in mid-March 2020, and resumed those activities for commercial and industrial customers in mid-July 2020 and residential customers in early August 2020. In October 2020, Ameren Missouri filed requests with the MoPSC for accounting authority orders related to its electric and natural gas services. If issued as requested, the orders would allow Ameren Missouri to accumulate certain costs incurred related to the COVID-19 pandemic, including bad debt write-offs, net of cost savings, as well as forgone customer late fee and reconnection fee revenues, for a specified time period, for potential recovery in future electric and natural gas service regulatory rate reviews. Costs incurred, net of savings, and forgone customer late fee and reconnection fee revenues related to the COVID-19 pandemic from March 2020 through December 2020 were immaterial. The MoPSC is under no deadline to issue orders, and Ameren Missouri cannot predict the ultimate outcome of these regulatory proceedings. Illinois Electric Distribution Service Rates In December 2020, the ICC issued an order in Ameren Illinois’ annual update filing that approved a $49 million decrease in Ameren Illinois’ electric distribution service rates beginning in January 2021. This order reflected a decrease to the annual formula rate based on 2019 actual costs, a decrease to include the 2019 revenue requirement reconciliation adjustment, and a decrease for the conclusion of the 2018 revenue requirement reconciliation adjustment, which was fully collected from customers in 2020, consistent with the ICC’s December 2019 annual update filing order. It also reflected an increase based on expected net plant additions for 2020. Request to Continue Electric Distribution Service Rate Reconciliation In April 2020, Ameren Illinois filed a request with the ICC, which, if approved, would allow Ameren Illinois to continue to reconcile electric distribution service rates to the last annual revenue requirement approved by the ICC under the performance-based formula ratemaking framework, for a period of up to two years after the framework expires or is no longer elected. Ameren Illinois expects a decision by the ICC in March 2021. Electric Customer Energy-Efficiency Investments In May 2020, Ameren Illinois filed its annual electric customer energy-efficiency formula rate update to establish the revenue requirement to be used for 2021 rates with the ICC. In December 2020, the ICC issued an order that approved 2021 electric customer energy-efficiency rates of $51 million, which represents an increase of $7 million from 2020 rates. January 2021 Natural Gas Delivery Service Rate Order In January 2021, the ICC issued an order in Ameren Illinois’ February 2020 natural gas delivery service regulatory rate review, which resulted in an increase to its annual revenues for natural gas delivery service of $76 million based on a 9.67% allowed ROE, a capital structure composed of 52% common equity, and a rate base of $2.1 billion. The new rates became effective in January 2021. As a result of this order, the rate base under the QIP was reset to zero. Ameren Illinois used a 2021 future test year in this proceeding. QIP Prudence Review In March 2019, Ameren Illinois filed a request for an ICC prudence review of natural gas infrastructure investments recovered under the QIP during 2018. In November 2019, the Illinois Attorney General’s office challenged the recovery of capital investments, among other things, that were made during 2018, alleging that the amount of investments is excessive based on a comparison to historical investment levels. The Illinois Attorney General’s office is not alleging imprudence or that the investments do not qualify for recovery. In March 2020, the ICC staff filed testimony that supports the prudence and reasonableness of the capital investments made during 2018. Ameren Illinois’ 2018 QIP rate recovery under review by the ICC is within the rate increase limitations allowed by law. Ameren Illinois expects a decision by the ICC in this proceeding within the first half of 2021. Service Disconnection Moratorium Proceeding In March 2020, the ICC issued an order requiring all Illinois electric distribution and natural gas utilities to suspend disconnections and late fees for customer nonpayment, on an interim basis, effective March 18, 2020. Pursuant to an ICC order issued in June 2020 and following a voluntary extension of the suspension of residential disconnections, Ameren Illinois resumed disconnection activities for commercial and industrial customers for nonpayment in early August 2020 and residential customers in mid-September 2020, with the exception of residential customers classified as low income, expressing a financial hardship, or relying on medical equipment. Disconnections for nonpayment for these and all other residential customers are expected to begin in April 2021, which is after the annual winter moratorium period on disconnections that ends March 31, 2021. Ameren Illinois began the winter moratorium on November 18, 2020, which has historically started at the beginning of December. Ameren Illinois also resumed charging late fees to all customers in late July 2020. The June 2020 order requires Ameren Illinois to implement more flexible credit and collection practices, on a temporary basis, including longer deferred payment arrangements, extending to 24 months in certain cases, and programs to provide financial assistance to customers. In addition, the order allows Ameren Illinois to recover up to $8 million in costs incurred related to the financial assistance programs. The portion of these costs 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 established by the order. The order also allows Ameren Illinois to recover forgone customer late fees and costs incurred related to the COVID-19 pandemic. The portion of these forgone late fees and costs associated with Ameren Illinois’ electric distribution business will be recovered through formula rates and the portion associated with its natural gas distribution business will be recovered through the special purpose rider. In addition, the order allows Ameren Illinois’ electric distribution business to recover bad debt expense for 2020, instead of write-offs net of subsequent recoveries, through future true-ups in its bad debt rider. The bad debt rider reverted to the recovery of bad debt write-offs, net of any subsequent recoveries, in 2021. 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 calculation inputs for materials and supplies. 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. This review could lead the FERC to ultimately require refunds for periods prior to 2019. 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. Pursuant to the May 2020 order, in the second quarter of 2020, Ameren and Ameren Illinois recorded a $2 million reduction to revenue for the period of January 2019 through May 2020. Regardless of the outcome of the appeal, the impacts of the May 2020 and Octobe

Property And Plant, Net

Property And Plant, Net	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
PROPERTY AND PLANT, NET	PROPERTY, PLANT, AND EQUIPMENT, NET The following table presents property, plant, and equipment, net, at December 31, 2020 and 2019: Ameren Missouri (a) Ameren Other Ameren (a) 2020 Property, plant, and equipment at original cost: (b) Electric generation: Coal (c) $ 4,875 $ — $ — $ 4,875 Natural gas and oil 1,097 — — 1,097 Nuclear 5,608 — — 5,608 Renewable (d) 1,301 — — 1,301 Electric distribution 6,784 6,649 — 13,433 Electric transmission 1,482 3,575 1,774 6,831 Natural gas 561 3,308 — 3,869 Other (e) 1,390 1,070 245 2,705 23,098 14,602 2,019 39,719 Less: Accumulated depreciation and amortization 9,689 3,780 304 13,773 13,409 10,822 1,715 25,946 Construction work in progress: Nuclear fuel in process 75 — — 75 Other 395 379 12 786 Property, plant, and equipment, net $ 13,879 $ 11,201 $ 1,727 $ 26,807 2019 Property, plant, and equipment at original cost: (b) Electric generation: Coal (c) $ 4,730 $ — $ — $ 4,730 Natural gas and oil 1,090 — — 1,090 Nuclear 5,414 — — 5,414 Renewable (d) 646 — — 646 Electric distribution 6,371 6,299 — 12,670 Electric transmission 1,405 3,101 1,642 6,148 Natural gas 528 3,024 — 3,552 Other (e) 1,173 993 236 2,402 21,357 13,417 1,878 36,652 Less: Accumulated depreciation and amortization 9,195 3,536 275 13,006 12,162 9,881 1,603 23,646 Construction work in progress: Nuclear fuel in process 135 — — 135 Other 338 202 55 595 Property, plant, and equipment, net $ 12,635 $ 10,083 $ 1,658 $ 24,376 (a) Amounts include two CTs that have related financing obligations. The gross cumulative asset value of those agreements was $240 million and $236 million at December 31, 2020 and 2019, respectively. The total accumulated depreciation associated with the two CTs was $99 million and $95 million at December 31, 2020 and 2019, respectively. See Note 5 – Long-term Debt and Equity Financings for additional information on these agreements. (b) The estimated lives for each asset group are as follows: 5 to 72 years for electric generation, excluding Ameren Missouri’s hydro generating assets, which have useful lives of up to 150 years, 20 to 80 years for electric distribution, 50 to 75 years for electric transmission, 20 to 80 years for natural gas, and 5 to 55 years for other. (c) Includes $36 million of oil-fired generation in both 2019 and 2020. (d) Renewable includes hydroelectric, wind, solar, and methane gas generation facilities. (e) Other property, plant, and equipment includes assets used to support electric and natural gas services. Capitalized software costs are classified within “Property, Plant, and Equipment, Net” on the balance sheet and are amortized on a straight-line basis over the expected period of benefit, ranging from 3 to 10 years. The following table presents the amortization, gross carrying value, and related accumulated amortization of capitalized software by year: Amortization Expense Gross Carrying Value Accumulated Amortization 2020 2019 2018 2020 2019 2020 2019 Ameren $ 93 $ 78 $ 71 $ 1,021 $ 901 $ (640) $ (584) Ameren Missouri 44 30 24 398 303 (189) (153) Ameren Illinois 45 45 44 397 377 (238) (221) Annual amortization expense for capitalized costs for software placed in service as of December 31, 2020, is estimated to be as follows: 2021 2022 2023 2024 2025 Ameren $ 105 $ 94 $ 81 $ 55 $ 23 Ameren Missouri 55 49 45 33 15 Ameren Illinois 46 42 34 21 7

Short-Term Debt And Liquidity

Short-Term Debt And Liquidity	12 Months Ended
	Dec. 31, 2020
Line of Credit Facility [Abstract]
SHORT-TERM DEBT AND LIQUIDITY	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/or, in the case of Ameren Missouri and Ameren Illinois, short-term affiliate borrowings. Short-Term Borrowings The Credit Agreements provide $2.3 billion of credit through maturity in December 2024. The total facility size of the Missouri Credit Agreement and Illinois Credit Agreement is $1.2 billion and $1.1 billion, respectively. The maturity date may be extended for two additional one-year periods upon mutual consent of the borrowers and lenders. Credit available under the agreements is provided by 22 international, national, and regional lenders, with no single lender providing more than $130 million of credit in aggregate. The obligations of each borrower under the respective Credit Agreements to which it is a party are several and not joint. Except under limited circumstances relating to expenses and indemnities, the obligations of Ameren Missouri and Ameren Illinois under the respective Credit Agreements are not guaranteed by Ameren (parent) or any other subsidiary of Ameren. The following table presents the maximum aggregate amount available to each borrower under each facility: Missouri Illinois Ameren (parent) $ 900 $ 500 Ameren Missouri 850 (a) Ameren Illinois (a) 800 (a) Not applicable. The borrowers have the option to seek additional commitments from existing or new lenders to increase the total facility size of the Credit Agreements to a maximum of $1.4 billion for the Missouri Credit Agreement and $1.3 billion for the Illinois Credit Agreement. Ameren (parent) borrowings are due and payable no later than the maturity date of the Credit Agreements. Ameren Missouri and Ameren Illinois borrowings under the applicable Credit Agreement are due and payable no later than the earlier of the maturity date or 364 days after the date of the borrowing. The obligations of the borrowers under the Credit Agreements are unsecured. Loans are available on a revolving basis under each of the Credit Agreements. Funds borrowed may be repaid and, subject to satisfaction of the conditions to borrowing, reborrowed from time to time. At the election of each borrower, the interest rates on such loans will be the alternate base rate plus the margin applicable to the particular borrower and/or the eurodollar rate plus the margin applicable to the particular borrower. The applicable margins will be determined by the borrower’s long-term unsecured credit ratings or, if no such ratings are in effect, the borrower’s corporate/issuer ratings then in effect. The borrowers have received commitments from the lenders to issue letters of credit up to $100 million under each of the Credit Agreements. In addition, the issuance of letters of credit is subject to the $2.3 billion overall combined facility borrowing limitations of the Credit Agreements. The borrowers will use the proceeds from any borrowings under the Credit Agreements for general corporate purposes, including working capital, commercial paper liquidity support, loan funding under the Ameren money pool arrangements, and other short-term affiliate loan arrangements. 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, as well as to support issuance of letters of credit for the borrowers. As of December 31, 2020, 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 $1.9 billion. The following table summarizes the borrowing activity and relevant interest rates under Ameren (parent)’s, Ameren Missouri’s, and Ameren Illinois’ commercial paper issuances and borrowings under the Credit Agreements in the aggregate for the years ended December 31, 2020 and 2019: Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2020 Average daily amount outstanding $ 108 $ 109 $ 46 $ 263 Commercial paper issuances outstanding at period-end 490 — — 490 Weighted-average interest rate 1.04 % 1.73 % 0.97 % 1.31 % Peak amount outstanding during period (a) $ 490 $ 573 $ 250 $ 908 Peak interest rate 3.30 % (b) 5.05 % (b) 3.40 % 5.05 % (b) 2019 Average daily amount outstanding $ 421 $ 122 $ 157 $ 700 Commercial paper issuances outstanding at period-end 153 234 53 440 Weighted-average interest rate 2.66 % 2.62 % 2.43 % 2.60 % Peak amount outstanding during period (a) $ 651 $ 549 $ 356 $ 1,113 Peak interest rate 3.80 % (c) 2.97 % 5.00 % (c) 5.00 % (c) (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 amount 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. (c) Ameren’s and Ameren Illinois’ peak interest rate was affected by temporary disruptions in the commercial paper market in the third quarter of 2019. Indebtedness Provisions and Other Covenants The information below is a summary of the Ameren Companies’ compliance with indebtedness provisions and other covenants. The Credit Agreements contain conditions for borrowings and issuances of letters of credit. These conditions include the absence of default or unmatured default, material accuracy of representations and warranties (excluding any representation after the closing date as to the absence of material adverse change and material litigation, and the absence of any notice of violation, liability, or requirement under any environmental laws that could have a material adverse effect), and obtaining required regulatory authorizations. In addition, it is a condition for any Ameren Illinois borrowing that, at the time of and after giving effect to such borrowing, Ameren Illinois not be in violation of any limitation on its ability to incur unsecured indebtedness contained in its articles of incorporation. The Credit Agreements also contain nonfinancial covenants, including restrictions on the ability to incur certain liens, to transact with affiliates, to dispose of assets, to make investments in or transfer assets to its affiliates, and to merge with other entities. The Credit Agreements require each of Ameren, Ameren Missouri, and Ameren Illinois to maintain consolidated indebtedness of not more than 65% of its consolidated total capitalization pursuant to a defined calculation set forth in the agreements. As of December 31, 2020, the ratios of consolidated indebtedness to total consolidated capitalization, calculated in accordance with the provisions of the Credit Agreements, were 56%, 48%, and 45%, for Ameren, Ameren Missouri, and Ameren Illinois, respectively. The Credit Agreements contain default provisions that apply separately to each borrower. However, a default of Ameren Missouri or Ameren Illinois under the applicable credit agreement is also deemed to constitute a default of Ameren (parent) under such agreement. Defaults include a cross-default resulting from a default of such borrower under any other agreement covering outstanding indebtedness of such borrower and certain subsidiaries (other than project finance subsidiaries and nonmaterial subsidiaries) in excess of $100 million in the aggregate (including under the other credit agreement). However, under the default provisions of the Credit Agreements, any default of Ameren (parent) under either credit agreement that results solely from a default of Ameren Missouri or Ameren Illinois does not result in a cross-default of Ameren (parent) under the other credit agreement. Further, the Credit Agreements default provisions provide that an Ameren (parent) default under either of the Credit Agreements does not constitute a default by Ameren Missouri or Ameren Illinois. None of the Credit Agreements or financing agreements contain credit rating triggers that would cause a default or acceleration of repayment of outstanding balances. The Ameren Companies were in compliance with the provisions and covenants of the Credit Agreements at December 31, 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. Ameren Missouri, Ameren Illinois, and ATXI may participate in the utility money pool as both lenders and borrowers. Ameren (parent) and Ameren Services may participate in the utility money pool only as lenders. Surplus internal funds are contributed to the money pool from participants. The primary sources of external funds for the utility money pool are the Credit Agreements and the commercial paper programs. The total amount available to the pool participants from the utility money pool at any given time is reduced by the amount of borrowings made by participants, but it is increased to the extent that the pool participants advance surplus funds to the utility money pool or remit funds from other external sources. The availability of funds is also determined by funding requirement limits established by regulatory authorizations. Participants receiving a loan under the utility money pool agreement must repay the principal amount of such loan, together with accrued interest. The rate of interest depends on the composition of internal and external funds in the utility money pool. The average interest rate for borrowing under the utility money pool for the year ended December 31, 2020, was 0.64% (2019 – 2.48%). See Note 13 – Related-party Transactions for the amount of interest income and expense from the utility money pool agreement recorded by Ameren Missouri and Ameren Illinois for the years ended December 31, 2020, 2019, and 2018.

Long-Term Debt And Equity Finan

Long-Term Debt And Equity Financings	12 Months Ended
	Dec. 31, 2020
Long-Term Debt And Equity Financings [Abstract]
LONG-TERM DEBT AND EQUITY FINANCINGS	LONG-TERM DEBT AND EQUITY FINANCINGS The following table presents long-term debt outstanding, including maturities due within one year, as of December 31, 2020 and 2019: 2020 2019 Ameren (Parent): 2.70% Senior unsecured notes due 2020 $ — $ 350 2.50% Senior unsecured notes due 2024 450 450 3.65% Senior unsecured notes due 2026 350 350 3.50% Senior unsecured notes due 2031 800 — Total long-term debt, gross 1,600 1,150 Less: Unamortized discount and premium (2) — Less: Unamortized debt issuance costs (10) (6) Less: Maturities due within one year — (350) Long-term debt, net $ 1,588 $ 794 Ameren Missouri: Bonds and notes: 5.00% Senior secured notes due 2020 (a) — 85 1.60% 1992 Series bonds due 2022 (b) 47 47 3.50% Senior secured notes due 2024 (a) 350 350 2.95% Senior secured notes due 2027 (a) 400 400 3.50% First mortgage bonds due 2029 (d) 450 450 2.95% First mortgage bonds due 2030 (d) 465 — 2.90% 1998 Series A bonds due 2033 (b) 60 60 2.90% 1998 Series B bonds due 2033 (b) 50 50 2.75% 1998 Series C bonds due 2033 (b) 50 50 5.50% Senior secured notes due 2034 (a) 184 184 5.30% Senior secured notes due 2037 (a) 300 300 8.45% Senior secured notes due 2039 (a)(c) 350 350 3.90% Senior secured notes due 2042 (a)(c) 485 485 3.65% Senior secured notes due 2045 (a) 400 400 4.00% First mortgage bonds due 2048 (d) 425 425 3.25% First mortgage bonds due 2049 (d) 330 330 2.625% First mortgage bonds due 2051 (green bonds) (d) 550 — Finance obligations: City of Bowling Green agreement (Peno Creek CT) due 2022 (e) 16 23 Audrain County agreement (Audrain County CT) due 2023 (e) 240 240 Total long-term debt, gross 5,152 4,229 Less: Unamortized discount and premium (12) (9) Less: Unamortized debt issuance costs (36) (30) Less: Maturities due within one year (8) (92) Long-term debt, net $ 5,096 $ 4,098 2020 2019 Ameren Illinois: Bonds and notes: 2.70% Senior secured notes due 2022 (f)(g) $ 400 $ 400 3.25% Senior secured notes due 2025 (f) 300 300 6.125% Senior secured notes due 2028 (f) 60 60 3.80% First mortgage bonds due 2028 (h) 430 430 1.55% First mortgage bonds due 2030 (h) 375 — 6.70% Senior secured notes due 2036 (f) 61 61 6.70% Senior secured notes due 2036 (f) 42 42 4.80% Senior secured notes due 2043 (f) 280 280 4.30% Senior secured notes due 2044 (f) 250 250 4.15% Senior secured notes due 2046 (f) 490 490 3.70% First mortgage bonds due 2047 (h) 500 500 4.50% First mortgage bonds due 2049 (h) 500 500 3.25% First mortgage bonds due 2050 (h) 300 300 Total long-term debt, gross 3,988 3,613 Less: Unamortized discount and premium (6) (4) Less: Unamortized debt issuance costs (36) (34) Long-term debt, net $ 3,946 $ 3,575 ATXI: 3.43% Senior unsecured notes due 2050 (i) $ 450 $ 450 Total long-term debt, gross 450 450 Less: Unamortized debt issuance costs (2) (2) Long-term debt, net $ 448 $ 448 Ameren consolidated long-term debt, net $ 11,078 $ 8,915 (a) These notes are collaterally secured by first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any first mortgage bonds issued under the Ameren Missouri mortgage indenture remain outstanding. Redemption, purchase, or maturity of all first mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the first mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the 2051 maturity of the 2.625% first mortgage bonds and the restrictions preventing a release date to occur that are attached to certain senior secured notes described in footnote (d) below, Ameren Missouri does not expect the first mortgage lien protection associated with these notes to fall away. (b) These bonds are collaterally secured by first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage indenture and have a fall-away lien provision similar to that of Ameren Missouri’s senior secured notes. (c) Ameren Missouri has agreed that so long as any of the 3.90% senior secured notes due 2042 are outstanding, Ameren Missouri will not permit a release date to occur, and so long as any of the 8.45% senior secured notes due 2039 are outstanding, Ameren Missouri will not optionally redeem, purchase, or otherwise retire in full the outstanding first mortgage bonds not subject to release provisions. (d) These bonds are first mortgage bonds issued by Ameren Missouri under the Ameren Missouri bond indenture. They are secured by substantially all Ameren Missouri property and franchises. (e) Payments due related to these financing obligations are paid to a trustee, which is authorized to utilize the cash only to pay equal amounts due to Ameren Missouri under related bonds issued by the city/county and held by Ameren Missouri. The timing and amounts of payments due from Ameren Missouri under the agreements are equal to the timing and amount of bond service payments due to Ameren Missouri, resulting in no net cash flow. The balance of both the financing obligations and the related investments in debt securities, recorded in “Other Assets,” was $256 million and $263 million, respectively, as of December 31, 2020 and 2019. (f) These notes are collaterally secured by first mortgage bonds issued by Ameren Illinois under the Ameren Illinois mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any first mortgage bonds issued under its mortgage indenture remain outstanding. Redemption, purchase, or maturity of all first mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the first mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the 2050 maturity date of the 3.25% first mortgage bonds, Ameren Illinois does not expect the first mortgage lien protection associated with these notes to fall away. (g) Ameren Illinois has agreed that so long as any of the 2.70% senior secured notes due 2022 are outstanding, Ameren Illinois will not permit a release date to occur. (h) These bonds are first mortgage bonds issued by Ameren Illinois under the Ameren Illinois mortgage indenture. They are secured by substantially all Ameren Illinois property and franchises. (i) The following table presents the principal maturities schedule for the 3.43% senior notes due 2050: Payment Date Principal Payment August 2022 $ 49.5 August 2024 49.5 August 2027 49.5 August 2030 49.5 August 2032 49.5 August 2038 49.5 August 2043 76.5 August 2050 76.5 Total $ 450.0 The following table presents the aggregate maturities of long-term debt, including current maturities, at December 31, 2020: Ameren (parent) (a) Ameren Missouri (a) Ameren Illinois (a) ATXI (a) Ameren Consolidated (a) 2021 $ — $ 8 $ — $ — $ 8 2022 — 55 400 50 505 2023 — 240 — — 240 2024 450 350 — 50 850 2025 — — 300 — 300 Thereafter 1,150 4,499 3,288 350 9,287 Total $ 1,600 $ 5,152 $ 3,988 $ 450 $ 11,190 (a) Excludes unamortized discount, unamortized premium, and debt issuance costs of $12 million, $48 million, $42 million, and $2 million at Ameren (parent), Ameren Missouri, Ameren Illinois, and ATXI, respectively. All classes of Ameren Missouri’s and Ameren Illinois’ preferred stock are entitled to cumulative dividends, have voting rights, and are not subject to mandatory redemption. The preferred stock of Ameren’s subsidiaries is included in “Noncontrolling Interests” on Ameren’s consolidated balance sheet. The following table presents the outstanding preferred stock of Ameren Missouri and Ameren Illinois, which is redeemable at the option of the issuer, at the prices shown below as of December 31, 2020 and 2019: Shares Outstanding Redemption Price (per share) 2020 2019 Ameren Missouri: Without par value and stated value of $100 per share, 25 million shares authorized $3.50 Series 130,000 shares $ 110.00 $ 13 $ 13 $3.70 Series 40,000 shares 104.75 4 4 $4.00 Series 150,000 shares 105.625 15 15 $4.30 Series 40,000 shares 105.00 4 4 $4.50 Series 213,595 shares 110.00 (a) 21 21 $4.56 Series 200,000 shares 102.47 20 20 $4.75 Series 20,000 shares 102.176 2 2 $5.50 Series A 14,000 shares 110.00 1 1 Total $ 80 $ 80 Ameren Illinois: With par value of $100 per share, 2 million shares authorized 4.00% Series 144,275 shares $ 101.00 $ 14 $ 14 4.08% Series 45,224 shares 103.00 5 5 4.20% Series 23,655 shares 104.00 2 2 4.25% Series 50,000 shares 102.00 5 5 4.26% Series 16,621 shares 103.00 2 2 4.42% Series 16,190 shares 103.00 2 2 4.70% Series 18,429 shares 104.30 2 2 4.90% Series 73,825 shares 102.00 7 7 4.92% Series 49,289 shares 103.50 5 5 5.16% Series 50,000 shares 102.00 5 5 6.625% Series 124,274 shares 100.00 12 12 7.75% Series 4,542 shares 100.00 1 1 Total $ 62 $ 62 Total Ameren $ 142 $ 142 (a) In the event of voluntary liquidation, $105.50. Ameren has 100 million shares of $0.01 par value preferred stock authorized, with no such shares outstanding. Ameren Missouri has 7.5 million shares of $1 par value preference stock authorized, with no such shares outstanding. Ameren Illinois has 2.6 million shares of no par value preferred stock authorized, with no such shares outstanding. Ameren Under the DRPlus and its 401(k) plan, Ameren issued 0.7 million, 0.9 million, and 1.2 million shares of common stock in 2020, 2019, and 2018, respectively, and received proceeds of $51 million, $68 million, and $74 million for the respective years. In addition, Ameren issued 0.5 million, 0.8 million, and 0.7 million shares of common stock valued at $38 million, $54 million, and $35 million in 2020, 2019, 2018, respectively, for no cash consideration in connection with stock-based compensation. In May 2020, Ameren filed a Form S-3 registration statement with the SEC, authorizing the offering of 4 million additional shares of its common stock under the DRPlus, which expires in May 2023. Shares of common stock sold under the DRPlus are, at Ameren’s option, newly issued shares, treasury shares, or shares purchased in the open market or in privately negotiated transactions. In October 2020, Ameren, Ameren Missouri, and Ameren Illinois filed a Form S-3 shelf registration statement with the SEC, registering the issuance of an unspecified amount of certain types of securities. This registration statement expires in October 2023. In October 2018, Ameren filed a Form S-8 registration statement with the SEC, authorizing the offering of 4 million additional shares of its common stock under its 401(k) plan. Shares of common stock issuable under the 401(k) plan are, at Ameren’s option, newly issued shares, treasury shares, or shares purchased in the open market or in privately negotiated transactions. In August 2019, Ameren entered into a forward sale agreement with a counterparty relating to 7.5 million shares of common stock. In December 2020, pursuant to the agreement terms, Ameren partially settled the forward sale agreement by physically delivering 5.9 million shares of common stock for cash proceeds of $425 million. In February 2021, Ameren settled the remainder of the forward sale agreement 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 for additional information about the wind generation facilities. In April 2020, Ameren (parent) issued $800 million of 3.50% senior unsecured notes due January 2031, with interest payable semiannually on January 15 and July 15 of each year, beginning July 15, 2020. Ameren received net proceeds of $793 million, which were used for general corporate purposes, including to repay outstanding short-term debt, and were used to fund the repayment of Ameren’s $350 million of 2.70% senior unsecured notes, which were redeemed at par plus accrued interest in October 2020. In September 2019, Ameren issued $450 million of 2.50% senior unsecured notes due September 2024, with interest payable semiannually on March 15 and September 15 of each year, beginning March 15, 2020. Ameren received net proceeds of $447 million, which were used to repay outstanding short-term debt. Ameren Missouri In March 2020, Ameren Missouri issued $465 million of 2.95% first mortgage bonds due March 2030, with interest payable semiannually on March 15 and September 15 of each year, beginning September 15, 2020. Ameren Missouri received net proceeds of $462 million, which were used to repay outstanding short-term debt, including short-term debt that Ameren Missouri incurred in connection with the repayment of $85 million of its 5.00% senior secured notes that matured in February 2020. In October 2020, Ameren Missouri issued $550 million of 2.625% first mortgage bonds due March 2051, with interest payable semiannually on March 15 and September 15 of each year, beginning March 15, 2021. The bonds were issued as green bonds; therefore, the proceeds will be used for eligible green projects. Ameren Missouri received net proceeds of $543 million, which were used to partially finance the acquisition of two wind generation facilities. See Note 2 – Rate and Regulatory Matters for information about the wind generation facilities. In March 2019, Ameren Missouri issued $450 million of 3.50% first mortgage bonds due March 2029, with interest payable semiannually on March 15 and September 15 of each year, beginning September 15, 2019. Ameren Missouri received net proceeds of $447 million, which were used to repay outstanding short-term debt, including short-term debt that Ameren Missouri incurred in connection with the repayment of $329 million of its 6.70% senior secured notes that matured February 1, 2019. In June and July 2019, all of the 1992 Series bonds, 1998 Series A bonds, 1998 Series B bonds, and 1998 Series C bonds issued by the Missouri Environmental Authority on behalf of Ameren Missouri were subject to purchase in lieu of redemption or a mandatory tender as a result of a change in the method of determining the interest rates on the bonds. The interest rate method of each of the series of bonds, as well as Ameren Missouri’s first mortgage bonds that collaterally secure each of the series of bonds, was changed from a variable rate to a fixed rate. Upon the change in the method of determining the interest rate, the bonds, totaling $207 million, were remarketed to new investors. The following table provides additional information on the bonds: 1992 Series 1998 Series A 1998 Series B 1998 Series C Transaction month June 2019 July 2019 July 2019 June 2019 Principal amount $47 $60 $50 $50 Fixed interest rate 1.60% 2.90% 2.90% 2.75% Variable interest rate (a) 2.58% 3.43% 3.57% 3.43% Maturity December 2022 September 2033 September 2033 September 2033 Interest payment dates June 1 and December 1 March 1 and September 1 March 1 and September 1 March 1 and September 1 Initial interest payment date December 2019 September 2019 September 2019 September 2019 (a) Represents the variable interest rate of the bonds effective prior to the change in method of determining the interest rate. In October 2019, Ameren Missouri issued $330 million of 3.25% first mortgage bonds due October 2049, with interest payable semiannually on April 1 and October 1 of each year, beginning April 1, 2020. Ameren Missouri received net proceeds of $326 million, which were used to repay $244 million of its 5.10% senior unsecured notes due October 1, 2019, with the remaining proceeds used to repay a portion of its short-term debt. In October 2019, Ameren Missouri redeemed the remaining amount outstanding of its 5.45% first mortgage bonds due 2028 for less than $1 million. For information on Ameren Missouri’s capital contributions, refer to Capital Contributions in Note 13 – Related-party Transactions. Ameren Illinois In November 2020, Ameren Illinois issued $375 million of 1.55% first mortgage bonds due November 2030, with interest payable semiannually on May 15 and November 15 of each year, beginning May 15, 2021. Ameren Illinois received net proceeds of $371 million, which were used to repay short-term debt. In November 2019, Ameren Illinois issued $300 million of 3.25% first mortgage bonds due March 2050, with interest payable semiannually on March 15 and September 15 of each year, beginning March 15, 2020. Ameren Illinois received net proceeds of $296 million, which were used to repay outstanding short-term debt. For information on Ameren Illinois’ capital contributions, refer to Capital Contributions in Note 13 – Related-party Transactions. Indenture Provisions and Other Covenants Ameren Missouri’s and Ameren Illinois’ indentures and articles of incorporation include covenants and provisions related to issuances of first mortgage bonds and preferred stock. Ameren Missouri and Ameren Illinois are required to meet certain ratios to issue additional first mortgage bonds and preferred stock. A failure to achieve these ratios would not result in a default under these covenants and provisions but would restrict the companies’ ability to issue bonds or preferred stock. The following table summarizes the required and actual interest coverage ratios for interest charges, dividend coverage ratios, and bonds and preferred stock issuable as of December 31, 2020, at an assumed interest rate of 5% and dividend rate of 6%. Required Interest Coverage Ratio (a) Actual Interest Bonds Issuable (b) Required Dividend Coverage Ratio (c) Actual Dividend Preferred Stock Ameren Missouri > 2.0 3.4 $5,025 > 2.5 128.5 $2,872 Ameren Illinois > 2.0 7.1 7,229 > 1.5 3.3 203 (d) (a) Coverage required on the annual interest charges on first mortgage bonds outstanding and to be issued. Coverage is not required in certain cases when additional first mortgage bonds are issued on the basis of retired bonds. (b) Amount of bonds issuable based either on required coverage ratios or unfunded property additions, whichever is more restrictive. The amounts shown also include bonds issuable based on retired bond capacity of $2,442 million and $643 million at Ameren Missouri and Ameren Illinois, respectively. (c) Coverage required on the annual dividend on preferred stock outstanding and to be issued, as required in the respective company’s articles of incorporation. (d) Preferred stock issuable is restricted by the amount of preferred stock that is currently authorized by Ameren Illinois’ articles of incorporation. Ameren’s indenture does not require Ameren to comply with any quantitative financial covenants. The indenture does, however, include certain cross-default provisions. Specifically, either (1) the failure by Ameren to pay when due and upon expiration of any applicable grace period any portion of any Ameren indebtedness in excess of $25 million, or (2) the acceleration upon default of the maturity of any Ameren indebtedness in excess of $25 million under any indebtedness agreement, including borrowings under the Credit Agreements or the Ameren commercial paper program, constitutes a default under the indenture, unless such past due or accelerated debt is discharged or the acceleration is rescinded or annulled within a specified period. Ameren Missouri and Ameren Illinois and certain other nonregistrant Ameren subsidiaries are subject to Section 305(a) of the Federal Power Act, which makes it unlawful for any officer or director of a public utility, as defined in the Federal Power Act, to participate in the making or paying of any dividend from any funds “properly included in capital account.” The FERC has consistently interpreted the provision to allow dividends to be paid as long as (1) the source of the dividends is clearly disclosed, (2) the dividends are not excessive, and (3) there is no self-dealing on the part of corporate officials. At a minimum, Ameren believes that dividends can be paid by its subsidiaries that are public utilities from net income and retained earnings. In addition, under Illinois law, Ameren Illinois and ATXI may not pay any dividend on their respective stock unless, among other things, their respective earnings and earned surplus are sufficient to declare and pay a dividend after provisions are made for reasonable and proper reserves, or unless Ameren Illinois or ATXI has specific authorization from the ICC. Ameren Illinois’ articles of incorporation require dividend payments on its common stock to be based on ratios of common stock to total capitalization and other provisions related to certain operating expenses and accumulations of earned surplus. Ameren Illinois has made a commitment to the FERC to maintain a minimum 30% ratio of common stock equity to total capitalization. As of December 31, 2020, using the FERC-agreed upon calculation method, Ameren Illinois’ ratio of common stock equity to total capitalization was 54%. ATXI’s note purchase agreement includes financial covenants that require ATXI not to permit at any time (1) debt to exceed 70% of total capitalization or (2) secured debt to exceed 10% of total assets. At December 31, 2020, 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. In order for the Ameren Companies to issue securities in the future, they will have to comply with all applicable requirements in effect at the time of any such issuances. Off-Balance-Sheet Arrangements At December 31, 2020, none of the Ameren Companies had any significant off-balance-sheet financing arrangements, other than the forward sale agreement relating to common stock, which was fully settled by mid-February 2021, and variable interest entities. See Note 1 – Summary of Significant Accounting Policies for further detail concerning variable interest entities.

Other Income, Net

Other Income, Net	12 Months Ended
	Dec. 31, 2020
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 years ended December 31, 2020, 2019, and 2018: 2020 2019 2018 Ameren: Other Income, Net Allowance for equity funds used during construction $ 32 $ 28 $ 36 Interest income on industrial development revenue bonds 25 25 26 Other interest income 4 8 7 Non-service cost components of net periodic benefit income (a) 116 90 70 Miscellaneous income 13 6 8 Donations (25) (b) (12) (33) Miscellaneous expense (14) (15) (12) Total Other Income, Net $ 151 $ 130 $ 102 Ameren Missouri: Other Income, Net Allowance for equity funds used during construction $ 19 $ 19 $ 27 Interest income on industrial development revenue bonds 25 25 26 Other interest income 1 1 2 Non-service cost components of net periodic benefit income (a) 46 18 17 Miscellaneous income 4 5 4 Donations (12) (b) (3) (14) Miscellaneous expense (7) (7) (6) Total Other Income, Net $ 76 $ 58 $ 56 Ameren Illinois: Other Income, Net Allowance for equity funds used during construction $ 13 $ 9 $ 9 Interest income 3 6 6 Non-service cost components of net periodic benefit income (a) 48 47 34 Miscellaneous income 6 3 3 Donations (5) (5) (6) Miscellaneous expense (6) (7) (4) Total Other Income, Net $ 59 $ 53 $ 42 (a) For the years ended December 31, 2020, 2019, and 2018, the non-service cost components of net periodic benefit income were adjusted by amounts deferred of $(4) million, $29 million, and $17 million, respectively, due to a tracker for the difference between the level of such costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. (b) Includes $8 million pursuant to Ameren Missouri’s March 2020 electric rate order. See Note 2 – Rate and Regulatory Matters for additional information.

Derivative Financial Instrument

Derivative Financial Instruments	12 Months Ended
	Dec. 31, 2020
Derivative Instrument Detail [Abstract]
DERIVATIVE FINANCIAL INSTRUMENTS	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. See Note 8 – Fair Value Measurements for discussion of our methods of assessing the fair value of derivative instruments. Many of our physical contracts, such as our purchased power contracts, qualify for the NPNS exception to derivative accounting rules. The revenue or expense on NPNS contracts is recognized at the contract price upon physical delivery. 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 December 31, 2020 and 2019, 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 December 31, 2020 and 2019. As of December 31, 2020, these contracts extended through October 2023, March 2026, May 2032, and March 2023 for fuel oils, natural gas, power, and uranium, respectively. Quantity (in millions, except as indicated) 2020 2019 Commodity Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils (in gallons) 43 — 43 58 — 58 Natural gas (in mmbtu) 33 114 147 20 136 156 Power (in MWhs) 6 7 13 5 7 12 Uranium (pounds in thousands) 365 — 365 565 — 565 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 December 31, 2020 and 2019: 2020 2019 Commodity Balance Sheet Location Ameren Ameren Ameren Ameren Ameren Ameren Fuel oils Other current assets $ 2 $ — $ 2 $ 4 $ — $ 4 Other assets — — — 2 — 2 Natural gas Other current assets 1 8 9 — 3 3 Other assets 2 2 4 — 1 1 Power Other current assets 7 — 7 14 — 14 Other assets — — — 2 — 2 Total assets $ 12 $ 10 $ 22 $ 22 $ 4 $ 26 Fuel oils Other current liabilities $ 7 $ — $ 7 $ 4 $ — $ 4 Other deferred credits and liabilities 2 — 2 3 — 3 Natural gas Other current liabilities 1 1 2 1 12 13 Other deferred credits and liabilities — 1 1 1 6 7 Power Other current liabilities 3 17 20 2 17 19 Other deferred credits and liabilities 8 181 189 1 207 208 Uranium Other deferred credits and liabilities — — — 1 — 1 Total liabilities $ 21 $ 200 $ 221 $ 13 $ 242 $ 255 The Ameren Companies elect to present the fair value amounts of derivative assets and derivative liabilities subject to an enforceable master netting arrangement or similar agreement at the gross amounts on the balance sheet. However, if the gross amounts recognized on the balance sheet were netted with derivative instruments and cash collateral received or posted, the net amounts would not be materially different from the gross amounts at December 31, 2020 and 2019. 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 December 31, 2020, if counterparty groups were to fail completely to perform on contracts, the Ameren Companies’ maximum exposure related to derivative assets would have been immaterial with or without consideration of the application of master netting arrangements or similar agreements and collateral held. Certain of our derivative instruments contain collateral provisions tied to the Ameren Companies’ credit ratings. If our credit ratings were downgraded below investment grade, or if a counterparty with reasonable grounds for uncertainty regarding our ability to satisfy an obligation requested adequate assurance of performance, additional collateral postings might be required. The additional collateral required is the net liability position allowed under the master netting arrangements or similar agreements, assuming (1) the credit risk-related contingent features underlying these arrangements were triggered and (2) those counterparties with rights to do so requested collateral. As of December 31, 2020, the aggregate fair value of derivative instruments with credit risk-related contingent features in a gross liability position, the cash collateral posted, and the aggregate amount of additional collateral that counterparties could require were each immaterial to Ameren, Ameren Missouri, and Ameren Illinois.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	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. We use various methods to determine fair value, including market, income, and cost approaches. With these approaches, we adopt certain assumptions that market participants would use in pricing the asset or liability, including assumptions about market risk or the risks inherent in the inputs to the valuation. Inputs to valuation can be readily observable, market-corroborated, or unobservable. We use valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs. Authoritative accounting guidance established a fair value hierarchy that prioritizes the inputs used to measure fair value. All financial assets and liabilities carried at fair value are classified and disclosed in one of the following three hierarchy levels: Level 1 (quoted prices in active markets for identical assets or liabilities): Inputs based on quoted prices in active markets for identical assets or liabilities. Level 1 assets and liabilities are primarily exchange-traded derivatives, cash and cash equivalents, and listed equity securities. The market approach is used to measure the fair value of equity securities held in Ameren Missouri’s nuclear decommissioning trust fund. Equity securities in this fund are representative of the S&P 500 index, excluding securities of Ameren Corporation, owners and/or operators of nuclear power plants, and the trustee and investment managers. The S&P 500 index comprises stocks of large-capitalization companies. Level 2 (significant other observable inputs) : Market-based inputs corroborated by third-party brokers or exchanges based on transacted market data. Level 2 assets and liabilities include certain assets held in Ameren Missouri’s nuclear decommissioning trust fund, including United States Treasury and agency securities, corporate bonds and other fixed-income securities, and certain over-the-counter derivative instruments, including natural gas and financial power transactions. Fixed income securities are valued by using prices from independent industry-recognized data vendors who provide values that are either exchange-based or matrix-based. The fair value measurements of fixed-income securities classified as Level 2 are based on inputs other than quoted prices that are observable for the asset or liability. Examples are matrix pricing, market corroborated pricing, and inputs such as yield curves and indices. Derivative instruments classified as Level 2 are valued by corroborated observable inputs, such as pricing services or prices from similar instruments that trade in liquid markets. Our development and corroboration process entails obtaining multiple quotes or prices from outside sources. To derive our forward view to price our derivative instruments at fair value, we average the bid/ask spreads to the midpoints. To validate forward prices obtained from outside parties, we compare the pricing to recently settled market transactions. Additionally, a review of all sources is performed to identify any anomalies or potential errors. Further, we consider the volume of transactions on certain trading platforms in our reasonableness assessment of the averaged midpoints. The value of natural gas derivative contracts is based upon exchange closing prices without significant unobservable adjustments. The value of power derivative contracts is based upon exchange closing prices or the use of multiple forward prices provided by third parties. The prices are averaged and shaped to a monthly profile when needed without significant unobservable adjustments. Level 3 (significant other unobservable inputs): Unobservable inputs that are not corroborated by market data. Level 3 assets and liabilities are valued by internally developed models and assumptions or methodologies that use significant unobservable inputs. Level 3 assets and liabilities include derivative instruments that trade in less liquid markets, where pricing is largely unobservable. We value Level 3 instruments by using pricing models with inputs that are often unobservable in the market, such as certain internal assumptions, quotes or prices from outside sources not supported by a liquid market, or trend rates. We perform an analysis each quarter to determine the appropriate hierarchy level of the assets and liabilities subject to fair value measurements. Financial assets and liabilities are classified in their entirety according to the lowest level of input that is significant to the fair value measurement. All assets and liabilities whose fair value measurement is based on significant unobservable inputs are classified as Level 3. 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 2020, 2019, or 2018. At December 31, 2020 and 2019, 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 December 31, 2020 and 2019: December 31, 2020 December 31, 2019 Level 1 Level 2 Level 3 Total Level 1 Level 2 Level 3 Total Assets: Ameren Missouri Derivative assets – commodity contracts: Fuel oils $ — $ — $ 2 $ 2 $ — $ — $ 6 $ 6 Natural gas — 3 — 3 — — — — Power 2 — 5 7 — 2 14 16 Total derivative assets – commodity contracts $ 2 $ 3 $ 7 $ 12 $ — $ 2 $ 20 $ 22 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 680 $ — $ — $ 680 $ 569 $ — $ — $ 569 Debt securities: U.S. Treasury and agency securities — 115 — 115 — 107 — 107 Corporate bonds — 115 — 115 — 93 — 93 Other — 67 — 67 — 73 — 73 Total nuclear decommissioning trust fund $ 680 $ 297 $ — $ 977 (a) $ 569 $ 273 $ — $ 842 (a) Total Ameren Missouri $ 682 $ 300 $ 7 $ 989 $ 569 $ 275 $ 20 $ 864 Ameren Illinois Derivative assets – commodity contracts: Natural gas $ — $ 6 $ 4 $ 10 $ — $ 1 $ 3 $ 4 Ameren Derivative assets – commodity contracts (b) $ 2 $ 9 $ 11 $ 22 $ — $ 3 $ 23 $ 26 Nuclear decommissioning trust fund (c) 680 297 — 977 (a) 569 273 — 842 (a) Total Ameren $ 682 $ 306 $ 11 $ 999 $ 569 $ 276 $ 23 $ 868 Liabilities: Ameren Missouri Derivative liabilities – commodity contracts: Fuel oils $ 6 $ — $ 3 $ 9 $ 1 $ — $ 6 $ 7 Natural gas — 1 — 1 — 2 — 2 Power 8 — 3 11 — 2 1 3 Uranium — — — — — — 1 1 Total Ameren Missouri $ 14 $ 1 $ 6 $ 21 $ 1 $ 4 $ 8 $ 13 Ameren Illinois Derivative liabilities – commodity contracts: Natural gas $ — $ 1 $ 1 $ 2 $ 3 $ 12 $ 3 $ 18 Power — — 198 198 — — 224 224 Total Ameren Illinois $ — $ 1 $ 199 $ 200 $ 3 $ 12 $ 227 $ 242 Ameren Derivative liabilities – commodity contracts (b) $ 14 $ 2 $ 205 $ 221 $ 4 $ 16 $ 235 $ 255 (a) Balance excludes $5 million and $5 million of cash and cash equivalents, receivables, payables, and accrued income, net for December 31, 2020 and 2019, 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 Ameren’s nuclear decommissioning trust fund by investment type. See Note 10 – Retirement Benefits for tables that set forth, by level within the fair value hierarchy, Ameren’s pension and postretirement plan assets as of December 31, 2020 and 2019. 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 years ended December 31, 2020 and 2019: 2020 2019 Ameren Ameren Ameren Ameren Ameren Ameren Beginning balance at January 1 $ 13 $ (224) $ (211) $ — $ (183) $ (183) Realized and unrealized gains (losses) included in regulatory assets/liabilities 15 8 23 23 (56) (33) Settlements (26) 18 (8) (7) 15 8 Transfers out of Level 3 — — — (3) — (3) Ending balance at December 31 $ 2 $ (198) $ (196) $ 13 $ (224) $ (211) Change in unrealized gains (losses) related to assets/liabilities held at December 31 $ 1 $ 9 $ 10 $ 12 $ (54) $ (42) 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 December 31, 2020 and 2019: Fair Value Weighted Average (b) Commodity Assets Liabilities Valuation Technique(s) Unobservable Input (a) Range 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 2019 Power (d) $ 14 $ (225) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) 22 – 34 25 Nodal basis ($/MWh) (6) – 0 (2) Trend rate (%) (1) – 0 0 (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. (d) Valuations through 2028 use visible forward prices adjusted for nodal-to-hub basis differentials. Valuations beyond 2028 use a trend rate factor and are similarly adjusted for nodal-to-hub basis differentials. The following table sets forth, by level within the fair value hierarchy, the carrying amount and fair value of financial assets and liabilities disclosed, but not carried, at fair value as of December 31, 2020 and 2019: Carrying Fair Value Level 1 Level 2 Level 3 Total Ameren: December 31, 2020 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 December 31, 2019 Ameren: Cash, cash equivalents, and restricted cash $ 176 $ 176 $ — $ — $ 176 Investments in industrial development revenue bonds (a) 263 — 263 — 263 Short-term debt 440 — 440 — 440 Long-term debt (including current portion) (a) 9,357 (b) — 9,957 484 (c) 10,441 Ameren Missouri: Cash, cash equivalents, and restricted cash $ 39 $ 39 $ — $ — $ 39 Investments in industrial development revenue bonds (a) 263 — 263 — 263 Short-term debt 234 — 234 — 234 Long-term debt (including current portion) (a) 4,190 (b) — 4,772 — 4,772 Ameren Illinois: Cash, cash equivalents, and restricted cash $ 125 $ 125 $ — $ — $ 125 Short-term debt 53 — 53 — 53 Long-term debt (including current portion) 3,575 (b) — 4,019 — 4,019 (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 December 31, 2020 and 2019, 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 $84 million, $36 million, and $36 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2020. Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $72 million, $30 million, and $34 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2019. (c) The Level 3 fair value amount consists of ATXI’s senior unsecured notes.

Callaway Energy Center

Callaway Energy Center	12 Months Ended
	Dec. 31, 2020
Nuclear Waste Matters [Abstract]
CALLAWAY ENERGY CENTER	CALLAWAY ENERGY CENTER 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. A thorough investigation of this matter was conducted. Work has begun to replace certain key components of the generator in order to return the energy center to service. Ameren Missouri expects generator repairs of $65 million, which are expected to be largely capital expenditures. Due to the long lead time for the manufacture, repair, and installation of the components, the energy center is expected to return to service in late June or early July 2021. Ameren Missouri will evaluate all of its options for losses, including the applicability of insurance coverages. See below for additional information about Ameren Missouri’s insurance coverage for the Callaway Energy Center. Spent Nuclear Fuel Under the Nuclear Waste Policy Act of 1982, as amended, the DOE is responsible for disposing of spent nuclear fuel from the Callaway Energy Center and other commercial nuclear energy centers. As required by the act, Ameren Missouri and other utilities have entered into standard contracts with the DOE, which stated that the DOE would begin to dispose of spent nuclear fuel by 1998. However, the DOE failed to fulfill its disposal obligations, and Ameren Missouri and other nuclear energy center owners sued the DOE to recover costs incurred for ongoing storage of their spent fuel. Ameren Missouri’s lawsuit against the DOE resulted in a settlement agreement that provides for annual reimbursement of additional spent fuel storage and related costs. Ameren Missouri received immaterial reimbursements from the DOE in the years ended December 31, 2020, 2019, and 2018. Ameren Missouri will continue to apply for reimbursement from the DOE for allowable costs associated with the ongoing storage of spent fuel. The DOE’s delay in carrying out its obligation to dispose of spent nuclear fuel from the Callaway Energy Center is not expected to adversely affect the continued operations of the energy center. Decommissioning Electric rates charged to customers provide for the recovery of the Callaway Energy Center’s decommissioning costs, which include decontamination, dismantling, and site restoration costs, over the expected life of the nuclear energy center. Amounts collected from customers are deposited into the external nuclear decommissioning trust fund to provide for the Callaway Energy Center’s decommissioning. It is assumed that the Callaway Energy Center site will be decommissioned after its retirement through the immediate dismantlement method and removed from service. The Callaway Energy Center’s operating license expires in 2044. 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. Ameren Missouri’s filing supported no change in electric service rates for decommissioning costs. There is no deadline by which the MoPSC must issue an order regarding the filing. Ameren and Ameren Missouri have classified the investments in debt and equity securities that are held in the nuclear decommissioning trust fund as available for sale, and have recorded all such investments at their fair market value at December 31, 2020 and 2019. Investments in the nuclear decommissioning trust fund have a target allocation of 60% to 70% in equity securities, with the balance invested in debt securities. 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 regulatory liability related to AROs. This reporting is consistent with the method used to account for the decommissioning costs recovered in rates. See Note 2 – Rate and Regulatory Matters for the regulatory liability recorded at December 31, 2020. If the assumed return on trust assets is not earned, Ameren Missouri believes that it is probable that any additional funding requirements resulting from such earnings deficiency will be recovered in customer rates. The following table presents proceeds from the sale and maturities of investments in Ameren Missouri’s nuclear decommissioning trust fund and the gross realized gains and losses resulting from those sales for the years ended December 31, 2020, 2019, and 2018: 2020 2019 2018 Proceeds from sales and maturities $ 183 $ 260 $ 299 Gross realized gains 10 10 18 Gross realized losses 3 2 5 The following table presents the cost and fair value of investments in debt and equity securities in Ameren’s and Ameren Missouri’s nuclear decommissioning trust fund at December 31, 2020 and December 31, 2019: Security Type Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value 2020 Debt securities $ 272 $ 25 $ — $ 297 Equity securities 198 491 9 680 Cash and cash equivalents 4 — — 4 Other (a) 1 — — 1 Total $ 475 $ 516 $ 9 $ 982 2019 Debt securities $ 262 $ 11 $ — $ 273 Equity securities 183 393 7 569 Cash and cash equivalents 26 — — 26 Other (a) (21) — — (21) Total $ 450 $ 404 $ 7 $ 847 (a) Represents net receivables and payables relating to pending securities sales, interest, and securities purchases. The following table presents the costs and fair values of investments in debt securities in Ameren’s and Ameren Missouri’s nuclear decommissioning trust fund according to their contractual maturities at December 31, 2020: Cost Fair Value Less than 5 years $ 120 $ 123 5 years to 10 years 69 75 Due after 10 years 83 99 Total $ 272 $ 297 Insurance The following table presents insurance coverage at Ameren Missouri’s Callaway Energy Center at December 31, 2020: Type and Source of Coverage Most Recent Maximum Coverages Maximum Assessments Public liability and nuclear worker liability: American Nuclear Insurers January 1, 2021 $ 450 $ — Pool participation (a) 13,348 (a) 138 (b) $ 13,798 (c) $ 138 Property damage: NEIL and EMANI April 1, 2020 $ 3,200 (d) $ 25 (e) Replacement power: NEIL April 1, 2020 $ 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) Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity up to $4.5 million for 52 weeks, which commences after the first 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.

Retirement Benefits

Retirement Benefits	12 Months Ended
	Dec. 31, 2020
Retirement Benefits [Abstract]
RETIREMENT BENEFITS	RETIREMENT BENEFITSThe primary objective of the Ameren pension and postretirement benefit plans is to provide eligible employees with pension and postretirement health care and life insurance benefits. Ameren has defined benefit pension plans covering substantially all of its employees and has a postretirement benefit plan covering non-union employees hired before October 2015 and union employees hired before January 2020. Ameren Missouri and Ameren Illinois each participate in Ameren’s single-employer pension and other postretirement plans. All non-union employees participate in a cash balance pension plan. Ameren Missouri union employees hired after July 1, 2013, and Ameren Illinois union employees hired after October 15, 2012, participate in a cash balance pension plan. Ameren uses a measurement date of December 31 for its pension and postretirement benefit plans. Ameren’s qualified pension plan is the Ameren Retirement Plan. Ameren’s other postretirement plan is the Ameren Retiree Welfare Benefit Plan. Ameren also has an unfunded nonqualified pension plan, the Ameren Supplemental Retirement Plan, which is available to provide certain management employees and retirees with a supplemental benefit when their qualified pension plan benefits are capped in compliance with Internal Revenue Code limitations. Only Ameren subsidiaries participate in the plans listed above. Ameren’s pension and other postretirement benefit plans were overfunded by $249 million in the aggregate as of December 31, 2020. Ameren’s total unfunded obligation under its pension and other postretirement benefit plans was $216 million as of December 31, 2019. These net assets and liabilities are recorded in “Other assets,” “Other current liabilities,” and “Pension and other postretirement benefits” on Ameren’s consolidated balance sheet. The increase in the overfunded pension and postretirement benefit plans during 2020 was primarily the result of an increase in the return on plan assets of the pension and postretirement trusts offset by a 75 basis point decrease in the pension and other postretirement benefit plan discount rates used to determine the present value of the obligation. The overfunded pension and other postretirement benefit plans also resulted in regulatory liabilities on Ameren’s, Ameren Missouri’s, and Ameren Illinois’ balance sheets. The following table presents the net benefit liability/(asset) recorded on the balance sheets as of December 31, 2020 and December 31, 2019: 2020 2019 Ameren (a) $ (249) $ 216 Ameren Missouri (a) (25) 142 Ameren Illinois (a) (210) (16) (a) Assets associated with pension and other postretirement benefits are recorded in “Other assets” on the balance sheet. Ameren recognizes the overfunded and underfunded status of its pension and postretirement plans as an asset or a liability on its consolidated balance sheet, with offsetting entries to accumulated OCI and regulatory assets or liabilities. The following table presents the funded status of Ameren’s pension and postretirement benefit plans as of December 31, 2020 and December 31, 2019. It also provides the amounts included in regulatory assets or liabilities and accumulated OCI at December 31, 2020 and December 31, 2019, that have not been recognized in net periodic benefit costs. 2020 2019 Pension Postretirement Pension Postretirement Accumulated benefit obligation at end of year $ 5,213 $ (a) $ 4,735 $ (a) Change in benefit obligation: Net benefit obligation at beginning of year $ 4,967 $ 1,110 $ 4,459 $ 1,034 Service cost 110 19 88 18 Interest cost 174 39 187 43 Plan amendments — — — 2 Participant contributions — 8 — 8 Actuarial loss 508 91 469 69 Benefits paid (249) (63) (236) (64) Net benefit obligation at end of year 5,510 1,204 4,967 1,110 Change in plan assets: Fair value of plan assets at beginning of year 4,564 1,297 3,899 1,113 Actual return on plan assets 1,143 209 878 237 Employer contributions 52 2 23 3 Participant contributions — 8 — 8 Benefits paid (249) (63) (236) (64) Fair value of plan assets at end of year 5,510 1,453 4,564 1,297 Funded status – deficiency (surplus) — (249) 403 (187) Accrued benefit cost (asset) at December 31 $ — $ (249) $ 403 $ (187) Amounts recognized in the balance sheet consist of: Noncurrent asset (b) $ (39) $ (249) $ — $ (187) Current liability (c) 2 — 2 — Noncurrent liability 37 — 401 — Net liability (asset) recognized $ — $ (249) $ 403 $ (187) Amounts recognized in regulatory assets or liabilities consist of: Net actuarial (gain) loss $ (138) $ (200) $ 244 $ (170) Prior service credit — (37) — (41) Amounts recognized in accumulated OCI (pretax) consist of: Net actuarial loss 5 6 26 4 Total $ (133) $ (231) $ 270 $ (207) (a) Not applicable. (b) Included in “Other assets” on Ameren’s consolidated balance sheet. (c) Included in “Other current liabilities” on Ameren’s consolidated balance sheet. The following table presents the assumptions used to determine our benefit obligations at December 31, 2020 and 2019: Pension Benefits Postretirement Benefits 2020 2019 2020 2019 Discount rate at measurement date 2.75 % 3.50 % 2.75 % 3.50 % Increase in future compensation 3.50 3.50 3.50 3.50 Cash balance pension plan interest crediting rate 5.00 5.00 (a) (a) Medical cost trend rate (initial) (b) (a) (a) 5.00 5.00 Medical cost trend rate (ultimate) (b) (a) (a) 5.00 5.00 (a) Not applicable. (b) Initial and ultimate medical cost trend rate for certain Medicare-eligible participants is 3.00%. Ameren determines discount rate assumptions by identifying a theoretical settlement portfolio of high-quality corporate bonds sufficient to provide for a plan’s projected benefit payments. The settlement portfolio of bonds is selected from a pool of more than 830 high-quality corporate bonds. A single discount rate is then determined; that rate results in a discounted value of the plan’s benefit payments that equates to the market value of the selected bonds. During 2020, Ameren adopted the updated Society of Actuaries mortality table and adopted the Society of Actuaries 2020 Mortality Improvement Scale. The updated mortality table reflects lower life expectancy in aggregate compared with the 2019 Society of Actuaries mortality table. The updated improvement scale assumes a lower rate of mortality improvement, compared with the 2019 Mortality Improvement Scale. The impact of the adoption of the table and the scale results in a decrease to our pension and other postretirement benefit obligations. Funding Pension benefits are based on the employees’ years of service, age, and compensation. Ameren’s pension plans are funded in compliance with income tax regulations, federal funding, and other regulatory requirements. As a result, Ameren expects to fund its pension plan at a level equal to the greater of the pension cost or the legally required minimum contribution. Based on its assumptions at December 31, 2020, its investment performance in 2020, and its pension funding policy, Ameren expects to make aggregate contributions of $60 million over the next five years. Ameren Missouri and Ameren Illinois estimate that their portion of the future funding requirements will be 30% and 60%, respectively. These estimates may change based on actual investment performance, changes in interest rates, changes in our assumptions, changes in government regulations, and any voluntary contributions. Our funding policy for postretirement benefits is primarily to fund the Voluntary Employee Beneficiary Association (VEBA) trusts to match the annual postretirement expense. The following table presents the cash contributions made to our defined benefit retirement plans and to our postretirement plan during 2020, 2019, and 2018: Pension Benefits Postretirement Benefits 2020 2019 2018 2020 2019 2018 Ameren Missouri $ 17 $ 3 $ 18 $ 1 $ 1 $ 1 Ameren Illinois 27 19 35 1 1 1 Other 8 1 7 — 1 — Ameren $ 52 $ 23 $ 60 $ 2 $ 3 $ 2 Investment Strategy and Policies Ameren manages plan assets in accordance with the “prudent investor” guidelines contained in ERISA. The investment committee, which includes members of senior management, approves and implements investment strategy and asset allocation guidelines for the plan assets. The investment committee’s goals are twofold: first, to ensure that sufficient funds are available to provide the benefits at the time they are payable; and second, to maximize total return on plan assets and to minimize expense volatility consistent with its tolerance for risk. Ameren delegates the task of investment management to specialists in each asset class. As appropriate, Ameren provides each investment manager with guidelines that specify allowable and prohibited investment types. The investment committee regularly monitors manager performance and compliance with investment guidelines. The expected return on plan assets assumption is based on historical and projected rates of return for current and planned asset classes in the investment portfolio. Projected rates of return for each asset class were estimated after an analysis of historical experience, future expectations, and the volatility of the various asset classes. After considering the target asset allocation for each asset class, we adjusted the overall expected rate of return for the portfolio for historical and expected experience of active portfolio management results compared with benchmark returns and for the effect of expenses paid from plan assets. Ameren will use an expected return on plan assets for its pension and postretirement plan assets of 6.50% in 2021. Ameren’s investment committee strives to assemble a portfolio of diversified assets that does not create a significant concentration of risks. The investment committee develops asset allocation guidelines between asset classes, and it creates diversification through investments in assets that differ by type (equity, debt, real estate), duration, market capitalization, country, style (growth or value), and industry, among other factors. The diversification of assets is displayed in the target allocation table below. The investment committee also routinely rebalances the plan assets to adhere to the diversification goals. The investment committee’s strategy reduces the concentration of investment risk; however, Ameren is still subject to overall market risk. Effective January 2020, Ameren’s investment committee developed and implemented a liability hedging investment strategy for its qualified pension plans designed to reduce interest rate risk as part of an objective for its long-term investment strategy. The plan invests in derivative instruments mainly consisting of interest rate futures intended to extend the duration of the pension plan assets so that the assets are more closely aligned with the duration of the liabilities. In addition, part of Ameren’s investment strategy includes participation in a securities lending program, which allows it to lend eligible securities to third party borrowers. All loans are collateralized by at least 102% of the loaned asset’s market value and the collateral is invested in the form of cash, government obligations, and U.S. agency obligations. Ameren’s fair value of securities loaned was $365 million and $246 million as of December 31, 2020, and 2019, respectively. Cash and securities obtained as collateral exceeded the fair value of the securities loaned as of December 31, 2020 and 2019. The following table presents our target allocations for 2021 and our pension and postretirement plans’ asset categories as of December 31, 2020 and 2019: Asset Target Allocation Percentage of Plan Assets at December 31, 2020 2019 Pension Plan: Cash and cash equivalents 0% – 5% 1 % 3 % Equity securities: U.S. large-capitalization 21% – 31% 26 % 27 % U.S. small- and mid-capitalization 3% – 13% 9 % 7 % International 9% – 19% 15 % 14 % Global 3% – 13% 9 % 9 % Total equity 51% – 61% 59 % 57 % Debt securities 35% – 45% 36 % 36 % Real estate 0% – 9% 4 % 4 % Private equity 0% – 5% (a) (a) Total 100 % 100 % Postretirement Plans: Cash and cash equivalents 0% – 7% 3 % 1 % Equity securities: U.S. large-capitalization 23% – 33% 31 % 31 % U.S. small- and mid-capitalization 3% – 13% 8 % 9 % International 9% – 19% 15 % 14 % Global 5% – 15% 10 % 11 % Total equity 55% – 65% 64 % 65 % Debt securities 33% – 43% 33 % 34 % Total 100 % 100 % (a) Less than 1% of plan assets. In general, the United States large-capitalization equity investments are passively managed or indexed, whereas the international, global, United States small-capitalization, and United States mid-capitalization equity investments are actively managed by investment managers. Debt securities include a broad range of fixed-income vehicles. Debt security investments in high-yield securities and non-United-States-dollar-denominated securities are owned by the plans, but in limited quantities to reduce risk. Most of the debt security investments are under active management by investment managers. Real estate investments include private real estate vehicles; however, Ameren does not, by policy, hold direct investments in real estate property. In addition to the derivative investments included in the liability hedging investment strategy described above, Ameren’s investment committee also allo ws investment managers to use derivatives, such as index futures, foreign exchange futures, and options, in certain situations to increase or to reduce market exposure in an efficient and timely manner. Fair Value Measurements of Plan Assets Investments in the pension and postretirement benefit plans were stated at fair value as of December 31, 2020. The fair value of an asset is the amount that would be received upon its sale in an orderly transaction between market participants at the measurement date. Cash and cash equivalents have initial maturities of three months or less and are recorded at cost plus accrued interest. Investments traded in active markets on national or international securities exchanges are valued at closing prices on the measurement date or, if that is not a business day, on the last business day before that date. Securities traded in over-the-counter markets are valued by quoted market prices, broker or dealer quotations, or alternative pricing sources with reasonable levels of price transparency. Investments measured under NAV as a practical expedient are based on the fair values of the underlying assets provided by the funds and their administrators. The fair value of real estate investments is based on NAV; it is determined by annual appraisal reports prepared by an independent real estate appraiser. Investments measured at NAV often provide for daily, monthly, or quarterly redemptions with 60 or less days of notice depending on the fund. For some funds, redemption may also require approval from the fund’s board of directors. Derivative contracts are valued at fair value, as determined by the investment managers (or independent third parties on behalf of the investment managers), who use proprietary models and take into consideration exchange quotations on underlying instruments, dealer quotations, and other market information. The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the pension plans’ assets measured at fair value and NAV as of December 31, 2020 and 2019: December 31, 2020 December 31, 2019 Level 1 Level 2 NAV Total Level 1 Level 2 NAV Total Cash and cash equivalents $ — $ — $ 145 $ 145 $ — $ — $ 139 $ 139 Equity securities: U.S. large-capitalization — — 1,511 1,511 — — 1,253 1,253 U.S. small- and mid-capitalization 513 — — 513 344 — — 344 International 375 — 492 867 296 — 363 659 Global — — 546 546 — — 407 407 Debt securities: Corporate bonds — 506 17 523 — 597 13 610 Municipal bonds — 50 — 50 — 75 — 75 U.S. Treasury and agency securities 3 1,325 — 1,328 5 1,010 — 1,015 Other (5) 8 — 3 — 8 — 8 Real estate — — 208 208 — — 211 211 Private equity — — 2 2 — — 2 2 Total $ 886 $ 1,889 $ 2,921 $ 5,696 $ 645 $ 1,690 $ 2,388 $ 4,723 Less: Medical benefit assets (a) (219) (176) Plus: Net receivables (b) 33 17 Fair value of pension plans’ assets $ 5,510 $ 4,564 (a) Medical benefit (health and welfare) component for accounts maintained in accordance with Section 401(h) of the Internal Revenue Code to fund a portion of the postretirement obligation. (b) Receivables related to pending securities sales, offset by payables related to pending securities purchases. The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the postretirement benefit plans’ assets measured at fair value and NAV as of December 31, 2020 and 2019: December 31, 2020 December 31, 2019 Level 1 Level 2 NAV Total Level 1 Level 2 NAV Total Cash and cash equivalents $ 38 $ — $ — $ 38 $ 12 $ — $ — $ 12 Equity securities: U.S. large-capitalization 279 — 107 386 238 — 112 350 U.S. small- and mid-capitalization 104 — — 104 93 — — 93 International 75 — 107 182 59 — 102 161 Global — — 120 120 — — 120 120 Debt securities: Municipal bonds — 106 — 106 — 107 — 107 Other — — 295 295 — — 277 277 Total $ 496 $ 106 $ 629 $ 1,231 $ 402 $ 107 $ 611 $ 1,120 Plus: Medical benefit assets (a) 219 176 Plus: Net receivables (b) 3 1 Fair value of postretirement benefit plans’ assets $ 1,453 $ 1,297 (a) Medical benefit (health and welfare) component for accounts maintained in accordance with Section 401(h) of the Internal Revenue Code to fund a portion of the postretirement obligation. These 401(h) assets are included in the pension plan assets shown above. (b) Receivables related to pending securities sales, offset by payables related to pending securities purchases. Net Periodic Benefit Cost The following table presents the components of the net periodic benefit cost of Ameren’s pension and postretirement benefit plans during 2020, 2019, and 2018: Pension Benefits Postretirement Benefits 2020 2019 2018 2020 2019 2018 Service cost (a) $ 110 $ 88 $ 100 $ 19 $ 18 $ 21 Non-service cost components: Interest cost 174 187 169 39 43 40 Expected return on plan assets (291) (276) (276) (80) (77) (77) Amortization of: Prior service credit (1) (1) (1) (4) (5) (4) Actuarial (gain) loss 60 25 68 (9) (15) (6) Total non-service cost components (b) $ (58) $ (65) $ (40) $ (54) $ (54) $ (47) Net periodic benefit cost (income) $ 52 $ 23 $ 60 $ (35) $ (36) $ (26) (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 consolidated statement of income. See Note 6 – Other Income, Net for additional information. The Ameren Companies are responsible for their share of the pension and postretirement benefit costs. The following table presents the pension costs and the postretirement benefit costs incurred for the years ended December 31, 2020, 2019, and 2018: Pension Costs Postretirement Costs 2020 2019 2018 2020 2019 2018 Ameren Missouri (a) $ 22 $ 5 $ 22 $ (5) $ (6) $ (1) Ameren Illinois 32 20 39 (31) (30) (25) Other (2) (2) (1) 1 — — Ameren $ 52 $ 23 $ 60 $ (35) $ (36) $ (26) (a) Does not include the impact of the tracker for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri and the level of such costs included in customer rates. The expected pension and postretirement benefit payments from qualified trust and company funds, which reflect expected future service, as of December 31, 2020, are as follows: Pension Benefits Postretirement Benefits Paid from Paid from Paid from Paid from 2021 $ 260 $ 2 $ 58 $ 2 2022 267 2 60 2 2023 274 3 61 2 2024 278 3 62 2 2025 283 3 61 2 2026 – 2030 1,454 12 303 11 The following table presents the assumptions used to determine net periodic benefit cost for our pension and postretirement benefit plans for the years ended December 31, 2020, 2019, and 2018: Pension Benefits Postretirement Benefits 2020 2019 2018 2020 2019 2018 Discount rate at measurement date 3.50 % 4.25 % 3.50 % 3.50 % 4.25 % 3.50 % Expected return on plan assets 7.00 7.00 7.00 7.00 7.00 7.00 Increase in future compensation 3.50 3.50 3.50 3.50 3.50 3.50 Cash balance pension plan interest crediting rate 5.00 5.00 5.00 (a) (a) (a) Medical cost trend rate (initial) (b) (a) (a) (a) 5.00 5.00 5.00 Medical cost trend rate (ultimate) (b) (a) (a) (a) 5.00 5.00 5.00 (a) Not applicable. (b) Initial and ultimate medical cost trend rate for certain Medicare-eligible participants is 3.00%. The table below reflects the sensitivity of Ameren’s plans to potential changes in key assumptions for the year ended December 31, 2020: Pension Benefits Postretirement Benefits Service Cost Expected Projected Service Cost Expected Postretirement 0.25% decrease in discount rate $ (1) $ — $ 195 $ — $ — $ 42 0.25% decrease in return on assets — 10 — — 3 — 0.25% increase in future compensation 2 — 20 — — — Other Ameren sponsors a 401(k) plan for eligible employees. The Ameren 401(k) plan covered all eligible Ameren employees at December 31, 2020. The plan allows employees to contribute a portion of their compensation in accordance with specific guidelines. Ameren matches a percentage of the employee contributions up to certain limits. The following table presents the portion of the matching contribution to the Ameren 401(k) plan attributable to each of the Ameren Companies for the years ended December 31, 2020, 2019, and 2018: 2020 2019 2018 Ameren Missouri $ 20 $ 19 $ 17 Ameren Illinois 17 16 15 Other 1 — 1 Ameren $ 38 $ 35 $ 33

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
STOCK-BASED COMPENSATION	STOCK-BASED COMPENSATION The 2014 Omnibus Incentive Compensation Plan is Ameren’s long-term stock-based compensation plan for eligible employees and directors. It provides for a maximum of 8 million common shares to be available for grant to eligible employees and directors. At December 31, 2020, there were 2.3 million common shares remaining for grant. Awards may be stock options, stock appreciation rights, restricted stock, restricted stock units, performance shares, performance share units, cash-based awards, and other stock-based awards. Ameren used newly issued shares to fulfill its stock-based compensation obligations for 2020, 2019, and 2018, and intends to use newly issued shares to fulfill its stock-based compensation obligations for 2021. The following table summarizes Ameren’s nonvested performance share unit and restricted stock unit activity for the year ended December 31, 2020: Performance Share Units – Market Condition (a) Performance Share Units – Performance Condition (b) Restricted Stock Units Share Weighted-average Fair Value per Share Unit Shares Weighted-average Fair Value per Share Unit Stock Weighted-average Fair Value per Stock Unit Nonvested at January 1, 2020 (c) 539,877 $ 63.79 — $ — 220,277 $ 61.13 Granted (d) 267,154 82.49 43,052 76.69 173,571 76.91 Forfeitures (16,303) 73.42 (1,155) 76.80 (7,507) 69.89 Vested and undistributed (e) (178,802) 69.87 (7,607) 76.78 (80,180) 66.72 Vested and distributed (147,787) 59.16 — — (2,466) 54.30 Performance share adjustment — — (2,394) 76.66 — — Nonvested at December 31, 2020 (f) 464,139 $ 73.34 31,896 $ 76.66 303,695 $ 68.52 (a) The exact number of shares issued pursuant to a share unit varies from 0% to 200% of the target award, depending on actual company performance relative to the specified market conditions. Compensation cost on nonforfeited awards is recognized regardless of whether Ameren achieves the specified market conditions. (b) The exact number of shares issued pursuant to a share unit varies from 0% to 200% of the target award, depending on actual company performance relative to the performance goals. Compensation cost is recognized ratably over the requisite service period only for awards for which it is probable that the performance condition will be satisfied. The performance share adjustment represents the change in the probability that a performance condition will be satisfied. (c) Does not include 503,283 performance share units for market performance and 79,854 restricted stock units that were vested and undistributed. (d) In September 2020, certain executive officers were granted an additional 37,104 restricted stock units with a grant date fair value of $3 million, which will vest after three years on September 17, 2023. The awards do not provide for pro rata vesting in connection with the executive officer’s retirement. (e) Vested and undistributed units are awards that vest on a pro-rata basis due to attainment of retirement eligibility by certain employees, but have not yet been distributed. For vested and undistributed performance share units, the number of shares issued for retirement-eligible employees will vary depending on actual performance over the three-year performance period. (f) Does not include 366,243 of performance share units for market performance, 7,607 of performance share units based on the achievement of renewable generation and energy storage installation targets, and 160,034 of restricted stock units that were vested and undistributed. Performance Share Units – Market Condition A market condition performance share unit vests and entitles an employee to receive shares of Ameren common stock (plus accumulated dividends) if, at the end of the three-year performance period, certain specified market conditions have been met and if the individual remains employed by Ameren through the required vesting period. The vesting period for share units awarded extends beyond the three-year performance period to the payout date, which is approximately 38 months after the grant date. In the event of a participant’s death or retirement at age 55 or older with five years or more of service, awards vest on a pro-rata basis over the three-year performance period. The exact number of shares issued pursuant to a share unit varies from 0% to 200% of the target award, depending on actual company performance relative to the specified market conditions. The fair value of each share unit is based on Ameren’s closing common share price at December 31st of the year prior to the award year and a Monte Carlo simulation. The Monte Carlo simulation is used to estimate expected share payout based on Ameren’s TSR for a three-year performance period relative to the designated peer group beginning January 1st of the award year. The simulation can produce a greater fair value for the share unit than the applicable closing common share price because it includes the weighted payout scenarios in which an increase in the share price has occurred. The significant assumptions used to calculate fair value also include a three-year risk-free rate, Ameren’s common stock volatility, volatility for the peer group, and Ameren’s attainment of a three-year average earnings per share threshold during the performance period. The following table presents the fair value of each share unit along with the significant assumptions used to calculate the fair value of each share unit for the years ended December 31, 2020, 2019, and 2018: 2020 2019 2018 Fair value of share units awarded $82.49 $67.42 $62.88 Three-year risk-free rate 1.62% 2.46% 1.98% Ameren’s common stock volatility (a) 15% 17% 17% Volatility range for the peer group (a) 14% – 28% 15% – 25% 15% – 23% (a) Based on a historical period that is equal to the remaining term of the performance period as of the grant date. Performance Share Units – Performance Condition A performance condition share unit vests and entitles an employee to receive shares of Ameren common stock (plus accumulated dividends) if, at the end of the three-year performance period, Ameren has met the specified performance condition and if the individual remains employed by Ameren through the required vesting period. The vesting period for share units awarded extends beyond the three-year performance period to the payout date, which is approximately 38 months after the grant date. In the event of a participant’s death or retirement at age 55 or older with five years or more of service, awards vest on a pro-rata basis over the three-year performance period. The exact number of shares issued pursuant to a share unit varies from 0% to 200% of the target award, depending on actual performance conditions achieved. The specified performance condition is based on Ameren’s clean energy goals, specifically the achievement of renewable generation and energy storage installation targets. The grant-date fair value for an individual outcome of a performance condition is determined by Ameren’s closing common share price on the grant date. Restricted Stock Units Restricted stock units vest and entitle an employee to receive shares of Ameren common stock (plus accumulated dividends) if the individual remains employed with Ameren through the payment date of the awards. Generally, in the event of a participant’s death or retirement at age 55 or older with five years or more of service, awards vest on a pro-rata basis, except for retention RSUs that are periodically granted, which do not provide for pro rata vesting in connection with the employee’s retirement. The payout date of the awards is approximately 38 months after the grant date. The fair value of each restricted stock unit is determined by Ameren’s closing common share price on the grant date. Stock-Based Compensation Expense The following table presents the stock-based compensation expense for the years ended December 31, 2020, 2019, and 2018: 2020 2019 2018 Ameren Missouri $ 5 $ 4 $ 4 Ameren Illinois 3 3 3 Other (a) 13 13 13 Ameren 21 20 20 Less income tax benefit 6 5 6 Stock-based compensation expense, net $ 15 $ 15 $ 14 (a) Represents compensation expense for employees of Ameren Services. These amounts are not included in the Ameren Missouri and Ameren Illinois amounts above. Ameren settled performance share units and restricted stock units of $58 million, $83 million, and $54 million for the years ended December 31, 2020, 2019, and 2018. There were no significant stock-based compensation costs capitalized during the years ended December 31, 2020, 2019, and 2018. As of December 31, 2020, total compensation cost of $34 million related to nonvested awards not yet recognized is expected to be recognized over a weighted-average period of 22 months. For the years ended December 31, 2020, 2019, and 2018, excess tax benefits associated with the settlement of stock-based compensation awards reduced income tax expense by $8 million, $15 million, and $6 million, respectively.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
INCOME TAXES	INCOME TAXES Missouri Income Tax Rate In 2018, legislation modifying Missouri tax law was enacted to decrease the state’s corporate income tax rate from 6.25% to 4%, effective January 1, 2020. As a result, in 2018, Ameren’s and Ameren Missouri’s accumulated deferred tax balances were revalued, resulting in a net decrease of $122 million to their accumulated deferred tax liability, which was offset by a regulatory liability. Additionally, Ameren recorded an immaterial amount to income tax expense. As a result of the March 2020 rate order, the effect of this tax decrease was reflected in customer rates on April 1, 2020. This income tax decrease did not have a material impact on the net income of Ameren (parent) and nonregistrant subsidiaries. The following table presents the principal reasons for the difference between the effective income tax rate and the federal statutory corporate income tax rate for the years ended December 31, 2020, 2019, and 2018: Ameren Missouri Ameren Illinois Ameren 2020 Federal statutory corporate income tax rate 21 % 21 % 21 % Increases (decreases) from: Amortization of excess deferred income taxes (16) (3) (9) Amortization of deferred investment tax credit (1) (1) (1) State tax 3 7 5 Stock-based compensation — — (1) Effective income tax rate 7 % 24 % 15 % 2019 Federal statutory corporate income tax rate 21 % 21 % 21 % Increases (decreases) from: Amortization of excess deferred income taxes (11) (4) (7) Amortization of deferred investment tax credit (1) — (1) State tax 5 7 6 Stock-based compensation — — (1) Effective income tax rate 14 % 24 % 18 % 2018 Federal statutory corporate income tax rate 21 % 21 % 21 % Increases (decreases) from: Amortization of excess deferred income taxes (4) (4) (4) Depreciation differences — (1) — Amortization of deferred investment tax credit (1) — (1) State tax 4 7 6 TCJA 1 1 1 Tax credits (1) — — Other permanent items — — (1) Effective income tax rate 20 % 24 % 22 % The following table presents the components of income tax expense for the years ended December 31, 2020, 2019, and 2018: Ameren Missouri Ameren Illinois Other Ameren 2020 Current taxes: Federal $ 14 $ 12 $ (24) $ 2 State 3 (6) 8 5 Deferred taxes: Federal 82 81 24 187 State 15 52 (10) 57 Amortization of excess deferred income taxes (75) (15) (1) (91) Amortization of deferred investment tax credits (5) — — (5) Total income tax expense $ 34 $ 124 $ (3) $ 155 2019 Current taxes: Federal $ 65 $ 19 $ (88) $ (4) State 22 11 (14) 19 Deferred taxes: Federal 37 66 82 185 State 5 29 25 59 Amortization of excess deferred income taxes (56) (15) (1) (72) Amortization of deferred investment tax credits (5) — — (5) Total income tax expense $ 68 $ 110 $ 4 $ 182 2018 Current taxes: Federal $ 104 $ 4 $ (118) $ (10) State 29 6 (12) 23 Deferred taxes: Federal 22 75 123 220 State (2) 28 23 49 Amortization of excess deferred income taxes (24) (15) (1) (40) Amortization of deferred investment tax credits (5) — — (5) Total income tax expense $ 124 $ 98 $ 15 $ 237 The following table presents the accumulated deferred income tax assets and liabilities recorded as a result of temporary differences and accumulated deferred investment tax credits at December 31, 2020 and 2019: Ameren Missouri Ameren Illinois Other Ameren 2020 Accumulated deferred income taxes, net liability (asset): Plant-related $ 2,112 $ 1,559 $ 205 $ 3,876 Regulatory assets and liabilities, net (285) (207) (23) (515) Deferred employee benefit costs (58) 8 (54) (104) Tax carryforwards (26) (6) (65) (97) Other (35) 13 39 17 Total net accumulated deferred income tax liabilities (assets) 1,708 1,367 102 3,177 Accumulated deferred investment tax credits 34 — — 34 Accumulated deferred income taxes and investment tax credits $ 1,742 $ 1,367 $ 102 $ 3,211 2019 Accumulated deferred income taxes, net liability (asset): Plant-related $ 2,000 $ 1,423 $ 193 $ 3,616 Regulatory assets and liabilities, net (310) (214) (24) (548) Deferred employee benefit costs (59) 7 (59) (111) Tax carryforwards (25) (3) (70) (98) Other (33) 11 43 21 Total net accumulated deferred income tax liabilities (assets) 1,573 1,224 83 2,880 Accumulated deferred investment tax credits 39 — — 39 Accumulated deferred income taxes and investment tax credits $ 1,612 $ 1,224 $ 83 $ 2,919 The following table presents the components of accumulated deferred income tax assets relating to net operating loss carryforwards, tax credit carryforwards, and charitable contribution carryforwards at December 31, 2020 and 2019: Ameren Missouri Ameren Illinois Other Ameren 2020 Net operating loss carryforwards: State (a) $ — $ 3 $ 4 $ 7 Total net operating loss carryforwards $ — $ 3 $ 4 $ 7 Tax credit carryforwards: Federal (b) $ 26 $ 3 $ 54 $ 83 State (c) — — 7 7 Total tax credit carryforwards $ 26 $ 3 $ 61 $ 90 Charitable contribution carryforwards (c) $ — $ — $ 3 $ 3 Valuation allowance (d) — — (3) (3) Total charitable contribution carryforwards $ — $ — $ — $ — 2019 Tax credit carryforwards: Federal $ 25 $ 3 $ 67 $ 95 State — — 3 3 Total tax credit carryforwards $ 25 $ 3 $ 70 $ 98 Charitable contribution carryforwards $ — $ — $ 3 $ 3 Valuation allowance — — (3) (3) Total charitable contribution carryforwards $ — $ — $ — $ — (a) Will expire 2032. (b) Will expire between 2030 and 2040. (c) Will expire between 2021 and 2025. (d) See Schedule II under Part IV, Item 15, in this report for information on changes in the valuation allowance. Uncertain Tax Positions As of December 31, 2020 and 2019, the Ameren Companies did not record any uncertain tax positions. Ameren is a part of the IRS’s compliance assurance process program, which involves real-time review of compliance with federal income tax law. State income tax returns are generally subject to examination for a period of three years after filing. The state impact of any federal changes remains subject to examination by various states for up to one year after formal notification to the states. Ameren’s federal tax returns for the 2015, 2017, 2018, and 2019 tax years are open, but, at the time of this filing, the Ameren Companies do not have material income tax issues under examination, administrative appeals, or litigation. Ameren Missouri has an uncertain tax position tracker. Under Missouri’s regulatory framework, uncertain tax positions do not reduce Ameren Missouri’s electric rate base. When an uncertain income tax position liability is resolved, the MoPSC requires, through the uncertain tax position tracker, the creation of a regulatory asset or regulatory liability to reflect the time value, with a return at the applicable WACC included in each of the electric rate orders in effect before the tax position was resolved, of the difference between the uncertain tax position liability that was excluded from rate base and the final tax liability. The resulting regulatory asset or liability will affect earnings in the year it is created. It will then be amortized over three years, beginning on the effective date of new rates established in the next electric service regulatory rate review.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2020
Related Party Transactions [Abstract]
RELATED PARTY TRANSACTIONS	RELATED-PARTY TRANSACTIONS In the normal course of business, Ameren Missouri and Ameren Illinois 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, except as noted in Software Licensing Agreement discussion below. Below are the material related-party agreements. Electric Power Supply Agreements Ameren Illinois must acquire capacity and energy sufficient to meet its obligations to customers. Ameren Illinois uses periodic RFP processes, administered by the IPA and approved by the ICC, to contract capacity and energy on behalf of its customers. Ameren Missouri participates in the RFP process and has been a winning supplier for certain periods. Capacity Supply Agreements In procurement events in 2020, Ameren Missouri contracted to supply a portion of Ameren Illinois’ capacity requirements for $3 million from June 2021 through May 2023. Energy Product Agreements Based on the outcome of IPA-administered procurement events, Ameren Missouri and Ameren Illinois have entered into energy product agreements by which Ameren Missouri agreed to sell, and Ameren Illinois agreed to purchase, a set amount of MWhs at a predetermined price over a specified period of time. The following table presents the specified performance period, price, and amount of MWhs included in the agreements: IPA Procurement Event Performance Period MWh Average Price per MWh April 2017 March 2019 – May 2020 85,600 $ 34 April 2018 June 2019 – September 2020 110,000 32 April 2019 January 2020 – December 2021 288,000 35 September 2019 April 2020 – November 2021 170,800 29 September 2020 September 2021 – November 2022 204,800 31 Collateral Postings Under the terms of the Illinois energy product agreements entered into through RFP processes administered by the IPA, suppliers must post collateral under certain market conditions to protect Ameren Illinois in the event of nonperformance. The collateral postings are unilateral, which means that only the suppliers can be required to post collateral. Therefore, Ameren Missouri, as a winning supplier in the RFP process, may be required to post collateral. As of December 31, 2020 and 2019, there were no collateral postings required of Ameren Missouri related to the Illinois energy product agreements. Interconnection Agreements Ameren Missouri and Ameren Illinois are parties to an interconnection agreement that governs the connection of their respective transmission lines and other facilities used for the distribution of power. These agreements have no contractual expiration date, but may be terminated by either party with three years’ notice. Ameren Missouri and ATXI are parties to an interconnection agreement that governs the connection of the High Prairie Renewable Energy Center to an ATXI transmission line that allows Ameren Missouri to distribute power generated from the High Prairie Renewable Energy Center. See Note 2 – Rate and Regulatory Matters for further information on the acquisition of the High Prairie Renewable Energy Center. Support Services Agreements Ameren Services provides support services to its affiliates. The costs of support services including wages, employee benefits, professional services, and other expenses, are based on, or are an allocation of, actual costs incurred. The support services agreement can be terminated at any time by the mutual agreement of Ameren Services and that affiliate or by either party with 60 days’ notice before the end of a calendar year. In addition, Ameren Missouri and Ameren Illinois provide affiliates with access to their facilities for administrative purposes and with use of other assets. The costs of the rent and facility services and other assets are based on, or are an allocation of, actual costs incurred. Ameren Missouri and Ameren Illinois also provide storm-related and miscellaneous support services to each other on an as-needed basis. Transmission Services Ameren Illinois receives transmission services from ATXI for its retail load. Electric Transmission Maintenance and Construction Agreements ATXI entered into separate agreements with Ameren Missouri and Ameren Illinois in which Ameren Missouri or Ameren Illinois, as applicable, may perform certain maintenance and construction services related to ATXI’s electric transmission assets. Money Pool See Note 4 – Short-term Debt and Liquidity for a discussion of affiliate borrowing arrangements. Software Licensing Agreement In September 2019, Ameren Missouri purchased a license for advanced metering infrastructure software from Ameren Illinois. The amount of the $24 million cost-based transaction price over the $5 million remaining carrying value of the software was recorded as revenue by Ameren Illinois, with $14 million of revenue recorded at Ameren Illinois Electric Distribution and $5 million recorded at Ameren Illinois Natural Gas. The revenue recorded at Ameren Illinois Electric Distribution was reflected in formula ratemaking, which resulted in no impact to net income. Per authoritative accounting guidance for sales to rate-regulated entities, the revenue recognized by Ameren Illinois was not eliminated upon consolidation by Ameren. Ameren Missouri’s $24 million software investment is included in “Property, Plant, and Equipment, Net.” Tax Allocation Agreement See Note 1 – Summary of Significant Accounting Policies 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 December 31, 2020 and December 31, 2019: 2020 2019 Ameren Missouri Ameren Illinois Ameren Missouri Ameren Illinois Income taxes payable to parent (a) $ — $ 6 $ 15 $ 43 Income taxes receivable from parent (b) 9 15 15 17 (a) Included in “Accounts payable – affiliates” on the balance sheet. (b) Included in “Accounts receivable – affiliates” on the balance sheet. Capital Contributions The following table presents cash capital contributions received from Ameren (parent) by Ameren Missouri and Ameren Illinois for the years ended December 31, 2020, 2019, and 2018: 2020 2019 2018 Ameren Missouri (a) $ 491 $ 124 $ 45 Ameren Illinois 464 (a) 15 (a) 160 (a) Includes capital contributions made as a result of the tax allocation agreement. Effects of Related-party Transactions on the Statement of Income The following table presents the impact on Ameren Missouri and Ameren Illinois of related-party transactions for the years ended December 31, 2020, 2019, and 2018. It is based primarily on the agreements discussed above and the money pool arrangements discussed in Note 4 – Short-term Debt and Liquidity. Agreement Income Statement Line Item Ameren Ameren Ameren Missouri power supply agreements Operating Revenues 2020 $ 11 $ (a) with Ameren Illinois 2019 3 (a) 2018 11 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2020 26 1 rent and facility services 2019 27 2 2018 22 3 Ameren Missouri and Ameren Illinois miscellaneous Operating Revenues 2020 3 1 support services and services provided to ATXI 2019 1 2 2018 1 1 Ameren Missouri software licensing Operating Revenues 2020 (a) (a) with Ameren Illinois 2019 (a) 19 2018 (a) (a) Total Operating Revenues 2020 $ 40 $ 2 2019 31 23 2018 34 4 Ameren Illinois power supply Purchased Power 2020 $ (a) $ 11 agreements with Ameren Missouri 2019 (a) 3 2018 (a) 11 Ameren Missouri and Ameren Illinois Purchased Power 2020 (a) 2 transmission services from ATXI 2019 (a) 2 2018 (a) 1 Total Purchased Power 2020 $ (a) $ 13 2019 (a) 5 2018 (a) 12 Ameren Missouri and Ameren Illinois Other Operations and 2020 $ (b) $ 4 rent and facility services Maintenance 2019 2 5 2018 3 6 Ameren Services support services Other Operations and 2020 140 133 agreement Maintenance 2019 135 127 2018 136 126 Total Other Operations and 2020 $ 140 $ 137 Maintenance Expenses 2019 137 132 2018 139 132 Money pool borrowings (advances) (Interest Charges) 2020 $ (b) $ (b) Other Income, Net 2019 (b) (b) 2018 1 (b) (a) Not applicable. (b) Amount less than $1 million.

Commitments And Contingencies

Commitments And Contingencies	12 Months Ended
	Dec. 31, 2020
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, will not have a material adverse effect on our results of operations, financial position, or liquidity. See also 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 15 – Supplemental Information in this report. Other Obligations To supply a portion of the fuel requirements of Ameren Missouri’s energy centers, Ameren Missouri has entered into various long-term commitments for the procurement of coal, natural gas, nuclear fuel, and methane gas. Ameren Missouri and Ameren Illinois also have entered into various long-term commitments for purchased power and natural gas for distribution. The table below presents our estimated minimum fuel, purchased power, and other commitments at December 31, 2020. Ameren’s and Ameren Illinois’ purchased power commitments include the Ameren Illinois agreements entered into as part of the IPA-administered power procurement process. Included in the Other column are minimum purchase commitments under contracts for equipment, design and construction, meter reading services, and service maintenance agreements related to Ameren Missouri’s High Prairie Renewable Energy Center, among other agreements, at December 31, 2020. Coal Natural Gas (a) Nuclear Purchased Power (b)(c)(d) Methane Other (e) Total Ameren: 2021 $ 321 $ 161 $ 56 $ 150 (f) $ 3 $ 97 $ 788 2022 194 111 12 64 3 29 413 2023 113 72 45 24 3 27 284 2024 94 31 26 7 3 25 186 2025 55 17 — — 3 28 103 Thereafter — 64 21 — 18 92 195 Total $ 777 $ 456 $ 160 $ 245 $ 33 $ 298 $ 1,969 Ameren Missouri: 2021 $ 321 $ 44 $ 56 $ — $ 3 $ 79 $ 503 2022 194 40 12 — 3 30 279 2023 113 35 45 — 3 28 224 2024 94 15 26 — 3 26 164 2025 55 7 — — 3 29 94 Thereafter — 17 21 — 18 70 126 Total $ 777 $ 158 $ 160 $ — $ 33 $ 262 $ 1,390 Ameren Illinois: 2021 $ — $ 117 $ — $ 160 (f) $ — $ 13 $ 290 2022 — 71 — 67 — — 138 2023 — 37 — 25 — — 62 2024 — 16 — 7 — — 23 2025 — 10 — — — — 10 Thereafter — 47 — — — — 47 Total $ — $ 298 $ — $ 259 $ — $ 13 $ 570 (a) Includes amounts for generation and for distribution. (b) The purchased power amounts for Ameren and Ameren Illinois exclude agreements for renewable energy credits through 2035 with various renewable energy suppliers due to the contingent nature of the payment amounts, with the exception of expected payments of $48 million through 2025. (c) The purchased power amounts for Ameren and Ameren Missouri exclude a 102-MW power purchase agreement with a wind farm operator, which expires in 2024, due to the contingent nature of the payment amounts. (d) The purchased power amounts for Ameren exclude obligations resulting from capacity supply and energy product agreements between Ameren Missouri and Ameren Illinois, as disclosed in Note 13 – Related-party Transactions. (e) The other amounts for Ameren exclude obligations resulting from interconnection agreements between Ameren Missouri and ATXI, as disclosed in Note 13 – Related-party Transactions. (f) In January 2018, as required by the FEJA, Ameren Illinois entered into agreements to acquire zero emission credits, through 2026. Annual zero emission credit commitment amounts will be published by the IPA each May prior to the start of the subsequent planning year, which begins each June. The amounts above reflect Ameren Illinois’ commitment to acquire approximately $26 million of zero emission credits through May 2021. Environmental Matters Our electric generation, transmission, and distribution operations and natural gas transmission, distribution, and storage operations must comply with a variety of environmental laws that are enforced through statutory and regulatory requirements including permitting programs implemented by federal, state, and local authorities. Depending upon the business activity of specific facilities, such laws address 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 ensure our business activities comply 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 SO 2 , particulate matter, NO x , mercury, toxic metals, and acid gases, and CO 2 emissions from new power plants. Clean Water Act regulations govern both water intake and discharges from power plants and 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 at Ameren Missouri’s energy centers. 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 as a solid waste 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, potential modifications to cooling water intake structures at existing power plants under Clean Water Act rules, and by effluent limitation guidelines applicable to steam electric generating units, all of which are discussed below. This estimate does not include capital expenditures that may be required as a result of the NSR and Clean Air Act litigation discussed below. 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. Clean Air Act Federal and state laws, including CSAPR, regulate emissions of SO 2 and NO x 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. To achieve compliance with the CSAPR, Ameren Missouri burns low-sulfur coal, operates two scrubbers at its Sioux Energy Center, and optimizes other existing air pollution control equipment. Ameren Missouri expects to incur additional costs to lower its emissions at one or more of its energy centers to comply with the CSAPR in future years. These higher costs are expected to be recovered from customers through the FAC or higher base rates. CO 2 Emissions Standards The EPA’s Affordable Clean Energy Rule repealed the Clean Power Plan and replaced it with a new rule that had established emission guidelines for states to follow in developing plans to limit CO 2 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. Additional litigation including reconsideration by the entire United States Court of Appeals for the District of Columbia Circuit or an appeal to the United States Supreme Court is possible. Regardless of the outcome of such potential legal challenges, the EPA is likely to develop new regulations to address carbon emissions from coal and natural gas electric generating units, which could take years to finalize. At this time, Ameren Missouri cannot predict the outcome of legal challenges 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 Act Litigation In January 2011, the Department of Justice, on behalf of the EPA, filed a complaint against Ameren Missouri in the United States District Court for the Eastern District of Missouri alleging that in performing projects at its 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 and, in September 2019, entered a final 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. There were no fines in the order. In October 2019, Ameren Missouri appealed the district court’s ruling to the United States Court of Appeals for the Eighth Circuit. The district court has stayed implementation of the majority of the requirements of its order while the case is under appeal. Ameren Missouri believes that the district court both misinterpreted and misapplied the law in its ruling. In December 2020, the court of appeals heard oral arguments presented by the parties. Ameren Missouri is unable to predict the ultimate resolution of this matter. The court is under no deadline to issue a ruling in this case; however, Ameren Missouri expects a ruling during 2021. The ultimate resolution of this matter could have a material adverse effect on the results of operations, financial position, and liquidity of Ameren and Ameren Missouri. Among other things and subject to economic and regulatory considerations, resolution of this matter could result in increased capital expenditures for the installation of air pollution control equipment, as well as increased operations and maintenance expenses. 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 are estimated at approximately $1 billion. Further, the flue gas desulfurization system would result in additional operation and maintenance expenses of $30 million to $50 million annually for the life of the energy center. Engineering studies required to develop estimated capital expenditures and estimated additional operation and maintenance expenses for the Labadie Energy Center to comply with the district court’s order will not be undertaken while the case is under appeal. As a result of the district court’s stay, Ameren Missouri does not expect to make significant capital expenditures or incur operations and maintenance expenses related to the district court’s order while the case is under appeal. Clean Water Act The EPA’s Section 316(b) Rule requires 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 being implemented through the permit renewal process of each energy center’s water discharge permit, which is expected to be completed by 2023. In 2015, the EPA issued a rule to revise the effluent limitation guidelines applicable to steam electric generating units. These guidelines established national standards for water discharges, prohibits effluent discharges of certain waste streams, and imposes 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 completed construction of wastewater treatment facilities at three of its four coal-fired energy centers in 2020. The Meramec Energy Center, which is the fourth coal-fired energy center and is scheduled to close permanently in 2022, will not have 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 is in the process of closing surface impoundments at three facilities, and is scheduled to complete the last of such closures at all of its energy centers in 2023. While the EPA has issued a series of revisions to the CCR rule, none of those revisions or proposals is expected to materially impact our closure schedule. Ameren and Ameren Missouri have AROs of $111 million recorded on their respective balance sheets as of December 31, 2020, associated with CCR storage facilities. Ameren Missouri estimates it will need to make total capital expenditures of $75 million to $100 million from 2021 through 2025 to implement its CCR management compliance plan, which includes installation of dry ash handling systems, wastewater treatment facilities, and groundwater monitoring equipment. Remediation The Ameren Companies are involved in a number of remediation actions to clean up sites impacted by the use or disposal of materials containing hazardous substances. Federal and state laws can require responsible parties to fund remediation regardless of their degree of fault, the legality of original disposal, or the ownership of a disposal site. As of December 31, 2020, Ameren Illinois has remediated the majority of the 44 former MGP sites in Illinois it owned or for which it was otherwise responsible. Ameren Illinois estimates it could substantially conclude remediation efforts at the remaining sites by 2023. The ICC allows Ameren Illinois to recover such remediation and related litigation costs from its electric and natural gas utility customers through environmental cost riders. Costs are subject to annual prudence review by the ICC. As of December 31, 2020, Ameren Illinois estimated the remaining obligation related to these former MGP sites at $94 million to $157 million. Ameren and Ameren Illinois recorded a liability of $94 million to represent the estimated minimum obligation for these sites, as no other amount within the range was a better estimate. The scope of the remediation activities at these former MGP sites may increase as remediation efforts continue. Considerable uncertainty remains in these estimates because many site-specific factors can influence the ultimate actual costs, including unanticipated underground structures, the degree to which groundwater is encountered, regulatory changes, local ordinances, and site accessibility. The actual costs and timing of completion may vary substantially from these estimates.

Supplemental Information

Supplemental Information	12 Months Ended
	Dec. 31, 2020
Supplemental Information [Abstract]
Supplemental Information	SUPPLEMENTAL INFORMATION Cash, Cash Equivalents, and Restricted Cash The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets and the statements of cash flows as of December 31, 2020 and 2019: December 31, 2020 December 31, 2019 Ameren Ameren Ameren Ameren Ameren Ameren Cash and cash equivalents $ 139 $ 136 $ — $ 16 $ 9 $ — Restricted cash included in “Other current assets” 17 5 6 14 4 5 Restricted cash included in “Other assets” 141 — 141 120 — 120 Restricted cash included in “Nuclear decommissioning trust fund” 4 4 — 26 26 — Total cash, cash equivalents, and restricted cash $ 301 $ 145 $ 147 $ 176 $ 39 $ 125 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 that are restricted for use in the procurement of renewable energy credits and amounts in a trust fund restricted for the use of funding certain asbestos-related claims. Accounts Receivable “Accounts receivable – trade” on Ameren’s and Ameren Illinois’ balance sheets include certain receivables purchased at a discount from alternative retail electric suppliers that elect to participate in the utility consolidated billing program. At December 31, 2020 and 2019, “Other current liabilities” on Ameren’s and Ameren Illinois’ balance sheets included payables for purchased receivables of $28 million and $32 million, respectively. The following table provides a reconciliation of the beginning and ending amount of the allowance for doubtful accounts for the years ended December 31, 2020 and 2019: December 31, 2020 December 31, 2019 Ameren Missouri Ameren Illinois (a) Ameren Ameren Missouri Ameren Illinois (a) Ameren Beginning of period $ 7 $ 10 $ 17 $ 7 $ 11 $ 18 Bad debt expense 15 33 48 9 21 30 Net write-offs (6) (9) (15) (9) (22) (31) End of period $ 16 $ 34 $ 50 $ 7 $ 10 $ 17 (a) Ameren Illinois has riders 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. In 2020, the rider for electric distribution allows for recovery of bad debt expense recognized under GAAP. See Note 2 – Rate and Regulatory Matters for additional information. Net write-offs decreased for the year ended December 31, 2020 due to the temporary suspension of disconnecting customers for nonpayment. See Note 2 – Rate and Regulatory Matters for additional information. Inventories The following table presents the components of inventories for each of the Ameren Companies at December 31, 2020 and 2019: December 31, 2020 December 31, 2019 Ameren Ameren Ameren Ameren Ameren Ameren Fuel (a) $ 115 $ — $ 115 $ 126 $ — $ 126 Natural gas stored underground 5 52 57 6 57 63 Materials, supplies, and other 266 83 349 241 64 305 Total inventories $ 386 $ 135 $ 521 $ 373 $ 121 $ 494 (a) Consists of coal, oil, and propane. Asset Retirement Obligations The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the years ended December 31, 2020 and 2019: December 31, 2020 December 31, 2019 Ameren Ameren Ameren Ameren Ameren Ameren Beginning balance at January 1 $ 687 $ 4 (a) $ 691 (b) $ 646 $ 4 $ 650 Liabilities incurred 36 (c) — 36 (c) — — — Liabilities settled (58) — (58) (20) — (20) Accretion (d) 29 1 30 28 — 28 Change in estimates 57 (e) — 57 (e) 33 (f) — 33 (f) Ending balance at December 31 $ 751 (g) $ 5 (a) $ 756 (b), (g) $ 687 $ 4 (a) $ 691 (b) (a) Included in “Other deferred credits and liabilities” on the balance sheet. (b) Balance included $60 million and $53 million in “Other current liabilities” on the balance sheet as of both December 31, 2020 and 2019, respectively. (c) In December 2020, Ameren Missouri recorded an ARO related to the decommissioning for the High Prairie Renewable Energy Center. (d) Accretion expense attributable to Ameren Missouri and Ameren Illinois was recorded as a decrease to regulatory liabilities and an increase to regulatory assets, respectively. (e) Ameren Missouri changed its fair value estimate primarily due to an update to the decommissioning of the Callaway Energy Center to reflect the cost study and funding analysis filed with the MoPSC in November 2020 and an increase in the cost estimate for closure of certain CCR storage facilities. (f) Ameren Missouri changed its fair value estimate primarily due to an increase in the cost estimate for closure of certain CCR storage facilities. (g) The balance as of December 31, 2020, includes an ARO related to the decommissioning of the Callaway Enter Center of $549 million. Noncontrolling Interests As of December 31, 2020 and 2019, Ameren’s noncontrolling interests included the preferred stock of Ameren Missouri and Ameren Illinois. Deferred Compensation As of December 31, 2020, and 2019, “Other current liabilities” and “Other deferred credits and liabilities” on Ameren’s balance sheet included deferred compensation obligations of $90 million and $86 million, respectively, recorded at the present value of future benefits to be paid. 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 years ended December 31, 2020, 2019, and 2018: 2020 2019 2018 Ameren Missouri $ 139 $ 147 $ 164 Ameren Illinois 115 117 118 Ameren $ 254 $ 264 $ 282 Allowance for Funds Used During Construction The following table presents the average rate that was applied to eligible construction work in progress and the amounts of allowance for funds used during construction capitalized in 2020, 2019, and 2018: 2020 2019 2018 Average rate: Ameren Missouri 5 % 6 % 7 % Ameren Illinois 5 % 5 % 5 % Ameren: Allowance for equity funds used during construction $ 32 $ 28 $ 36 Allowance for borrowed funds used during construction 16 20 21 Total Ameren $ 48 $ 48 $ 57 Ameren Missouri: Allowance for equity funds used during construction $ 19 $ 19 $ 27 Allowance for borrowed funds used during construction 10 12 14 Total Ameren Missouri $ 29 $ 31 $ 41 Ameren Illinois: Allowance for equity funds used during construction $ 13 $ 9 $ 9 Allowance for borrowed funds used during construction 6 8 7 Total Ameren Illinois $ 19 $ 17 $ 16 Earnings per Share Earnings per basic and diluted share are computed by dividing “Net Income Attributable to Ameren Common Shareholders” by the weighted-average number of basic and diluted common shares outstanding, respectively, during the applicable period. The weighted-average shares outstanding for earnings per diluted share includes the incremental effects resulting from performance share units, restricted stock units, and the forward sale agreement relating to common stock when the impact would be dilutive, as calculated using the treasury stock method. For information regarding performance share units and restricted stock units, see Note 11 – Stock-based Compensation. For information regarding the forward sale agreement, see Note 5 – Long-term Debt and Equity Financings. The following table reconciles the weighted-average number of common shares outstanding to the diluted weighted-average number of common shares outstanding for the years ended December 31, 2020, 2019, and 2018: 2020 2019 2018 Weighted-average Common Shares Outstanding – Basic 247.0 245.6 243.8 Assumed settlement of performance share units and restricted stock units 1.2 1.4 2.0 Dilutive effect of forward sale agreement 0.5 0.1 — Weighted-average Common Shares Outstanding – Diluted (a) 248.7 247.1 245.8 (a) There were no potentially dilutive securities excluded from the earnings per diluted share calculations for the years ended December 31, 2020, 2019, and 2018. Supplemental Cash Flow Information The following table provides noncash financing and investing activity excluded from the statements of cash flows for the years ended December 31, 2020, 2019, and 2018: December 31, 2020 December 31, 2019 December 31, 2018 Ameren Ameren Ameren Ameren Ameren Ameren Ameren Ameren Ameren Investing Accrued capital expenditures $ 400 $ 183 $ 218 $ 333 $ 140 $ 163 $ 272 $ 121 $ 138 Accrued nuclear fuel expenditures — — — 19 19 — 20 20 — Accrued wind generation expenditures 46 46 — — — — — — — Net realized and unrealized gain (loss) – nuclear decommissioning trust fund 116 116 — 143 143 — (38) (38) — Exchange of bond investments for the extinguishment of senior unsecured notes (a) — — — 17 — 17 — — — Financing Issuance of common stock for stock-based compensation $ 38 $ — $ — $ 54 $ — $ — $ 35 $ — $ — Exchange of bond investments for the extinguishment of senior unsecured notes (a) — — — (17) — (17) — — — (a) See Note 5 – Long-term Debt and Equity Financings for additional information.

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2020
Segment Reporting [Abstract]
SEGMENT INFORMATION	SEGMENT INFORMATION Ameren has four segments: Ameren Missouri, Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Transmission. The Ameren Missouri segment includes all of the operations of Ameren Missouri. Ameren Illinois Electric Distribution consists of the electric distribution business of Ameren Illinois. Ameren Illinois Natural Gas consists of the natural gas business of Ameren Illinois. Ameren Transmission primarily consists of the aggregated electric transmission businesses of Ameren Illinois and ATXI. The category called Other primarily includes Ameren (parent) activities and Ameren Services. Ameren Missouri has one segment. Ameren Illinois has three segments: Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Illinois Transmission. See Note 1 – Summary of Significant Accounting Policies for additional information regarding the operations of Ameren Missouri, Ameren Illinois, and ATXI. Segment operating revenues and a majority of operating expenses are directly recognized and incurred by Ameren Illinois to each Ameren Illinois segment. Common operating expenses, miscellaneous income and expenses, interest charges, and income tax expense are allocated by Ameren Illinois to each Ameren Illinois segment based on certain factors, which primarily relate to the nature of the cost. Additionally, Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution, other retail electric suppliers, and wholesale customers. The transmission expense for Illinois customers who have elected to purchase their power from Ameren Illinois is recovered through a cost recovery mechanism with no net effect on Ameren Illinois Electric Distribution earnings, as costs are offset by corresponding revenues. Transmission revenues from these transactions are reflected in Ameren Transmission’s and Ameren Illinois Transmission’s operating revenues. An intersegment elimination at Ameren and Ameren Illinois occurs to eliminate these transmission revenues and expenses. The following tables present information about the reported revenue and specified items reflected in net income attributable to common shareholders and capital expenditures by segment at Ameren and Ameren Illinois for the years ended December 31, 2020, 2019, and 2018. Ameren, Ameren Missouri, and Ameren Illinois management review segment capital expenditure information rather than any individual or total asset amount. Ameren Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Other Intersegment Eliminations Ameren 2020 External revenues $ 3,069 $ 1,496 $ 760 $ 469 $ — $ — $ 5,794 Intersegment revenues 40 2 — 54 — (96) — Depreciation and amortization 604 288 81 98 4 — 1,075 Interest income 26 2 — 1 4 (4) 29 Interest charges 190 72 41 78 (a) 42 (4) 419 Income taxes (benefit) 34 42 36 78 (35) — 155 Net income (loss) attributable to Ameren common shareholders 436 143 99 216 (23) — 871 Capital expenditures 1,666 (b) 543 301 716 5 2 3,233 (b) 2019 External revenues $ 3,212 $ 1,487 $ 791 $ 401 $ — $ — $ 5,891 Intersegment revenues 31 17 6 63 — (98) 19 (c) Depreciation and amortization 556 273 78 84 4 — 995 Interest income 26 6 — 1 5 (5) 33 Interest charges 178 71 38 74 (a) 25 (5) 381 Income taxes (benefit) 68 45 30 64 (25) — 182 Net income (loss) attributable to Ameren common shareholders 426 146 84 185 (13) — 828 Capital expenditures 1,076 518 318 528 3 (32) (d) 2,411 2018 External revenues $ 3,555 $ 1,544 $ 814 $ 378 $ — $ — $ 6,291 Intersegment revenues 34 3 1 55 — (93) — Depreciation and amortization 550 259 65 77 4 — 955 Interest income 28 6 — — 4 (5) 33 Interest charges 200 73 38 75 (a) 19 (4) 401 Income taxes (benefit) 124 41 25 56 (9) — 237 Net income (loss) attributable to Ameren common shareholders 478 136 70 164 (33) — 815 Capital expenditures 914 503 311 562 5 (9) 2,286 (a) Ameren Transmission interest charges include an allocation of financing costs from Ameren (parent). (b) Includes $564 million at Ameren and Ameren Missouri for the acquisition of the High Prairie Renewable Energy Center for the year ended December 31, 2020. (c) Intersegment revenues at Ameren include $14 million and $5 million of revenue from Ameren Illinois Electric Distribution and Ameren Illinois Natural Gas, respectively, for the year ended December 31, 2019, for a software licensing agreement with Ameren Missouri. Under authoritative accounting guidance for rate-regulated entities, the revenue recognized by Ameren Illinois was not eliminated upon consolidation. See Note 13 – Related-party Transactions for additional information. (d) Intersegment capital expenditure eliminations include $24 million of eliminations for the year ended December 31, 2019, for a software licensing agreement between Ameren Illinois and Ameren Missouri. See Note 13 – Related-party Transactions for additional information. Ameren Illinois Ameren Illinois Electric Distribution Ameren Illinois Ameren Illinois Transmission Intersegment Eliminations Ameren Illinois 2020 External revenues $ 1,498 $ 760 $ 277 $ — $ 2,535 Intersegment revenues — — 52 (52) — Depreciation and amortization 288 81 65 — 434 Interest income 2 — 1 — 3 Interest charges 72 41 42 — 155 Income taxes 42 36 46 — 124 Net income available to common shareholder 143 99 137 — 379 Capital expenditures 543 301 603 — 1,447 2019 External revenues $ 1,504 $ 797 $ 226 $ — $ 2,527 Intersegment revenues — — 62 (62) — Depreciation and amortization 273 78 55 — 406 Interest income 6 — — — 6 Interest charges 71 38 38 — 147 Income taxes 45 30 35 — 110 Net income available to common shareholder 146 84 113 — 343 Capital expenditures 518 318 372 — 1,208 2018 External revenues $ 1,547 $ 815 $ 214 $ — $ 2,576 Intersegment revenues — — 53 (53) — Depreciation and amortization 259 65 50 — 374 Interest income 6 — — — 6 Interest charges 73 38 38 — 149 Income taxes 41 25 32 — 98 Net income available to common shareholder 136 70 98 — 304 Capital expenditures 503 311 444 — 1,258 The following tables present disaggregated revenues by segment at Ameren and Ameren Illinois for the years ended December 31, 2020, 2019, and 2018. Economic factors affect the nature, timing, amount, and uncertainty of revenues and cash flows in a similar manner across customer classes. Revenues from alternative revenue programs have a similar distribution among customer classes as revenues from contracts with customers. Other revenues not associated with contracts with customers are presented in the Other customer classification, along with electric transmission and off-system revenues. Ameren Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Intersegment Eliminations Ameren 2020 Residential $ 1,373 $ 867 $ — $ — $ — $ 2,240 Commercial 1,025 486 — — — 1,511 Industrial 261 124 — — — 385 Other 325 21 — 523 (94) 775 Total electric revenues $ 2,984 $ 1,498 $ — $ 523 $ (94) $ 4,911 Residential $ 76 $ — $ 541 $ — $ — $ 617 Commercial 29 — 136 — — 165 Industrial 4 — 14 — — 18 Other 16 — 69 — (2) 83 Total gas revenues $ 125 $ — $ 760 $ — $ (2) $ 883 Total revenues (a) $ 3,109 $ 1,498 $ 760 $ 523 $ (96) $ 5,794 2019 Residential $ 1,403 $ 848 $ — $ — $ — $ 2,251 Commercial 1,157 497 — — — 1,654 Industrial 278 127 — — — 405 Other 271 32 (b) — 464 (96) 671 Total electric revenues $ 3,109 $ 1,504 $ — $ 464 $ (96) $ 4,981 Residential $ 81 $ — $ 570 $ — $ — $ 651 Commercial 34 — 154 — — 188 Industrial 4 — 13 — — 17 Other 15 — 60 (b) — (2) 73 Total gas revenues $ 134 $ — $ 797 $ — $ (2) $ 929 Total revenues (a) $ 3,243 $ 1,504 $ 797 $ 464 $ (98) $ 5,910 2018 Residential $ 1,560 $ 867 $ — $ — $ — $ 2,427 Commercial 1,271 511 — — — 1,782 Industrial 312 130 — — — 442 Other 308 (c) 39 — 433 (92) 688 (c) Total electric revenues $ 3,451 $ 1,547 $ — $ 433 $ (92) $ 5,339 Residential $ 90 $ — $ 581 $ — $ — $ 671 Commercial 37 — 159 — — 196 Industrial 4 — 17 — — 21 Other 7 — 58 — (1) 64 Total gas revenues $ 138 $ — $ 815 $ — $ (1) $ 952 Total revenues (a) $ 3,589 $ 1,547 $ 815 $ 433 $ (93) $ 6,291 (a) The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the years ended December 31, 2020, 2019, and 2018: Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Ameren 2020 Revenues from alternative revenue programs $ (14) $ (20) $ 20 $ 50 $ 36 Other revenues not from contracts with customers 25 8 2 1 36 2019 Revenues from alternative revenue programs $ 35 $ (74) $ — $ (31) $ (70) Other revenues not from contracts with customers 19 7 2 — 28 2018 Revenues from alternative revenue programs $ (8) $ (3) $ (23) $ (25) $ (59) Other revenues not from contracts with customers 24 16 2 — 42 (b) Includes $14 million and $5 million for Ameren Illinois Electric Distribution and Ameren Illinois Natural Gas, respectively, for the year ended December 31, 2019, for a software licensing agreement with Ameren Missouri. See Note 13 – Related-party Transactions for additional information. (c) Includes $60 million for the year ended December 31, 2018, for the reduction to revenue for the excess amounts collected in rates to be refunded related to the TCJA from January 1, 2018, through July 31, 2018. See Note 2 – Rate and Regulatory Matters for additional information. Ameren Illinois Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Ameren Illinois 2020 Residential $ 867 $ 541 $ — $ — $ 1,408 Commercial 486 136 — — 622 Industrial 124 14 — — 138 Other 21 69 329 (52) 367 Total revenues (a) $ 1,498 $ 760 $ 329 $ (52) $ 2,535 2019 Residential $ 848 $ 570 $ — $ — $ 1,418 Commercial 497 154 — — 651 Industrial 127 13 — — 140 Other 32 (b) 60 (b) 288 (62) 318 Total revenues (a) $ 1,504 $ 797 $ 288 $ (62) $ 2,527 2018 Residential $ 867 $ 581 $ — $ — $ 1,448 Commercial 511 159 — — 670 Industrial 130 17 — — 147 Other 39 58 267 (53) 311 Total revenues (a) $ 1,547 $ 815 $ 267 $ (53) $ 2,576 (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 years ended December 31, 2020, 2019, and 2018: Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Ameren Illinois 2020 Revenues from alternative revenue programs $ (20) $ 20 $ 42 $ 42 Other revenues not from contracts with customers 8 2 — 10 2019 Revenues from alternative revenue programs $ (74) $ — $ (33) $ (107) Other revenues not from contracts with customers 7 2 — 9 2018 Revenues from alternative revenue programs $ (3) $ (23) $ (25) $ (51) Other revenues not from contracts with customers 16 2 — 18 (b) Includes $14 million and $5 million for Ameren Illinois Electric Distribution and Ameren Illinois Natural Gas, respectively, for the year ended December 31, 2019, for a software licensing agreement with Ameren Missouri. See Note 13 – Related-party Transactions for additional information.

Schedule I - Condensed Financia

Schedule I - Condensed Financial Information Of Parent	12 Months Ended
	Dec. 31, 2020
Condensed Financial Information Disclosure [Abstract]
Condensed Financial Information Of Parent	SCHEDULE I – CONDENSED FINANCIAL INFORMATION OF PARENT AMEREN CORPORATION CONDENSED STATEMENT OF INCOME AND COMPREHENSIVE INCOME For the Years Ended December 31, 2020, 2019, and 2018 (In millions) 2020 2019 2018 Operating revenues $ — $ — $ — Operating expenses 12 15 11 Operating loss (12) (15) (11) Equity in earnings of subsidiaries 908 850 857 Interest income from affiliates 4 5 3 Total other expense, net (8) (2) (12) Interest charges (57) (39) (34) Income tax benefit 36 29 12 Net Income Attributable to Ameren Common Shareholders $ 871 $ 828 $ 815 Net Income Attributable to Ameren Common Shareholders $ 871 $ 828 $ 815 Other Comprehensive Income (Loss), Net of Taxes: Pension and other postretirement benefit plan activity, net of income taxes (benefit) of $5, $1, and $(1), respectively 16 5 (4) Comprehensive Income Attributable to Ameren Common Shareholders $ 887 $ 833 $ 811 SCHEDULE I – CONDENSED FINANCIAL INFORMATION OF PARENT (In millions, except per share amounts) December 31, 2020 December 31, 2019 Assets: Cash and cash equivalents $ — $ — Advances to money pool 16 102 Accounts receivable – affiliates 12 73 Miscellaneous accounts and notes receivable 15 4 Other current assets 4 3 Total current assets 47 182 Investments in subsidiaries 10,872 9,108 Note receivable – ATXI 75 75 Accumulated deferred income taxes, net 42 49 Other assets 167 145 Total assets $ 11,203 $ 9,559 Liabilities and Shareholders’ Equity: Current maturities of long-term debt $ — $ 350 Short-term debt 490 153 Borrowings from money pool — 24 Accounts payable – affiliates 41 39 Other current liabilities 34 23 Total current liabilities 565 589 Long-term debt 1,588 794 Pension and other postretirement benefits 27 37 Other deferred credits and liabilities 85 80 Total liabilities 2,265 1,500 Commitments and Contingencies (Note 5) Shareholders’ Equity: Common stock, $.01 par value, 400.0 shares authorized – shares outstanding of 253.3 and 246.2, respectively 3 2 Other paid-in capital, principally premium on common stock 6,179 5,694 Retained earnings 2,757 2,380 Accumulated other comprehensive loss (1) (17) Total shareholders’ equity 8,938 8,059 Total liabilities and shareholders’ equity $ 11,203 $ 9,559 SCHEDULE I – CONDENSED FINANCIAL INFORMATION OF PARENT AMEREN CORPORATION CONDENSED STATEMENT OF CASH FLOWS For the Years Ended December 31, 2020, 2019, and 2018 (In millions) 2020 2019 2018 Net cash flows provided by operating activities $ 147 $ 491 $ 550 Cash flows from investing activities: Money pool advances, net 86 (26) (63) Investments in subsidiaries (956) (142) (208) Other 8 5 5 Net cash flows used in investing activities (862) (163) (266) Cash flows from financing activities: Dividends on common stock (494) (472) (451) Short-term debt, net 337 (317) 87 Money pool borrowings, net (24) (22) 18 Maturities of long-term debt (350) — — Issuances of long-term debt 798 450 — Issuances of common stock 476 68 74 Employee payroll taxes related to stock-based compensation (20) (29) (19) Debt issuance costs (7) (4) — Net cash flows provided by (used in) financing activities 716 (326) (291) Net change in cash, cash equivalents, and restricted cash $ 1 $ 2 $ (7) Cash, cash equivalents, and restricted cash at beginning of year 3 1 8 Cash, cash equivalents, and restricted cash at end of year $ 4 $ 3 $ 1 Supplemental information: Cash dividends received from consolidated subsidiaries $ 105 $ 445 $ 450 Noncash financing activity – Issuance of common stock for stock-based compensation 38 54 35 AMEREN CORPORATION (parent company only) NOTES TO CONDENSED FINANCIAL STATEMENTS December 31, 2020 NOTE 1 – BASIS OF PRESENTATION Ameren Corporation (parent company only) is a public utility holding company that conducts substantially all of its business operations through its subsidiaries. Ameren Corporation (parent company only) has accounted for its subsidiaries using the equity method. These financial statements are presented on a condensed basis. See Note 1 – Summary of Significant Accounting Policies under Part II, Item 8, of this report for additional information. See Note 13 – Related-party Transactions under Part II, Item 8, of this report for information on the tax allocation agreement between Ameren Corporation (parent company only) and its subsidiaries. NOTE 2 – CASH AND CASH EQUIVALENTS The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheet and the statement of cash flows as of December 31, 2020 and 2019: (In millions) 2020 2019 Cash and cash equivalents $ — $ — Restricted cash included in “Other current assets” 4 3 Total cash, cash equivalents, and restricted cash $ 4 $ 3 See Note 1 – Summary of Significant Accounting Policies under Part II, Item 8, of this report for additional information. NOTE 3 – SHORT-TERM DEBT AND LIQUIDITY Ameren, Ameren Services, and other non-state-regulated Ameren subsidiaries have the ability, subject to Ameren parent company and applicable regulatory short-term borrowing authorizations, to access funding from the Credit Agreements and the commercial paper programs through a non-state-regulated subsidiary money pool agreement. All participants may borrow from or lend to the non-state-regulated money pool. The total amount available to pool participants from the non-state-regulated subsidiary money pool at any given time is reduced by the amount of borrowings made by participants, but is increased to the extent that the pool participants advance surplus funds to the non-state- regulated subsidiary money pool or remit funds from other external sources. The non-state-regulated subsidiary money pool was established to coordinate and to provide short-term cash and working capital for the participants. Participants receiving a loan under the non-state-regulated subsidiary money pool agreement must repay the principal amount of such loan, together with accrued interest. The rate of interest depends on the composition of internal and external funds in the non-state-regulated subsidiary money pool. Interest revenues and interest charges related to non-state-regulated money pool advances and borrowings were immaterial in 2018, 2019, and 2020. Ameren Corporation (parent company only) had a total of $3 million in guarantees outstanding, primarily for ATXI, that were not recorded on its December 31, 2020 balance sheet. The ATXI guarantees were issued to local governments as assurance for potential remediation of damage caused by ATXI construction. See Note 4 – Short-term Debt and Liquidity under Part II, Item 8, of this report for a description and details of short-term debt and liquidity needs of Ameren Corporation (parent company only). NOTE 4 – LONG-TERM OBLIGATIONS See Note 5 – Long-term Debt and Equity Financings under Part II, Item 8, of this report for additional information on Ameren Corporation’s (parent company only) long-term debt, indenture provisions, and forward sale agreement related to common stock. NOTE 5 – COMMITMENTS AND CONTINGENCIES See Note 14 – Commitments and Contingencies under Part II, Item 8, of this report for a description of all material contingencies of Ameren Corporation (parent company only). NOTE 6 – OTHER EXPENSE, NET The following table presents the components of “Other Expense, Net” in the Condensed Statement of Income and Comprehensive Income for the years ended December 31, 2020, 2019, and 2018: (In millions) 2020 2019 2018 Other Expense, Net Non-service cost components of net periodic benefit income $ 1 $ 2 $ 2 Donations (8) (3) (13) Other expense, net (1) (1) (1) Total Other Expense, Net $ (8) $ (2) $ (12)
Schedule of Cash and Cash Equivalents Including Restricted Cash	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets and the statements of cash flows as of December 31, 2020 and 2019: December 31, 2020 December 31, 2019 Ameren Ameren Ameren Ameren Ameren Ameren Cash and cash equivalents $ 139 $ 136 $ — $ 16 $ 9 $ — Restricted cash included in “Other current assets” 17 5 6 14 4 5 Restricted cash included in “Other assets” 141 — 141 120 — 120 Restricted cash included in “Nuclear decommissioning trust fund” 4 4 — 26 26 — Total cash, cash equivalents, and restricted cash $ 301 $ 145 $ 147 $ 176 $ 39 $ 125 The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheet and the statement of cash flows as of December 31, 2020 and 2019: (In millions) 2020 2019 Cash and cash equivalents $ — $ — Restricted cash included in “Other current assets” 4 3 Total cash, cash equivalents, and restricted cash $ 4 $ 3
Other Income And Expenses	The following table presents the components of “Other Income, Net” in the Ameren Companies’ statements of income for the years ended December 31, 2020, 2019, and 2018: 2020 2019 2018 Ameren: Other Income, Net Allowance for equity funds used during construction $ 32 $ 28 $ 36 Interest income on industrial development revenue bonds 25 25 26 Other interest income 4 8 7 Non-service cost components of net periodic benefit income (a) 116 90 70 Miscellaneous income 13 6 8 Donations (25) (b) (12) (33) Miscellaneous expense (14) (15) (12) Total Other Income, Net $ 151 $ 130 $ 102 Ameren Missouri: Other Income, Net Allowance for equity funds used during construction $ 19 $ 19 $ 27 Interest income on industrial development revenue bonds 25 25 26 Other interest income 1 1 2 Non-service cost components of net periodic benefit income (a) 46 18 17 Miscellaneous income 4 5 4 Donations (12) (b) (3) (14) Miscellaneous expense (7) (7) (6) Total Other Income, Net $ 76 $ 58 $ 56 Ameren Illinois: Other Income, Net Allowance for equity funds used during construction $ 13 $ 9 $ 9 Interest income 3 6 6 Non-service cost components of net periodic benefit income (a) 48 47 34 Miscellaneous income 6 3 3 Donations (5) (5) (6) Miscellaneous expense (6) (7) (4) Total Other Income, Net $ 59 $ 53 $ 42 (a) For the years ended December 31, 2020, 2019, and 2018, the non-service cost components of net periodic benefit income were adjusted by amounts deferred of $(4) million, $29 million, and $17 million, respectively, due to a tracker for the difference between the level of such costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. (b) Includes $8 million pursuant to Ameren Missouri’s March 2020 electric rate order. See Note 2 – Rate and Regulatory Matters for additional information. The following table presents the components of “Other Expense, Net” in the Condensed Statement of Income and Comprehensive Income for the years ended December 31, 2020, 2019, and 2018: (In millions) 2020 2019 2018 Other Expense, Net Non-service cost components of net periodic benefit income $ 1 $ 2 $ 2 Donations (8) (3) (13) Other expense, net (1) (1) (1) Total Other Expense, Net $ (8) $ (2) $ (12)

Schedule II - Valuation And Qua

Schedule II - Valuation And Qualifying Accounts	12 Months Ended
	Dec. 31, 2020
SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract]
Valuation And Qualifying Accounts	SCHEDULE II – VALUATION AND QUALIFYING ACCOUNTS FOR THE YEARS ENDED DECEMBER 31, 2020, 2019, AND 2018 (In millions) Column A Column B Column C Column D Column E Description Balance at (1) (2) Charged to Other Accounts (a) Deductions (b) Balance at End Ameren: Deducted from assets – allowance for doubtful accounts: 2020 $ 17 $ 42 $ 6 $ 15 $ 50 2019 18 26 4 31 17 2018 19 27 4 32 18 Deferred tax valuation allowance: 2020 $ 3 $ — $ — $ — $ 3 2019 5 (2) — — 3 2018 11 (6) — — 5 Ameren Missouri: Deducted from assets – allowance for doubtful accounts: 2020 $ 7 $ 15 $ — $ 6 $ 16 2019 7 9 — 9 7 2018 7 9 — 9 7 Ameren Illinois: Deducted from assets – allowance for doubtful accounts: 2020 $ 10 $ 27 $ 6 $ 9 $ 34 2019 11 17 4 22 10 2018 12 18 4 23 11 (a) Amounts associated with the allowance for doubtful accounts relate to the uncollectible account reserve associated with receivables purchased by Ameren Illinois from alternative retail electric suppliers, as required by the Illinois Public Utilities Act. (b) Uncollectible accounts charged off, less recoveries.

Summary Of Significant Accoun_2

Summary Of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Nature of Operations	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 also 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 Missouri was incorporated in Missouri in 1922 and is successor to a number of companies, the oldest of which was organized in 1881. It is the largest electric utility in the state of Missouri. It supplies electric and natural gas service to a 24,000-square-mile area in central and eastern Missouri, which includes the Greater St. Louis area. Ameren Missouri supplies electric service to 1.2 million customers and natural gas service to 0.1 million customers. • Ameren Illinois Company, doing business as Ameren Illinois, operates rate-regulated electric transmission, electric distribution, and natural gas distribution businesses in Illinois. Ameren Illinois was incorporated in Illinois in 1923 and is the successor to a number of companies, the oldest of which was organized in 1902. Ameren Illinois supplies electric and natural gas utility service to a 43,700 square mile area in central and southern Illinois. Ameren Illinois supplies electric service to 1.2 million customers and natural gas service to 0.8 million customers. • Ameren Transmission Company of Illinois, doing business as ATXI, operates a FERC rate-regulated electric transmission business in the MISO. ATXI was incorporated in Illinois in 2006. In December 2020, ATXI completed construction of the ninth and final line segment of the Illinois Rivers transmission line, a MISO-approved electric transmission line. ATXI also operates the Spoon River and Mark Twain transmission lines, which were placed in service in February 2018 and December 2019, respectively.
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, except as disclosed in Note 13 – Related-party Transactions. Ameren Missouri and Ameren Illinois have no subsidiaries. All tabular dollar amounts are in millions, unless otherwise indicated. Our accounting policies conform to GAAP. Our financial statements reflect all adjustments (which include normal, recurring adjustments) that are necessary, in our opinion, for a fair presentation 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.
Public Utilities	Regulation Our customer rates are regulated by the MoPSC, the ICC, and the FERC. We defer certain costs as assets pursuant to actions of rate regulators or because of expectations that we will be able to recover such costs in future rates charged to customers. We also defer certain amounts as liabilities pursuant to actions of rate regulators or based on the expectation that such amounts will be returned to customers in future rates. Regulatory assets and liabilities are amortized consistent with the period of expected regulatory treatment. See Note 2 – Rate and Regulatory Matters for additional information on our regulatory frameworks, regulatory recovery mechanisms, and regulatory assets and liabilities recorded at December 31, 2020 and 2019. We continually assess the recoverability of our respective regulatory assets. Regulatory assets are charged to earnings when it is no longer probable that such amounts will be recovered through future revenues. To the extent that reductions in customers’ rates or refunds to customers related to regulatory liabilities are no longer probable, the amounts are credited to earnings. Environmental Costs Liabilities for environmental costs are recorded on an undiscounted basis when it is probable that a liability has been incurred and the amount of the liability can be reasonably estimated. Costs are expensed or deferred as a regulatory asset when it is expected that the costs will be recovered from customers in future rates. See Note 14 – Commitments and Contingencies for additional information on liabilities for environmental costs.
Cash and Cash Equivalents	Cash, Cash Equivalents, and Restricted Cash Cash and cash equivalents include short-term, highly liquid investments purchased with an original maturity of three months or less. Cash and cash equivalents subject to legal or contractual restrictions and not readily available for use for general corporate purposes are classified as restricted cash. See Note 15 – Supplemental Information for a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets and the statements of cash flows.
Allowance for Doubtful Accounts Receivable	Allowance for Doubtful Accounts Receivable The allowance for doubtful accounts represents our estimate of existing accounts receivable that will ultimately be uncollectible. The allowance is calculated by applying estimated loss factors to various classes of outstanding receivables, including unbilled revenue. The loss factors used to estimate uncollectible accounts are based upon both historical collections experience and management’s estimate of future collections success given the existing and anticipated future collections environment. Ameren Illinois has bad debt riders that adjust rates for net write-offs of customer accounts receivable above or below those being collected in rates. In 2020, the rider for electric distribution allowed for recovery of bad debt expense recognized under GAAP. See Note 2 – Rate and Regulatory Matters for additional information.
Inventories	Inventories Inventories are recorded at the lower of weighted-average cost or net realizable value. Inventories are capitalized when purchased and then expensed as consumed or capitalized as property, plant, and equipment when installed, as appropriate. See Note 15 – Supplemental Information for the components of inventories.
Property and Plant	Property, Plant, and Equipment, Net We capitalize the cost of additions to, and betterments of, units of property, plant, and equipment. The cost includes labor, material, applicable taxes, and overhead. An allowance for funds used during construction, as discussed below, is also capitalized as a cost of our rate-regulated assets. Maintenance expenditures are expensed as incurred. Beginning in 2020, maintenance expenses related to scheduled Callaway nuclear refueling and maintenance outages, which were previously expensed as incurred, are deferred and amortized over the number of expected months until the completion of the next refueling outage, which historically has been approximately 18 months . When units of depreciable property are retired, the original costs, and the associated removal cost, net of salvage, are charged to accumulated depreciation. If environmental expenditures are related to assets currently in use, as in the case of the installation of pollution control equipment, the cost is capitalized and depreciated over the expected life of the asset. See Asset Retirement Obligations section below and Note 3 – Property, Plant, and Equipment, Net for additional information. Ameren Missouri’s cost of nuclear fuel is capitalized as a part of “Property, Plant, and Equipment, Net” on the balance sheet and then amortized to “Operating Expenses – Fuel” in the statement of income on a unit-of-production basis. Depreciation Depreciation is provided over the estimated lives of the various classes of depreciable property by applying composite rates on a straight-line basis to the cost basis of such property. The composite rates include a provision for the estimated removal cost of property, plant, and equipment retired from service, net of salvage. The provision for depreciation for the Ameren Companies in 2020, 2019, and 2018 ranged from 3% to 4% of the average depreciable cost. See Note 3 – Property, Plant, and Equipment, Net for additional information on estimated depreciable lives.
Allowance for Funds Used During Construction	Allowance for Funds Used During Construction As a part of “Property, Plant, and Equipment, Net” on the balance sheet, we capitalize allowance for funds used during construction, which is the cost of borrowed funds and the cost of equity funds (preferred and common shareholders’ equity) applicable to eligible rate-regulated construction work in progress, in accordance with the utility industry’s accounting practice and GAAP. The amount of allowance for funds used during construction is calculated using a FERC-prescribed formula based on a rate, which incorporates the average cost of short-term debt, the average cost of long-term debt, and the cost of equity funds. The portion attributable to borrowed funds is recorded as a reduction of “Interest Charges” on the statements of income. The portion attributable to equity funds is recorded within “Other Income, Net” on the statements of income. This accounting practice offsets the effect on earnings of the cost of financing during construction. See Note 15 – Supplemental Information for the amount of allowance for funds used during construction capitalized and the average rate applied to eligible construction work in progress. Allowance for funds used during construction does not represent a current source of cash funds. Under accepted ratemaking practice, cash recovery of allowance for funds used during construction and other construction costs occurs when completed projects are placed in service and reflected in customer rates.
Goodwill	Goodwill Goodwill represents the excess of the purchase price of an acquisition over the fair value of the net assets acquired. Ameren and Ameren Illinois had goodwill of $411 million at December 31, 2020 and 2019. Ameren has four reporting units: Ameren Missouri, Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Transmission. Ameren Illinois has three reporting units: Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Illinois Transmission. Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Illinois Transmission had goodwill of $238 million, $80 million, and $93 million, respectively, at December 31, 2020 and 2019. The Ameren Transmission reporting unit had the same $93 million of goodwill as the Ameren Illinois Transmission reporting unit at December 31, 2020 and 2019. Ameren and Ameren Illinois evaluate goodwill for impairment in each of their reporting units as of October 31 each year, or more frequently if events occur or circumstances change that would more likely than not reduce the fair value of their reporting units below their carrying amounts. To determine whether the fair value of a reporting unit is more likely than not greater than its carrying amount, Ameren and Ameren Illinois elect to perform either a qualitative assessment or to bypass the qualitative assessment and perform a quantitative test.
Impairment of Long-lived Assets	Impairment of Long-lived Assets We evaluate long-lived assets classified as held and used for impairment when events or changes in circumstances indicate that the carrying value of such assets may not be recoverable. Whether an impairment has occurred is determined by comparing the estimated undiscounted cash flows attributable to the assets to the carrying value of the assets. If the carrying value exceeds the undiscounted cash flows, we recognize an impairment charge equal to the amount by which the carrying value exceeds the estimated fair value of the assets. In the period in which we determine that an asset meets held for sale criteria, we record an impairment charge to the extent the book value exceeds its estimated fair value less cost to sell. We did not identify any events or changes in circumstances that indicated that the carrying value of long-lived assets may not be recoverable in 2020 or 2019.
Variable Interest Entities	Variable Interest Entities As of December 31, 2020 and 2019, Ameren had unconsolidated variable interests as a limited partner in various equity method investments, totaling $37 million and $28 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 December 31, 2020, the maximum exposure to loss related to these variable interest entities is limited to the investment in these partnerships of $37 million plus associated outstanding funding commitments of $35 million .
Asset Retirement Obligations	Asset Retirement Obligations We record the estimated fair value of legal obligations associated with the retirement of tangible long-lived assets in the period in which the liabilities are incurred and capitalize a corresponding amount as part of the book value of the related long-lived asset. In subsequent periods, we adjust AROs for accretion and changes in the estimated fair values of the obligations, with a corresponding increase or decrease in the asset book value for the fair value changes. Asset book values, reflected within “Property, Plant, and Equipment, Net” on the balance sheet, are depreciated over the remaining useful life of the related asset. Due to regulatory recovery, that depreciation is deferred as a regulatory balance. The depreciation of the asset book values at Ameren Missouri was $28 million , $18 million, and $14 million for the years ended December 31, 2020, 2019, and 2018, respectively, which was deferred as a reduction to the net regulatory liability. The net regulatory liability also reflects a deferral for the nuclear decommissioning trust fund balance for the Callaway Energy Center. The depreciation deferred to the regulatory asset at Ameren Illinois was immaterial in each respective period. Uncertainties as to the probability, timing, or amount of cash expenditures associated with AROs affect our estimates of fair value. Ameren and Ameren Missouri have recorded AROs for retirement costs associated with decommissioning of Ameren Missouri’s Callaway and High Prairie Renewable energy centers, CCR facilities, and river structures. Additionally, Ameren, Ameren Missouri, and Ameren Illinois have recorded AROs for retirement costs associated with asbestos removal and the disposal of certain transformers. See Note 15 – Supplemental Information for a reconciliation of the beginning and ending carrying amounts of AROs. Estimated funds collected from customers to pay for the future removal cost of property, plant, and equipment retired from service, net of salvage, represent a cost of removal regulatory liability. See the cost of removal regulatory liability balance in Note 2 – Rate and Regulatory Matters.
Company-owned Life Insurance	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 December 31, 2020, the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $272 million (December 31, 2019 – $264 million) and $115 million (December 31, 2019 – $123 million), respectively, while total borrowings against the policies were $107 million (December 31, 2019 – $114 million) at both Ameren and Ameren Illinois. Ameren and Ameren Illinois have the right to offset the borrowings
Operating Revenues	Operating Revenues We record revenues from contracts with customers for various electric and natural gas services, which primarily consist of retail distribution, electric transmission, and off-system arrangements. When more than one performance obligation exists in a contract, the consideration under the contract is allocated to the performance obligations based on the relative standalone selling price. Electric and natural gas retail distribution revenues are earned when the commodity is delivered to our customers. We accrue an estimate of electric and natural gas retail distribution revenues for service provided but unbilled at the end of each accounting period. Electric transmission revenues are earned as electric transmission services are provided. Off-system revenues are primarily comprised of MISO revenues and wholesale bilateral revenues. MISO revenues include the sale of electricity, capacity, and ancillary services. Wholesale bilateral revenues include the sale of electricity and capacity. MISO-related electricity and wholesale bilateral electricity revenues are earned as electricity is delivered. Capacity and ancillary service revenues are earned as services are provided. Retail distribution, electric transmission, and off-system revenues, including the underlying components described above, represent a series of goods or services that are substantially the same and have the same pattern of transfer over time to our customers. Revenues from contracts with customers are equal to the amounts billed and our estimate of electric and natural gas retail distribution services provided but unbilled at the end of each accounting period. Customers are billed at least monthly, and payments are due less than one month after goods and/or services are provided. See Note 16 – Segment Information for disaggregated revenue information. For certain regulatory recovery mechanisms that are alternative revenue programs rather than revenues from contracts with customers, we recognize revenues that have been authorized for rate recovery, are objectively determinable and probable of recovery, and are expected to be collected from customers within two years from the end of the year. Our alternative revenue programs include revenue requirement reconciliations, the MEEIA, the VBA, and the DCA. These revenues are subsequently recognized as revenues from contracts with customers when billed, with an offset to alternative revenue program revenues. As of December 31, 2020 and 2019, our remaining performance obligations 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.
Cost Of Sales	Accounting for MISO Transactions MISO-related purchase and sale transactions are recorded by Ameren, Ameren Missouri, and Ameren Illinois using settlement information provided by the MISO. Ameren Missouri records these purchase and sale transactions on a net hourly position. Ameren Missouri records net purchases in a single hour in “Operating Expenses – Purchased power” and net sales in a single hour in “Operating Revenues – Electric” in its statement of income. Ameren Illinois records net purchases in “Operating Expenses – Purchased power” in its statement of income to reflect all of its MISO transactions relating to the procurement of power for its customers. On occasion, Ameren Missouri’s and Ameren Illinois’ prior-period transactions will be resettled outside the routine settlement process because of a change in the MISO’s tariff or a material interpretation thereof. In these cases, Ameren Missouri and Ameren Illinois recognize revenues and expenses associated with resettlements once the resettlement is probable and the resettlement amount can be estimated. There were no material MISO resettlements in 2020, 2019, or 2018.
Stock-Based Compensation	Stock-based Compensation Stock-based compensation cost is measured at the grant date based on the fair value of the award, net of an assumed forfeiture rate. Ameren recognizes as compensation expense the estimated fair value of stock-based compensation on a straight-line basis over the requisite vesting period. See Note 11 – Stock-based Compensation for additional information.
Unamortized Debt Discount, Premium, And Expense	Unamortized Debt Discounts, Premiums, and Issuance Costs Long-term debt discounts, premiums, and issuance costs are amortized over the lives of the related issuances. Credit agreement fees are amortized over the term of the agreement.
Income Taxes	Income Taxes Ameren uses an asset and liability approach for its financial accounting and reporting of income taxes. Deferred tax assets and liabilities are recognized for transactions that are treated differently for financial reporting and income tax return purposes. These deferred tax assets and liabilities are based on statutory tax rates. We expect that regulators will reduce future revenues for deferred tax liabilities that were initially recorded at rates in excess of the current statutory rate. Therefore, reductions in certain deferred tax liabilities that were recorded because of decreases in the statutory rate have been credited to a regulatory liability. A regulatory asset has been established to recognize the probable recovery through future customer rates of tax benefits related to the equity component of allowance for funds used during construction, as well as the effects of tax rate increases. To the extent deferred tax balances are included in rate base, the revaluation of deferred taxes is recorded as a regulatory asset or liability on the balance sheet and will be collected from, or refunded to, customers. For deferred tax balances not included in rate base, the revaluation of deferred taxes is recorded as an adjustment to income tax expense on the income statement. See Note 12 – Income Taxes for further information regarding the revaluation of deferred taxes related to Missouri state corporate income tax rate changes. Ameren Missouri, Ameren Illinois, and all the other Ameren subsidiary companies are parties to a tax allocation agreement with Ameren (parent) that provides for the allocation of consolidated tax liabilities. The tax allocation agreement specifies that each party be allocated an amount of tax using a stand-alone calculation, which is similar to what would be owed or refunded had the party been separately subject to tax without considering the impact of consolidation. Any net benefit attributable to Ameren (parent) is reallocated to the other parties. This reallocation is treated as a capital contribution to the party receiving the benefit. See Note 13 – Related-party Transactions for information regarding capital contributions under the tax allocation agreement.
Accounting Changes and Other Matters	Accounting Changes and Other Matters In the first quarter of 2020, the Ameren Companies adopted authoritative accounting guidance that requires credit losses on most financial assets carried at amortized cost and off-balance sheet credit exposures, such as financial guarantees or loan commitments, to be measured using a current expected credit loss (CECL) model. The guidance requires an entity to measure expected credit losses using relevant information about past events, current conditions, and reasonable and supportable forecasts that affect the collectibility of the reported amount. In addition, the guidance made certain changes to the impairment model applicable to available-for-sale debt securities, such as requiring credit losses to be presented as an allowance rather than a write-down on impaired debt securities for which there is neither an intent nor a more-likely-than-not requirement to sell. Our adoption of this guidance did not have a material impact on the Ameren Companies’ financial statements and did not result in a cumulative effect adjustment to retained earnings as of the adoption date. See Note 15 – Supplemental Information for additional information regarding credit losses on accounts receivable. In the first quarter of 2020, the Ameren Companies adopted authoritative accounting guidance that altered certain disclosure requirements in relation to fair value measurements. See Note 8 – Fair Value Measurements for our fair value measurement disclosures.
Derivatives, Policy	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
Noncontrolling Interests	Noncontrolling Interests As of December 31, 2020 and 2019, Ameren’s noncontrolling interests included the preferred stock of Ameren Missouri and Ameren Illinois.
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.
Earnings Per Share, Policy	Earnings per ShareEarnings per basic and diluted share are computed by dividing “Net Income Attributable to Ameren Common Shareholders” by the weighted-average number of basic and diluted common shares outstanding, respectively, during the applicable period. The weighted-average shares outstanding for earnings per diluted share includes the incremental effects resulting from performance share units, restricted stock units, and the forward sale agreement relating to common stock when the impact would be dilutive, as calculated using the treasury stock method.

Rate And Regulatory Matters (Ta

Rate And Regulatory Matters (Tables)	12 Months Ended
	Dec. 31, 2020
Public Utilities, General Disclosures [Abstract]
Schedule Of Regulatory Assets And Liabilities	The following table presents our regulatory assets and regulatory liabilities at December 31, 2020 and 2019: 2020 2019 Ameren Ameren Ameren Ameren Ameren Ameren Regulatory assets: Under-recovered FAC (a) $ 48 $ — $ 48 $ — $ — $ — Under-recovered Illinois electric power costs (b) — 4 4 — 4 4 MTM derivative losses (c) 21 200 221 12 242 254 IEIMA revenue requirement reconciliation adjustment (d)(e) — — — — 17 17 FERC revenue requirement reconciliation adjustment (f) — 28 50 — 1 16 Under-recovered VBA (g) — 11 11 — — — Pension and postretirement benefit costs (h) — — — 7 26 33 Income taxes (i) 117 65 183 114 61 177 Bad debt rider (j) — 11 11 — — — Callaway costs (e)(k) 14 — 14 18 — 18 Callaway refueling and maintenance outage costs (l) 39 — 39 — — — Unamortized loss on reacquired debt (m) 52 22 74 55 31 86 Environmental cost riders (n) — 93 93 — 127 127 Storm costs (e)(o) — 7 7 — 7 7 Workers’ compensation claims (p) 4 5 9 4 7 11 Allowance for funds used during construction for pollution control equipment (e)(q) 15 — 15 15 — 15 Customer generation rebate program (e)(r) — 17 17 — 1 1 Solar rebate program (s) 5 — 5 5 — 5 PISA (e)(t) 78 — 78 41 — 41 RESRAM (u) 2 — 2 9 — 9 FEJA energy-efficiency rider (e)(v) — 283 283 — 211 211 Other 12 33 45 13 16 29 Total regulatory assets $ 407 $ 779 $ 1,209 $ 293 $ 751 $ 1,061 Less: current regulatory assets (60) (37) (109) (8) (57) (69) Noncurrent regulatory assets $ 347 $ 742 $ 1,100 $ 285 $ 694 $ 992 Regulatory liabilities: Over-recovered FAC (a) $ 10 $ — $ 10 $ 39 $ — $ 39 Over-recovered Illinois electric power costs (b) — 15 15 — 11 11 Over-recovered PGA (b) 7 15 22 8 14 22 Over-recovered VBA (g) — — — — 8 8 MTM derivative gains (c) 11 10 21 18 3 21 IEIMA revenue requirement reconciliation adjustment (d) — 22 22 — 18 18 FERC revenue requirement reconciliation adjustment (f) — 21 21 — 37 38 Estimated refund for FERC complaint cases (w) — 7 15 — 23 40 Income taxes (i) 1,317 790 2,192 1,428 813 2,326 Cost of removal (x) 1,027 873 1,923 1,041 827 1,884 AROs (y) 436 — 436 303 — 303 Pension and postretirement benefit costs (h) 198 177 375 — — — Pension and postretirement benefit costs tracker (z) 55 — 55 72 — 72 Renewable energy credits and zero emission credits (aa) — 200 200 — 155 155 Excess income taxes collected in 2018 (ab) 45 — 45 60 — 60 Other 30 21 51 30 24 54 Total regulatory liabilities $ 3,136 $ 2,151 $ 5,403 $ 2,999 $ 1,933 $ 5,051 Less: current regulatory liabilities (26) (88) (121) (62) (84) $ (164) Noncurrent regulatory liabilities $ 3,110 $ 2,063 $ 5,282 $ 2,937 $ 1,849 $ 4,887 (a) Under-recovered or over-recovered fuel costs to be recovered or refunded through the FAC. Specific accumulation periods aggregate the under-recovered or over-recovered costs over four months, any related adjustments that occur over the following four months, and the recovery from, or refund to, customers that occurs over the next eight months. (b) Under-recovered or over-recovered costs from utility customers. Amounts will be recovered from, or refunded to, customers within one year of the deferral. (c) Deferral of commodity-related derivative MTM losses or gains. See Note 7 – Derivative Financial Instruments for additional information. (d) The difference between Ameren Illinois’ electric distribution service annual revenue requirement calculated under the performance-based formula ratemaking framework and the revenue requirement included in customer rates for that year. Any under-recovery or over-recovery will be recovered from, or refunded to, customers with interest within two years. (e) These assets earn a return at the applicable WACC. (f) Ameren Illinois’ and ATXI’s annual revenue requirement reconciliation calculated pursuant to the FERC’s electric transmission formula ratemaking framework. Any under-recovery or over-recovery will be recovered from, or refunded to, customers within two years. (g) Under-recovered or over-recovered natural gas revenue caused by sales volume deviations from weather normalized sales approved by the ICC in rate regulatory reviews. Each year’s amount will be recovered from or refunded to customers from April through December of the following year. (h) Under-recovered or over-recovered costs are being amortized in proportion to the recognition of prior service costs (credits) and actuarial losses (gains) attributable to Ameren’s pension plan and postretirement benefit plans. See Note 10 – Retirement Benefits for additional information. (i) The regulatory assets represent amounts that will be recovered from customers for deferred income taxes related to the equity component of allowance for funds used during construction and the effects of tax rate changes. The regulatory liabilities represent amounts that will be refunded to customers for deferred income taxes related to depreciation differences, other tax liabilities, and the unamortized portion of investment tax credits recorded at rates in excess of current statutory rates. Amounts associated with the equity component of allowance for funds used during construction, and the unamortized portion of investment tax credits will be amortized over the expected life of the related assets. For net regulatory liabilities related to deferred income taxes recorded at rates other than the current statutory rate, the weighted-average remaining amortization periods at Ameren, Ameren Missouri, and Ameren Illinois are 34, 26, and 42 years. (j) A rider for the difference between the level of bad debt write-offs, net of any subsequent recoveries, incurred by Ameren Illinois and the level of such costs included in electric distribution and natural gas delivery service rates. Pursuant to the June 2020 ICC order in the service disconnection moratorium proceeding discussed above, Ameren Illinois’ electric distribution bad debt rider provided for the recovery of bad debt expense in 2020. The under-recovery or over-recovery for each year is recovered from, or refunded to, customers over a twelve-month period beginning June the following year. (k) Ameren Missouri’s Callaway Energy Center operations and maintenance expenses, property taxes, and carrying costs incurred between the plant in-service date and the date the plant was reflected in rates. These costs are being amortized over the original remaining life of the energy center. (l) In February 2020, the MoPSC issued an order approving a stipulation and agreement allowing Ameren Missouri to defer and amortize maintenance expenses related to scheduled refueling and maintenance outages at its Callaway Energy Center. Maintenance expenses are amortized over the period between refueling and maintenance outages, which has historically been approximately 18 months. Amortization began in January 2021 and will continue until the completion of the next refueling and maintenance outage. (m) Losses related to reacquired debt. These amounts are being amortized over the lives of the related new debt issuances or the original lives of the old debt issuances if no new debt was issued. (n) The recoverable portion of accrued environmental site liabilities that will be collected from electric and natural gas customers through ICC-approved cost recovery riders. The period of recovery will depend on the timing of remediation expenditures. See Note 14 – Commitments and Contingencies for additional information. (o) Storm costs from 2016, 2018, and 2020 deferred in accordance with the IEIMA. These costs are being amortized over five-year periods beginning in the year the storm occurred. (p) The period of recovery will depend on the timing of actual expenditures. (q) The MoPSC’s May 2010 electric rate order allowed Ameren Missouri to record an allowance for funds used during construction for pollution control equipment at its Sioux Energy Center until the cost of that equipment was included in customer rates beginning in 2011. These costs are being amortized over the expected life of the Sioux Energy Center, currently through 2033. (r) Costs associated with Ameren Illinois’ customer generation rebate program. Costs are amortized over a 15-year period, beginning in the year rebates are paid. (s) Costs associated with Ameren Missouri’s solar rebate program. The amortization period for these assets will be determined in a future electric service regulatory rate review. (t) Under the PISA, Ameren Missouri is permitted to defer and recover 85% of the depreciation expense and earn a return at the applicable WACC on investments in certain property, plant, and equipment placed in service after September 1, 2018, and not included in base rates. Accumulated PISA deferrals are added to rate base prospectively and amortized over a period of 20 years following a regulatory rate review. (u) Costs associated with Ameren Missouri’s compliance with the state of Missouri’s renewable energy standard. Costs incurred over a twelve-month period beginning each August are amortized over a twelve-month period beginning February the following year. (v) The electric energy-efficiency investments are being amortized over their weighted-average useful lives beginning in the period in which they were made, with current remaining amortization periods ranging from 6 to 13 years. (w) The 2020 balances represent the estimated refunds to transmission customers related to the May 2020 FERC order in the November 2013 FERC complaint case. The 2019 balances represent the estimated refunds to transmission customers related to the November 2019 FERC order in the November 2013 FERC complaint case. See further discussion of the FERC ROE complaint cases above. (x) Estimated funds collected from customers to pay for the future removal cost of property, plant, and equipment retired from service, net of salvage. (y) The ARO regulatory liability includes the nuclear decommissioning trust fund balance (December 31, 2020 - $982 million), net of recoverable removal costs for AROs (December 31, 2020 - $546 million). See Note 1 – Summary of Significant Accounting Policies – Asset Retirement Obligations. (z) A regulatory recovery mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri and the level of such costs included in customer rates. The period of refund varies based on MoPSC approval in a regulatory rate review. For costs incurred prior to 2019, the weighted-average remaining amortization period is four years. For costs incurred during 2019 and after, the amortization period will be determined in a future electric service regulatory rate review. (aa) Funds collected for the purchase of renewable energy credits and zero emission credits through IPA procurements. The balance will be amortized as the credits are purchased. (ab) The excess amount collected in rates related to the TCJA from January 1, 2018, through July 31, 2018. The regulatory liability is being amortized over a three-year period, which began in April 2020.

Property And Plant, Net (Tables

Property And Plant, Net (Tables)	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Schedule Of Property And Plant, Net	The following table presents property, plant, and equipment, net, at December 31, 2020 and 2019: Ameren Missouri (a) Ameren Other Ameren (a) 2020 Property, plant, and equipment at original cost: (b) Electric generation: Coal (c) $ 4,875 $ — $ — $ 4,875 Natural gas and oil 1,097 — — 1,097 Nuclear 5,608 — — 5,608 Renewable (d) 1,301 — — 1,301 Electric distribution 6,784 6,649 — 13,433 Electric transmission 1,482 3,575 1,774 6,831 Natural gas 561 3,308 — 3,869 Other (e) 1,390 1,070 245 2,705 23,098 14,602 2,019 39,719 Less: Accumulated depreciation and amortization 9,689 3,780 304 13,773 13,409 10,822 1,715 25,946 Construction work in progress: Nuclear fuel in process 75 — — 75 Other 395 379 12 786 Property, plant, and equipment, net $ 13,879 $ 11,201 $ 1,727 $ 26,807 2019 Property, plant, and equipment at original cost: (b) Electric generation: Coal (c) $ 4,730 $ — $ — $ 4,730 Natural gas and oil 1,090 — — 1,090 Nuclear 5,414 — — 5,414 Renewable (d) 646 — — 646 Electric distribution 6,371 6,299 — 12,670 Electric transmission 1,405 3,101 1,642 6,148 Natural gas 528 3,024 — 3,552 Other (e) 1,173 993 236 2,402 21,357 13,417 1,878 36,652 Less: Accumulated depreciation and amortization 9,195 3,536 275 13,006 12,162 9,881 1,603 23,646 Construction work in progress: Nuclear fuel in process 135 — — 135 Other 338 202 55 595 Property, plant, and equipment, net $ 12,635 $ 10,083 $ 1,658 $ 24,376 (a) Amounts include two CTs that have related financing obligations. The gross cumulative asset value of those agreements was $240 million and $236 million at December 31, 2020 and 2019, respectively. The total accumulated depreciation associated with the two CTs was $99 million and $95 million at December 31, 2020 and 2019, respectively. See Note 5 – Long-term Debt and Equity Financings for additional information on these agreements. (b) The estimated lives for each asset group are as follows: 5 to 72 years for electric generation, excluding Ameren Missouri’s hydro generating assets, which have useful lives of up to 150 years, 20 to 80 years for electric distribution, 50 to 75 years for electric transmission, 20 to 80 years for natural gas, and 5 to 55 years for other. (c) Includes $36 million of oil-fired generation in both 2019 and 2020. (d) Renewable includes hydroelectric, wind, solar, and methane gas generation facilities. (e) Other property, plant, and equipment includes assets used to support electric and natural gas services.
Schedule of Capitalized Software	Capitalized software costs are classified within “Property, Plant, and Equipment, Net” on the balance sheet and are amortized on a straight-line basis over the expected period of benefit, ranging from 3 to 10 years. The following table presents the amortization, gross carrying value, and related accumulated amortization of capitalized software by year: Amortization Expense Gross Carrying Value Accumulated Amortization 2020 2019 2018 2020 2019 2020 2019 Ameren $ 93 $ 78 $ 71 $ 1,021 $ 901 $ (640) $ (584) Ameren Missouri 44 30 24 398 303 (189) (153) Ameren Illinois 45 45 44 397 377 (238) (221)
Schedule of Capitalized Software, Future Amortization Expense	Annual amortization expense for capitalized costs for software placed in service as of December 31, 2020, is estimated to be as follows: 2021 2022 2023 2024 2025 Ameren $ 105 $ 94 $ 81 $ 55 $ 23 Ameren Missouri 55 49 45 33 15 Ameren Illinois 46 42 34 21 7

Short-Term Debt And Liquidity (

Short-Term Debt And Liquidity (Tables)	12 Months Ended
	Dec. 31, 2020
Line of Credit Facility [Abstract]
Schedule Of Maximum Aggregate Amount Available On Credit Agreements	The following table presents the maximum aggregate amount available to each borrower under each facility: Missouri Illinois Ameren (parent) $ 900 $ 500 Ameren Missouri 850 (a) Ameren Illinois (a) 800 (a) Not applicable.
Schedule of Commercial Paper	The following table summarizes the borrowing activity and relevant interest rates under Ameren (parent)’s, Ameren Missouri’s, and Ameren Illinois’ commercial paper issuances and borrowings under the Credit Agreements in the aggregate for the years ended December 31, 2020 and 2019: Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2020 Average daily amount outstanding $ 108 $ 109 $ 46 $ 263 Commercial paper issuances outstanding at period-end 490 — — 490 Weighted-average interest rate 1.04 % 1.73 % 0.97 % 1.31 % Peak amount outstanding during period (a) $ 490 $ 573 $ 250 $ 908 Peak interest rate 3.30 % (b) 5.05 % (b) 3.40 % 5.05 % (b) 2019 Average daily amount outstanding $ 421 $ 122 $ 157 $ 700 Commercial paper issuances outstanding at period-end 153 234 53 440 Weighted-average interest rate 2.66 % 2.62 % 2.43 % 2.60 % Peak amount outstanding during period (a) $ 651 $ 549 $ 356 $ 1,113 Peak interest rate 3.80 % (c) 2.97 % 5.00 % (c) 5.00 % (c) (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 amount 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. (c) Ameren’s and Ameren Illinois’ peak interest rate was affected by temporary disruptions in the commercial paper market in the third quarter of 2019.

Long-Term Debt And Equity Fin_2

Long-Term Debt And Equity Financings (Tables)	12 Months Ended
	Dec. 31, 2020
Debt Instrument [Line Items]
Schedule of Long-term Debt Instruments	The following table presents long-term debt outstanding, including maturities due within one year, as of December 31, 2020 and 2019: 2020 2019 Ameren (Parent): 2.70% Senior unsecured notes due 2020 $ — $ 350 2.50% Senior unsecured notes due 2024 450 450 3.65% Senior unsecured notes due 2026 350 350 3.50% Senior unsecured notes due 2031 800 — Total long-term debt, gross 1,600 1,150 Less: Unamortized discount and premium (2) — Less: Unamortized debt issuance costs (10) (6) Less: Maturities due within one year — (350) Long-term debt, net $ 1,588 $ 794 Ameren Missouri: Bonds and notes: 5.00% Senior secured notes due 2020 (a) — 85 1.60% 1992 Series bonds due 2022 (b) 47 47 3.50% Senior secured notes due 2024 (a) 350 350 2.95% Senior secured notes due 2027 (a) 400 400 3.50% First mortgage bonds due 2029 (d) 450 450 2.95% First mortgage bonds due 2030 (d) 465 — 2.90% 1998 Series A bonds due 2033 (b) 60 60 2.90% 1998 Series B bonds due 2033 (b) 50 50 2.75% 1998 Series C bonds due 2033 (b) 50 50 5.50% Senior secured notes due 2034 (a) 184 184 5.30% Senior secured notes due 2037 (a) 300 300 8.45% Senior secured notes due 2039 (a)(c) 350 350 3.90% Senior secured notes due 2042 (a)(c) 485 485 3.65% Senior secured notes due 2045 (a) 400 400 4.00% First mortgage bonds due 2048 (d) 425 425 3.25% First mortgage bonds due 2049 (d) 330 330 2.625% First mortgage bonds due 2051 (green bonds) (d) 550 — Finance obligations: City of Bowling Green agreement (Peno Creek CT) due 2022 (e) 16 23 Audrain County agreement (Audrain County CT) due 2023 (e) 240 240 Total long-term debt, gross 5,152 4,229 Less: Unamortized discount and premium (12) (9) Less: Unamortized debt issuance costs (36) (30) Less: Maturities due within one year (8) (92) Long-term debt, net $ 5,096 $ 4,098 2020 2019 Ameren Illinois: Bonds and notes: 2.70% Senior secured notes due 2022 (f)(g) $ 400 $ 400 3.25% Senior secured notes due 2025 (f) 300 300 6.125% Senior secured notes due 2028 (f) 60 60 3.80% First mortgage bonds due 2028 (h) 430 430 1.55% First mortgage bonds due 2030 (h) 375 — 6.70% Senior secured notes due 2036 (f) 61 61 6.70% Senior secured notes due 2036 (f) 42 42 4.80% Senior secured notes due 2043 (f) 280 280 4.30% Senior secured notes due 2044 (f) 250 250 4.15% Senior secured notes due 2046 (f) 490 490 3.70% First mortgage bonds due 2047 (h) 500 500 4.50% First mortgage bonds due 2049 (h) 500 500 3.25% First mortgage bonds due 2050 (h) 300 300 Total long-term debt, gross 3,988 3,613 Less: Unamortized discount and premium (6) (4) Less: Unamortized debt issuance costs (36) (34) Long-term debt, net $ 3,946 $ 3,575 ATXI: 3.43% Senior unsecured notes due 2050 (i) $ 450 $ 450 Total long-term debt, gross 450 450 Less: Unamortized debt issuance costs (2) (2) Long-term debt, net $ 448 $ 448 Ameren consolidated long-term debt, net $ 11,078 $ 8,915 (a) These notes are collaterally secured by first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any first mortgage bonds issued under the Ameren Missouri mortgage indenture remain outstanding. Redemption, purchase, or maturity of all first mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the first mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the 2051 maturity of the 2.625% first mortgage bonds and the restrictions preventing a release date to occur that are attached to certain senior secured notes described in footnote (d) below, Ameren Missouri does not expect the first mortgage lien protection associated with these notes to fall away. (b) These bonds are collaterally secured by first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage indenture and have a fall-away lien provision similar to that of Ameren Missouri’s senior secured notes. (c) Ameren Missouri has agreed that so long as any of the 3.90% senior secured notes due 2042 are outstanding, Ameren Missouri will not permit a release date to occur, and so long as any of the 8.45% senior secured notes due 2039 are outstanding, Ameren Missouri will not optionally redeem, purchase, or otherwise retire in full the outstanding first mortgage bonds not subject to release provisions. (d) These bonds are first mortgage bonds issued by Ameren Missouri under the Ameren Missouri bond indenture. They are secured by substantially all Ameren Missouri property and franchises. (e) Payments due related to these financing obligations are paid to a trustee, which is authorized to utilize the cash only to pay equal amounts due to Ameren Missouri under related bonds issued by the city/county and held by Ameren Missouri. The timing and amounts of payments due from Ameren Missouri under the agreements are equal to the timing and amount of bond service payments due to Ameren Missouri, resulting in no net cash flow. The balance of both the financing obligations and the related investments in debt securities, recorded in “Other Assets,” was $256 million and $263 million, respectively, as of December 31, 2020 and 2019. (f) These notes are collaterally secured by first mortgage bonds issued by Ameren Illinois under the Ameren Illinois mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any first mortgage bonds issued under its mortgage indenture remain outstanding. Redemption, purchase, or maturity of all first mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the first mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the 2050 maturity date of the 3.25% first mortgage bonds, Ameren Illinois does not expect the first mortgage lien protection associated with these notes to fall away. (g) Ameren Illinois has agreed that so long as any of the 2.70% senior secured notes due 2022 are outstanding, Ameren Illinois will not permit a release date to occur. (h) These bonds are first mortgage bonds issued by Ameren Illinois under the Ameren Illinois mortgage indenture. They are secured by substantially all Ameren Illinois property and franchises.
Schedule Of Maturities Of Long-Term Debt	The following table presents the principal maturities schedule for the 3.43% senior notes due 2050: Payment Date Principal Payment August 2022 $ 49.5 August 2024 49.5 August 2027 49.5 August 2030 49.5 August 2032 49.5 August 2038 49.5 August 2043 76.5 August 2050 76.5 Total $ 450.0 The following table presents the aggregate maturities of long-term debt, including current maturities, at December 31, 2020: Ameren (parent) (a) Ameren Missouri (a) Ameren Illinois (a) ATXI (a) Ameren Consolidated (a) 2021 $ — $ 8 $ — $ — $ 8 2022 — 55 400 50 505 2023 — 240 — — 240 2024 450 350 — 50 850 2025 — — 300 — 300 Thereafter 1,150 4,499 3,288 350 9,287 Total $ 1,600 $ 5,152 $ 3,988 $ 450 $ 11,190 (a) Excludes unamortized discount, unamortized premium, and debt issuance costs of $12 million, $48 million, $42 million, and $2 million at Ameren (parent), Ameren Missouri, Ameren Illinois, and ATXI, respectively.
Schedule Of Outstanding Preferred Stock	The following table presents the outstanding preferred stock of Ameren Missouri and Ameren Illinois, which is redeemable at the option of the issuer, at the prices shown below as of December 31, 2020 and 2019: Shares Outstanding Redemption Price (per share) 2020 2019 Ameren Missouri: Without par value and stated value of $100 per share, 25 million shares authorized $3.50 Series 130,000 shares $ 110.00 $ 13 $ 13 $3.70 Series 40,000 shares 104.75 4 4 $4.00 Series 150,000 shares 105.625 15 15 $4.30 Series 40,000 shares 105.00 4 4 $4.50 Series 213,595 shares 110.00 (a) 21 21 $4.56 Series 200,000 shares 102.47 20 20 $4.75 Series 20,000 shares 102.176 2 2 $5.50 Series A 14,000 shares 110.00 1 1 Total $ 80 $ 80 Ameren Illinois: With par value of $100 per share, 2 million shares authorized 4.00% Series 144,275 shares $ 101.00 $ 14 $ 14 4.08% Series 45,224 shares 103.00 5 5 4.20% Series 23,655 shares 104.00 2 2 4.25% Series 50,000 shares 102.00 5 5 4.26% Series 16,621 shares 103.00 2 2 4.42% Series 16,190 shares 103.00 2 2 4.70% Series 18,429 shares 104.30 2 2 4.90% Series 73,825 shares 102.00 7 7 4.92% Series 49,289 shares 103.50 5 5 5.16% Series 50,000 shares 102.00 5 5 6.625% Series 124,274 shares 100.00 12 12 7.75% Series 4,542 shares 100.00 1 1 Total $ 62 $ 62 Total Ameren $ 142 $ 142 (a) In the event of voluntary liquidation, $105.50.
Schedule of Remarketed Bonds	The following table provides additional information on the bonds: 1992 Series 1998 Series A 1998 Series B 1998 Series C Transaction month June 2019 July 2019 July 2019 June 2019 Principal amount $47 $60 $50 $50 Fixed interest rate 1.60% 2.90% 2.90% 2.75% Variable interest rate (a) 2.58% 3.43% 3.57% 3.43% Maturity December 2022 September 2033 September 2033 September 2033 Interest payment dates June 1 and December 1 March 1 and September 1 March 1 and September 1 March 1 and September 1 Initial interest payment date December 2019 September 2019 September 2019 September 2019 (a) Represents the variable interest rate of the bonds effective prior to the change in method of determining the interest rate.
Schedule of Required and Actual Debt Ratios	The following table summarizes the required and actual interest coverage ratios for interest charges, dividend coverage ratios, and bonds and preferred stock issuable as of December 31, 2020, at an assumed interest rate of 5% and dividend rate of 6%. Required Interest Coverage Ratio (a) Actual Interest Bonds Issuable (b) Required Dividend Coverage Ratio (c) Actual Dividend Preferred Stock Ameren Missouri > 2.0 3.4 $5,025 > 2.5 128.5 $2,872 Ameren Illinois > 2.0 7.1 7,229 > 1.5 3.3 203 (d) (a) Coverage required on the annual interest charges on first mortgage bonds outstanding and to be issued. Coverage is not required in certain cases when additional first mortgage bonds are issued on the basis of retired bonds. (b) Amount of bonds issuable based either on required coverage ratios or unfunded property additions, whichever is more restrictive. The amounts shown also include bonds issuable based on retired bond capacity of $2,442 million and $643 million at Ameren Missouri and Ameren Illinois, respectively. (c) Coverage required on the annual dividend on preferred stock outstanding and to be issued, as required in the respective company’s articles of incorporation. (d) Preferred stock issuable is restricted by the amount of preferred stock that is currently authorized by Ameren Illinois’ articles of incorporation.

Other Income, Net (Tables)

Other Income, Net (Tables)	12 Months Ended
	Dec. 31, 2020
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 years ended December 31, 2020, 2019, and 2018: 2020 2019 2018 Ameren: Other Income, Net Allowance for equity funds used during construction $ 32 $ 28 $ 36 Interest income on industrial development revenue bonds 25 25 26 Other interest income 4 8 7 Non-service cost components of net periodic benefit income (a) 116 90 70 Miscellaneous income 13 6 8 Donations (25) (b) (12) (33) Miscellaneous expense (14) (15) (12) Total Other Income, Net $ 151 $ 130 $ 102 Ameren Missouri: Other Income, Net Allowance for equity funds used during construction $ 19 $ 19 $ 27 Interest income on industrial development revenue bonds 25 25 26 Other interest income 1 1 2 Non-service cost components of net periodic benefit income (a) 46 18 17 Miscellaneous income 4 5 4 Donations (12) (b) (3) (14) Miscellaneous expense (7) (7) (6) Total Other Income, Net $ 76 $ 58 $ 56 Ameren Illinois: Other Income, Net Allowance for equity funds used during construction $ 13 $ 9 $ 9 Interest income 3 6 6 Non-service cost components of net periodic benefit income (a) 48 47 34 Miscellaneous income 6 3 3 Donations (5) (5) (6) Miscellaneous expense (6) (7) (4) Total Other Income, Net $ 59 $ 53 $ 42 (a) For the years ended December 31, 2020, 2019, and 2018, the non-service cost components of net periodic benefit income were adjusted by amounts deferred of $(4) million, $29 million, and $17 million, respectively, due to a tracker for the difference between the level of such costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. (b) Includes $8 million pursuant to Ameren Missouri’s March 2020 electric rate order. See Note 2 – Rate and Regulatory Matters for additional information. The following table presents the components of “Other Expense, Net” in the Condensed Statement of Income and Comprehensive Income for the years ended December 31, 2020, 2019, and 2018: (In millions) 2020 2019 2018 Other Expense, Net Non-service cost components of net periodic benefit income $ 1 $ 2 $ 2 Donations (8) (3) (13) Other expense, net (1) (1) (1) Total Other Expense, Net $ (8) $ (2) $ (12)

Derivative Financial Instrume_2

Derivative Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2020
Derivative Instrument Detail [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 December 31, 2020 and 2019. As of December 31, 2020, these contracts extended through October 2023, March 2026, May 2032, and March 2023 for fuel oils, natural gas, power, and uranium, respectively. Quantity (in millions, except as indicated) 2020 2019 Commodity Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils (in gallons) 43 — 43 58 — 58 Natural gas (in mmbtu) 33 114 147 20 136 156 Power (in MWhs) 6 7 13 5 7 12 Uranium (pounds in thousands) 365 — 365 565 — 565
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 December 31, 2020 and 2019: 2020 2019 Commodity Balance Sheet Location Ameren Ameren Ameren Ameren Ameren Ameren Fuel oils Other current assets $ 2 $ — $ 2 $ 4 $ — $ 4 Other assets — — — 2 — 2 Natural gas Other current assets 1 8 9 — 3 3 Other assets 2 2 4 — 1 1 Power Other current assets 7 — 7 14 — 14 Other assets — — — 2 — 2 Total assets $ 12 $ 10 $ 22 $ 22 $ 4 $ 26 Fuel oils Other current liabilities $ 7 $ — $ 7 $ 4 $ — $ 4 Other deferred credits and liabilities 2 — 2 3 — 3 Natural gas Other current liabilities 1 1 2 1 12 13 Other deferred credits and liabilities — 1 1 1 6 7 Power Other current liabilities 3 17 20 2 17 19 Other deferred credits and liabilities 8 181 189 1 207 208 Uranium Other deferred credits and liabilities — — — 1 — 1 Total liabilities $ 21 $ 200 $ 221 $ 13 $ 242 $ 255

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2020
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 December 31, 2020 and 2019: December 31, 2020 December 31, 2019 Level 1 Level 2 Level 3 Total Level 1 Level 2 Level 3 Total Assets: Ameren Missouri Derivative assets – commodity contracts: Fuel oils $ — $ — $ 2 $ 2 $ — $ — $ 6 $ 6 Natural gas — 3 — 3 — — — — Power 2 — 5 7 — 2 14 16 Total derivative assets – commodity contracts $ 2 $ 3 $ 7 $ 12 $ — $ 2 $ 20 $ 22 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 680 $ — $ — $ 680 $ 569 $ — $ — $ 569 Debt securities: U.S. Treasury and agency securities — 115 — 115 — 107 — 107 Corporate bonds — 115 — 115 — 93 — 93 Other — 67 — 67 — 73 — 73 Total nuclear decommissioning trust fund $ 680 $ 297 $ — $ 977 (a) $ 569 $ 273 $ — $ 842 (a) Total Ameren Missouri $ 682 $ 300 $ 7 $ 989 $ 569 $ 275 $ 20 $ 864 Ameren Illinois Derivative assets – commodity contracts: Natural gas $ — $ 6 $ 4 $ 10 $ — $ 1 $ 3 $ 4 Ameren Derivative assets – commodity contracts (b) $ 2 $ 9 $ 11 $ 22 $ — $ 3 $ 23 $ 26 Nuclear decommissioning trust fund (c) 680 297 — 977 (a) 569 273 — 842 (a) Total Ameren $ 682 $ 306 $ 11 $ 999 $ 569 $ 276 $ 23 $ 868 Liabilities: Ameren Missouri Derivative liabilities – commodity contracts: Fuel oils $ 6 $ — $ 3 $ 9 $ 1 $ — $ 6 $ 7 Natural gas — 1 — 1 — 2 — 2 Power 8 — 3 11 — 2 1 3 Uranium — — — — — — 1 1 Total Ameren Missouri $ 14 $ 1 $ 6 $ 21 $ 1 $ 4 $ 8 $ 13 Ameren Illinois Derivative liabilities – commodity contracts: Natural gas $ — $ 1 $ 1 $ 2 $ 3 $ 12 $ 3 $ 18 Power — — 198 198 — — 224 224 Total Ameren Illinois $ — $ 1 $ 199 $ 200 $ 3 $ 12 $ 227 $ 242 Ameren Derivative liabilities – commodity contracts (b) $ 14 $ 2 $ 205 $ 221 $ 4 $ 16 $ 235 $ 255 (a) Balance excludes $5 million and $5 million of cash and cash equivalents, receivables, payables, and accrued income, net for December 31, 2020 and 2019, 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 Ameren’s nuclear decommissioning trust fund by investment type.
Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level 3 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 years ended December 31, 2020 and 2019: 2020 2019 Ameren Ameren Ameren Ameren Ameren Ameren Beginning balance at January 1 $ 13 $ (224) $ (211) $ — $ (183) $ (183) Realized and unrealized gains (losses) included in regulatory assets/liabilities 15 8 23 23 (56) (33) Settlements (26) 18 (8) (7) 15 8 Transfers out of Level 3 — — — (3) — (3) Ending balance at December 31 $ 2 $ (198) $ (196) $ 13 $ (224) $ (211) Change in unrealized gains (losses) related to assets/liabilities held at December 31 $ 1 $ 9 $ 10 $ 12 $ (54) $ (42)
Fair Value Inputs, Assets and Liabilities, Quantitative Information	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 December 31, 2020 and 2019: Fair Value Weighted Average (b) Commodity Assets Liabilities Valuation Technique(s) Unobservable Input (a) Range 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 2019 Power (d) $ 14 $ (225) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) 22 – 34 25 Nodal basis ($/MWh) (6) – 0 (2) Trend rate (%) (1) – 0 0 (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. (d) Valuations through 2028 use visible forward prices adjusted for nodal-to-hub basis differentials. Valuations beyond 2028 use a trend rate factor and are similarly adjusted for nodal-to-hub basis differentials.
Schedule of Carrying Values and Estimated Fair Values of Debt Instruments [Table Text Block]	The following table sets forth, by level within the fair value hierarchy, the carrying amount and fair value of financial assets and liabilities disclosed, but not carried, at fair value as of December 31, 2020 and 2019: Carrying Fair Value Level 1 Level 2 Level 3 Total Ameren: December 31, 2020 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 December 31, 2019 Ameren: Cash, cash equivalents, and restricted cash $ 176 $ 176 $ — $ — $ 176 Investments in industrial development revenue bonds (a) 263 — 263 — 263 Short-term debt 440 — 440 — 440 Long-term debt (including current portion) (a) 9,357 (b) — 9,957 484 (c) 10,441 Ameren Missouri: Cash, cash equivalents, and restricted cash $ 39 $ 39 $ — $ — $ 39 Investments in industrial development revenue bonds (a) 263 — 263 — 263 Short-term debt 234 — 234 — 234 Long-term debt (including current portion) (a) 4,190 (b) — 4,772 — 4,772 Ameren Illinois: Cash, cash equivalents, and restricted cash $ 125 $ 125 $ — $ — $ 125 Short-term debt 53 — 53 — 53 Long-term debt (including current portion) 3,575 (b) — 4,019 — 4,019 (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 December 31, 2020 and 2019, 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 $84 million, $36 million, and $36 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2020. Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $72 million, $30 million, and $34 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2019. (c) The Level 3 fair value amount consists of ATXI’s senior unsecured notes.

Callaway Energy Center (Tables)

Callaway Energy Center (Tables)	12 Months Ended
	Dec. 31, 2020
Nuclear Waste Matters [Abstract]
Proceeds From Sale Of Investments In Nuclear Decommissioning Trust Fund And Gross Realized Gains And Losses	The following table presents proceeds from the sale and maturities of investments in Ameren Missouri’s nuclear decommissioning trust fund and the gross realized gains and losses resulting from those sales for the years ended December 31, 2020, 2019, and 2018: 2020 2019 2018 Proceeds from sales and maturities $ 183 $ 260 $ 299 Gross realized gains 10 10 18 Gross realized losses 3 2 5
Fair Value Of Securities In Nuclear Decommissioning Trust Fund	The following table presents the cost and fair value of investments in debt and equity securities in Ameren’s and Ameren Missouri’s nuclear decommissioning trust fund at December 31, 2020 and December 31, 2019: Security Type Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value 2020 Debt securities $ 272 $ 25 $ — $ 297 Equity securities 198 491 9 680 Cash and cash equivalents 4 — — 4 Other (a) 1 — — 1 Total $ 475 $ 516 $ 9 $ 982 2019 Debt securities $ 262 $ 11 $ — $ 273 Equity securities 183 393 7 569 Cash and cash equivalents 26 — — 26 Other (a) (21) — — (21) Total $ 450 $ 404 $ 7 $ 847 (a) Represents net receivables and payables relating to pending securities sales, interest, and securities purchases.
Fair Value Of Securities In Nuclear Decommissioning Trust Fund Classified by Contractual Maturity Date	The following table presents the costs and fair values of investments in debt securities in Ameren’s and Ameren Missouri’s nuclear decommissioning trust fund according to their contractual maturities at December 31, 2020: Cost Fair Value Less than 5 years $ 120 $ 123 5 years to 10 years 69 75 Due after 10 years 83 99 Total $ 272 $ 297
Schedule of Insurance Coverage	The following table presents insurance coverage at Ameren Missouri’s Callaway Energy Center at December 31, 2020: Type and Source of Coverage Most Recent Maximum Coverages Maximum Assessments Public liability and nuclear worker liability: American Nuclear Insurers January 1, 2021 $ 450 $ — Pool participation (a) 13,348 (a) 138 (b) $ 13,798 (c) $ 138 Property damage: NEIL and EMANI April 1, 2020 $ 3,200 (d) $ 25 (e) Replacement power: NEIL April 1, 2020 $ 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) Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity up to $4.5 million for 52 weeks, which commences after the first 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)

Retirement Benefits (Tables)	12 Months Ended
	Dec. 31, 2020
Summary Of Benefit Liability Recorded	The following table presents the net benefit liability/(asset) recorded on the balance sheets as of December 31, 2020 and December 31, 2019: 2020 2019 Ameren (a) $ (249) $ 216 Ameren Missouri (a) (25) 142 Ameren Illinois (a) (210) (16) (a) Assets associated with pension and other postretirement benefits are recorded in “Other assets” on the balance sheet.
Funded Status Of Benefit Plans And Amounts Included In Regulatory Assets And OCI	The following table presents the funded status of Ameren’s pension and postretirement benefit plans as of December 31, 2020 and December 31, 2019. It also provides the amounts included in regulatory assets or liabilities and accumulated OCI at December 31, 2020 and December 31, 2019, that have not been recognized in net periodic benefit costs. 2020 2019 Pension Postretirement Pension Postretirement Accumulated benefit obligation at end of year $ 5,213 $ (a) $ 4,735 $ (a) Change in benefit obligation: Net benefit obligation at beginning of year $ 4,967 $ 1,110 $ 4,459 $ 1,034 Service cost 110 19 88 18 Interest cost 174 39 187 43 Plan amendments — — — 2 Participant contributions — 8 — 8 Actuarial loss 508 91 469 69 Benefits paid (249) (63) (236) (64) Net benefit obligation at end of year 5,510 1,204 4,967 1,110 Change in plan assets: Fair value of plan assets at beginning of year 4,564 1,297 3,899 1,113 Actual return on plan assets 1,143 209 878 237 Employer contributions 52 2 23 3 Participant contributions — 8 — 8 Benefits paid (249) (63) (236) (64) Fair value of plan assets at end of year 5,510 1,453 4,564 1,297 Funded status – deficiency (surplus) — (249) 403 (187) Accrued benefit cost (asset) at December 31 $ — $ (249) $ 403 $ (187) Amounts recognized in the balance sheet consist of: Noncurrent asset (b) $ (39) $ (249) $ — $ (187) Current liability (c) 2 — 2 — Noncurrent liability 37 — 401 — Net liability (asset) recognized $ — $ (249) $ 403 $ (187) Amounts recognized in regulatory assets or liabilities consist of: Net actuarial (gain) loss $ (138) $ (200) $ 244 $ (170) Prior service credit — (37) — (41) Amounts recognized in accumulated OCI (pretax) consist of: Net actuarial loss 5 6 26 4 Total $ (133) $ (231) $ 270 $ (207) (a) Not applicable. (b) Included in “Other assets” on Ameren’s consolidated balance sheet. (c) Included in “Other current liabilities” on Ameren’s consolidated balance sheet.
Assumptions Used To Determine Benefit Obligations	The following table presents the assumptions used to determine our benefit obligations at December 31, 2020 and 2019: Pension Benefits Postretirement Benefits 2020 2019 2020 2019 Discount rate at measurement date 2.75 % 3.50 % 2.75 % 3.50 % Increase in future compensation 3.50 3.50 3.50 3.50 Cash balance pension plan interest crediting rate 5.00 5.00 (a) (a) Medical cost trend rate (initial) (b) (a) (a) 5.00 5.00 Medical cost trend rate (ultimate) (b) (a) (a) 5.00 5.00 (a) Not applicable. (b) Initial and ultimate medical cost trend rate for certain Medicare-eligible participants is 3.00%.
Schedule Of Cash Contributions Made To Benefit Plans	The following table presents the cash contributions made to our defined benefit retirement plans and to our postretirement plan during 2020, 2019, and 2018: Pension Benefits Postretirement Benefits 2020 2019 2018 2020 2019 2018 Ameren Missouri $ 17 $ 3 $ 18 $ 1 $ 1 $ 1 Ameren Illinois 27 19 35 1 1 1 Other 8 1 7 — 1 — Ameren $ 52 $ 23 $ 60 $ 2 $ 3 $ 2
Target Allocation Of The Plans' Asset Categories	The following table presents our target allocations for 2021 and our pension and postretirement plans’ asset categories as of December 31, 2020 and 2019: Asset Target Allocation Percentage of Plan Assets at December 31, 2020 2019 Pension Plan: Cash and cash equivalents 0% – 5% 1 % 3 % Equity securities: U.S. large-capitalization 21% – 31% 26 % 27 % U.S. small- and mid-capitalization 3% – 13% 9 % 7 % International 9% – 19% 15 % 14 % Global 3% – 13% 9 % 9 % Total equity 51% – 61% 59 % 57 % Debt securities 35% – 45% 36 % 36 % Real estate 0% – 9% 4 % 4 % Private equity 0% – 5% (a) (a) Total 100 % 100 % Postretirement Plans: Cash and cash equivalents 0% – 7% 3 % 1 % Equity securities: U.S. large-capitalization 23% – 33% 31 % 31 % U.S. small- and mid-capitalization 3% – 13% 8 % 9 % International 9% – 19% 15 % 14 % Global 5% – 15% 10 % 11 % Total equity 55% – 65% 64 % 65 % Debt securities 33% – 43% 33 % 34 % Total 100 % 100 % (a) Less than 1% of plan assets.
Components Of Net Periodic Benefit Cost	The following table presents the components of the net periodic benefit cost of Ameren’s pension and postretirement benefit plans during 2020, 2019, and 2018: Pension Benefits Postretirement Benefits 2020 2019 2018 2020 2019 2018 Service cost (a) $ 110 $ 88 $ 100 $ 19 $ 18 $ 21 Non-service cost components: Interest cost 174 187 169 39 43 40 Expected return on plan assets (291) (276) (276) (80) (77) (77) Amortization of: Prior service credit (1) (1) (1) (4) (5) (4) Actuarial (gain) loss 60 25 68 (9) (15) (6) Total non-service cost components (b) $ (58) $ (65) $ (40) $ (54) $ (54) $ (47) Net periodic benefit cost (income) $ 52 $ 23 $ 60 $ (35) $ (36) $ (26) (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 consolidated statement of income. See Note 6 – Other Income, Net for additional information.
Summary Of Benefit Plan Costs Incurred	The Ameren Companies are responsible for their share of the pension and postretirement benefit costs. The following table presents the pension costs and the postretirement benefit costs incurred for the years ended December 31, 2020, 2019, and 2018: Pension Costs Postretirement Costs 2020 2019 2018 2020 2019 2018 Ameren Missouri (a) $ 22 $ 5 $ 22 $ (5) $ (6) $ (1) Ameren Illinois 32 20 39 (31) (30) (25) Other (2) (2) (1) 1 — — Ameren $ 52 $ 23 $ 60 $ (35) $ (36) $ (26) (a) Does not include the impact of the tracker for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri and the level of such costs included in customer rates.
Schedule Of Expected Payments From Qualified Trust And Company Funds	The expected pension and postretirement benefit payments from qualified trust and company funds, which reflect expected future service, as of December 31, 2020, are as follows: Pension Benefits Postretirement Benefits Paid from Paid from Paid from Paid from 2021 $ 260 $ 2 $ 58 $ 2 2022 267 2 60 2 2023 274 3 61 2 2024 278 3 62 2 2025 283 3 61 2 2026 – 2030 1,454 12 303 11
Assumptions Used To Determine Net Periodic Benefit Cost	The following table presents the assumptions used to determine net periodic benefit cost for our pension and postretirement benefit plans for the years ended December 31, 2020, 2019, and 2018: Pension Benefits Postretirement Benefits 2020 2019 2018 2020 2019 2018 Discount rate at measurement date 3.50 % 4.25 % 3.50 % 3.50 % 4.25 % 3.50 % Expected return on plan assets 7.00 7.00 7.00 7.00 7.00 7.00 Increase in future compensation 3.50 3.50 3.50 3.50 3.50 3.50 Cash balance pension plan interest crediting rate 5.00 5.00 5.00 (a) (a) (a) Medical cost trend rate (initial) (b) (a) (a) (a) 5.00 5.00 5.00 Medical cost trend rate (ultimate) (b) (a) (a) (a) 5.00 5.00 5.00 (a) Not applicable. (b) Initial and ultimate medical cost trend rate for certain Medicare-eligible participants is 3.00%.
Schedule Of Potential Changes In Key Assumptions	The table below reflects the sensitivity of Ameren’s plans to potential changes in key assumptions for the year ended December 31, 2020: Pension Benefits Postretirement Benefits Service Cost Expected Projected Service Cost Expected Postretirement 0.25% decrease in discount rate $ (1) $ — $ 195 $ — $ — $ 42 0.25% decrease in return on assets — 10 — — 3 — 0.25% increase in future compensation 2 — 20 — — —
Schedule Of Matching Contributions	The following table presents the portion of the matching contribution to the Ameren 401(k) plan attributable to each of the Ameren Companies for the years ended December 31, 2020, 2019, and 2018: 2020 2019 2018 Ameren Missouri $ 20 $ 19 $ 17 Ameren Illinois 17 16 15 Other 1 — 1 Ameren $ 38 $ 35 $ 33
Pension Benefits
Target Allocation Of The Plans' Asset Categories	The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the pension plans’ assets measured at fair value and NAV as of December 31, 2020 and 2019: December 31, 2020 December 31, 2019 Level 1 Level 2 NAV Total Level 1 Level 2 NAV Total Cash and cash equivalents $ — $ — $ 145 $ 145 $ — $ — $ 139 $ 139 Equity securities: U.S. large-capitalization — — 1,511 1,511 — — 1,253 1,253 U.S. small- and mid-capitalization 513 — — 513 344 — — 344 International 375 — 492 867 296 — 363 659 Global — — 546 546 — — 407 407 Debt securities: Corporate bonds — 506 17 523 — 597 13 610 Municipal bonds — 50 — 50 — 75 — 75 U.S. Treasury and agency securities 3 1,325 — 1,328 5 1,010 — 1,015 Other (5) 8 — 3 — 8 — 8 Real estate — — 208 208 — — 211 211 Private equity — — 2 2 — — 2 2 Total $ 886 $ 1,889 $ 2,921 $ 5,696 $ 645 $ 1,690 $ 2,388 $ 4,723 Less: Medical benefit assets (a) (219) (176) Plus: Net receivables (b) 33 17 Fair value of pension plans’ assets $ 5,510 $ 4,564 (a) Medical benefit (health and welfare) component for accounts maintained in accordance with Section 401(h) of the Internal Revenue Code to fund a portion of the postretirement obligation. (b) Receivables related to pending securities sales, offset by payables related to pending securities purchases.
Postretirement Benefits
Target Allocation Of The Plans' Asset Categories	The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the postretirement benefit plans’ assets measured at fair value and NAV as of December 31, 2020 and 2019: December 31, 2020 December 31, 2019 Level 1 Level 2 NAV Total Level 1 Level 2 NAV Total Cash and cash equivalents $ 38 $ — $ — $ 38 $ 12 $ — $ — $ 12 Equity securities: U.S. large-capitalization 279 — 107 386 238 — 112 350 U.S. small- and mid-capitalization 104 — — 104 93 — — 93 International 75 — 107 182 59 — 102 161 Global — — 120 120 — — 120 120 Debt securities: Municipal bonds — 106 — 106 — 107 — 107 Other — — 295 295 — — 277 277 Total $ 496 $ 106 $ 629 $ 1,231 $ 402 $ 107 $ 611 $ 1,120 Plus: Medical benefit assets (a) 219 176 Plus: Net receivables (b) 3 1 Fair value of postretirement benefit plans’ assets $ 1,453 $ 1,297 (a) Medical benefit (health and welfare) component for accounts maintained in accordance with Section 401(h) of the Internal Revenue Code to fund a portion of the postretirement obligation. These 401(h) assets are included in the pension plan assets shown above. (b) Receivables related to pending securities sales, offset by payables related to pending securities purchases.

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Summary Of Nonvested Shares Related To Long-Term Incentive Plan	The following table summarizes Ameren’s nonvested performance share unit and restricted stock unit activity for the year ended December 31, 2020: Performance Share Units – Market Condition (a) Performance Share Units – Performance Condition (b) Restricted Stock Units Share Weighted-average Fair Value per Share Unit Shares Weighted-average Fair Value per Share Unit Stock Weighted-average Fair Value per Stock Unit Nonvested at January 1, 2020 (c) 539,877 $ 63.79 — $ — 220,277 $ 61.13 Granted (d) 267,154 82.49 43,052 76.69 173,571 76.91 Forfeitures (16,303) 73.42 (1,155) 76.80 (7,507) 69.89 Vested and undistributed (e) (178,802) 69.87 (7,607) 76.78 (80,180) 66.72 Vested and distributed (147,787) 59.16 — — (2,466) 54.30 Performance share adjustment — — (2,394) 76.66 — — Nonvested at December 31, 2020 (f) 464,139 $ 73.34 31,896 $ 76.66 303,695 $ 68.52 (a) The exact number of shares issued pursuant to a share unit varies from 0% to 200% of the target award, depending on actual company performance relative to the specified market conditions. Compensation cost on nonforfeited awards is recognized regardless of whether Ameren achieves the specified market conditions. (b) The exact number of shares issued pursuant to a share unit varies from 0% to 200% of the target award, depending on actual company performance relative to the performance goals. Compensation cost is recognized ratably over the requisite service period only for awards for which it is probable that the performance condition will be satisfied. The performance share adjustment represents the change in the probability that a performance condition will be satisfied. (c) Does not include 503,283 performance share units for market performance and 79,854 restricted stock units that were vested and undistributed. (d) In September 2020, certain executive officers were granted an additional 37,104 restricted stock units with a grant date fair value of $3 million, which will vest after three years on September 17, 2023. The awards do not provide for pro rata vesting in connection with the executive officer’s retirement. (e) Vested and undistributed units are awards that vest on a pro-rata basis due to attainment of retirement eligibility by certain employees, but have not yet been distributed. For vested and undistributed performance share units, the number of shares issued for retirement-eligible employees will vary depending on actual performance over the three-year performance period. (f) Does not include 366,243 of performance share units for market performance, 7,607 of performance share units based on the achievement of renewable generation and energy storage installation targets, and 160,034 of restricted stock units that were vested and undistributed.
Disclosure of Share-based Compensation Arrangements by Share-based Payment Award	The following table presents the fair value of each share unit along with the significant assumptions used to calculate the fair value of each share unit for the years ended December 31, 2020, 2019, and 2018: 2020 2019 2018 Fair value of share units awarded $82.49 $67.42 $62.88 Three-year risk-free rate 1.62% 2.46% 1.98% Ameren’s common stock volatility (a) 15% 17% 17% Volatility range for the peer group (a) 14% – 28% 15% – 25% 15% – 23% (a) Based on a historical period that is equal to the remaining term of the performance period as of the grant date. The following table presents the stock-based compensation expense for the years ended December 31, 2020, 2019, and 2018: 2020 2019 2018 Ameren Missouri $ 5 $ 4 $ 4 Ameren Illinois 3 3 3 Other (a) 13 13 13 Ameren 21 20 20 Less income tax benefit 6 5 6 Stock-based compensation expense, net $ 15 $ 15 $ 14 (a) Represents compensation expense for employees of Ameren Services. These amounts are not included in the Ameren Missouri and Ameren Illinois amounts above.

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Schedule Of Effective Income Tax Rate Reconciliation	The following table presents the principal reasons for the difference between the effective income tax rate and the federal statutory corporate income tax rate for the years ended December 31, 2020, 2019, and 2018: Ameren Missouri Ameren Illinois Ameren 2020 Federal statutory corporate income tax rate 21 % 21 % 21 % Increases (decreases) from: Amortization of excess deferred income taxes (16) (3) (9) Amortization of deferred investment tax credit (1) (1) (1) State tax 3 7 5 Stock-based compensation — — (1) Effective income tax rate 7 % 24 % 15 % 2019 Federal statutory corporate income tax rate 21 % 21 % 21 % Increases (decreases) from: Amortization of excess deferred income taxes (11) (4) (7) Amortization of deferred investment tax credit (1) — (1) State tax 5 7 6 Stock-based compensation — — (1) Effective income tax rate 14 % 24 % 18 % 2018 Federal statutory corporate income tax rate 21 % 21 % 21 % Increases (decreases) from: Amortization of excess deferred income taxes (4) (4) (4) Depreciation differences — (1) — Amortization of deferred investment tax credit (1) — (1) State tax 4 7 6 TCJA 1 1 1 Tax credits (1) — — Other permanent items — — (1) Effective income tax rate 20 % 24 % 22 %
Schedule Of Components Of Income Tax Expense (Benefit)	The following table presents the components of income tax expense for the years ended December 31, 2020, 2019, and 2018: Ameren Missouri Ameren Illinois Other Ameren 2020 Current taxes: Federal $ 14 $ 12 $ (24) $ 2 State 3 (6) 8 5 Deferred taxes: Federal 82 81 24 187 State 15 52 (10) 57 Amortization of excess deferred income taxes (75) (15) (1) (91) Amortization of deferred investment tax credits (5) — — (5) Total income tax expense $ 34 $ 124 $ (3) $ 155 2019 Current taxes: Federal $ 65 $ 19 $ (88) $ (4) State 22 11 (14) 19 Deferred taxes: Federal 37 66 82 185 State 5 29 25 59 Amortization of excess deferred income taxes (56) (15) (1) (72) Amortization of deferred investment tax credits (5) — — (5) Total income tax expense $ 68 $ 110 $ 4 $ 182 2018 Current taxes: Federal $ 104 $ 4 $ (118) $ (10) State 29 6 (12) 23 Deferred taxes: Federal 22 75 123 220 State (2) 28 23 49 Amortization of excess deferred income taxes (24) (15) (1) (40) Amortization of deferred investment tax credits (5) — — (5) Total income tax expense $ 124 $ 98 $ 15 $ 237
Schedule Of Deferred Tax Assets And Liabilities Resulting From Temporary Differences	The following table presents the accumulated deferred income tax assets and liabilities recorded as a result of temporary differences and accumulated deferred investment tax credits at December 31, 2020 and 2019: Ameren Missouri Ameren Illinois Other Ameren 2020 Accumulated deferred income taxes, net liability (asset): Plant-related $ 2,112 $ 1,559 $ 205 $ 3,876 Regulatory assets and liabilities, net (285) (207) (23) (515) Deferred employee benefit costs (58) 8 (54) (104) Tax carryforwards (26) (6) (65) (97) Other (35) 13 39 17 Total net accumulated deferred income tax liabilities (assets) 1,708 1,367 102 3,177 Accumulated deferred investment tax credits 34 — — 34 Accumulated deferred income taxes and investment tax credits $ 1,742 $ 1,367 $ 102 $ 3,211 2019 Accumulated deferred income taxes, net liability (asset): Plant-related $ 2,000 $ 1,423 $ 193 $ 3,616 Regulatory assets and liabilities, net (310) (214) (24) (548) Deferred employee benefit costs (59) 7 (59) (111) Tax carryforwards (25) (3) (70) (98) Other (33) 11 43 21 Total net accumulated deferred income tax liabilities (assets) 1,573 1,224 83 2,880 Accumulated deferred investment tax credits 39 — — 39 Accumulated deferred income taxes and investment tax credits $ 1,612 $ 1,224 $ 83 $ 2,919
Schedule Of Net Operating Loss Carryforwards And Tax Credit Carryforwards	The following table presents the components of accumulated deferred income tax assets relating to net operating loss carryforwards, tax credit carryforwards, and charitable contribution carryforwards at December 31, 2020 and 2019: Ameren Missouri Ameren Illinois Other Ameren 2020 Net operating loss carryforwards: State (a) $ — $ 3 $ 4 $ 7 Total net operating loss carryforwards $ — $ 3 $ 4 $ 7 Tax credit carryforwards: Federal (b) $ 26 $ 3 $ 54 $ 83 State (c) — — 7 7 Total tax credit carryforwards $ 26 $ 3 $ 61 $ 90 Charitable contribution carryforwards (c) $ — $ — $ 3 $ 3 Valuation allowance (d) — — (3) (3) Total charitable contribution carryforwards $ — $ — $ — $ — 2019 Tax credit carryforwards: Federal $ 25 $ 3 $ 67 $ 95 State — — 3 3 Total tax credit carryforwards $ 25 $ 3 $ 70 $ 98 Charitable contribution carryforwards $ — $ — $ 3 $ 3 Valuation allowance — — (3) (3) Total charitable contribution carryforwards $ — $ — $ — $ — (a) Will expire 2032. (b) Will expire between 2030 and 2040. (c) Will expire between 2021 and 2025. (d) See Schedule II under Part IV, Item 15, in this report for information on changes in the valuation allowance.

Related Party Transactions (Tab

Related Party Transactions (Tables)	12 Months Ended
	Dec. 31, 2020
Related Party Transactions [Abstract]
Schedule of Related Party Electric Power Supply Agreements	The following table presents the specified performance period, price, and amount of MWhs included in the agreements: IPA Procurement Event Performance Period MWh Average Price per MWh April 2017 March 2019 – May 2020 85,600 $ 34 April 2018 June 2019 – September 2020 110,000 32 April 2019 January 2020 – December 2021 288,000 35 September 2019 April 2020 – November 2021 170,800 29 September 2020 September 2021 – November 2022 204,800 31
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 December 31, 2020 and December 31, 2019: 2020 2019 Ameren Missouri Ameren Illinois Ameren Missouri Ameren Illinois Income taxes payable to parent (a) $ — $ 6 $ 15 $ 43 Income taxes receivable from parent (b) 9 15 15 17 (a) Included in “Accounts payable – affiliates” on the balance sheet.
Schedule of Capital Contributions	The following table presents cash capital contributions received from Ameren (parent) by Ameren Missouri and Ameren Illinois for the years ended December 31, 2020, 2019, and 2018: 2020 2019 2018 Ameren Missouri (a) $ 491 $ 124 $ 45 Ameren Illinois 464 (a) 15 (a) 160 (a) Includes capital contributions made as a result of the tax allocation agreement.
Schedule of Related Party Transactions	The following table presents the impact on Ameren Missouri and Ameren Illinois of related-party transactions for the years ended December 31, 2020, 2019, and 2018. It is based primarily on the agreements discussed above and the money pool arrangements discussed in Note 4 – Short-term Debt and Liquidity. Agreement Income Statement Line Item Ameren Ameren Ameren Missouri power supply agreements Operating Revenues 2020 $ 11 $ (a) with Ameren Illinois 2019 3 (a) 2018 11 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2020 26 1 rent and facility services 2019 27 2 2018 22 3 Ameren Missouri and Ameren Illinois miscellaneous Operating Revenues 2020 3 1 support services and services provided to ATXI 2019 1 2 2018 1 1 Ameren Missouri software licensing Operating Revenues 2020 (a) (a) with Ameren Illinois 2019 (a) 19 2018 (a) (a) Total Operating Revenues 2020 $ 40 $ 2 2019 31 23 2018 34 4 Ameren Illinois power supply Purchased Power 2020 $ (a) $ 11 agreements with Ameren Missouri 2019 (a) 3 2018 (a) 11 Ameren Missouri and Ameren Illinois Purchased Power 2020 (a) 2 transmission services from ATXI 2019 (a) 2 2018 (a) 1 Total Purchased Power 2020 $ (a) $ 13 2019 (a) 5 2018 (a) 12 Ameren Missouri and Ameren Illinois Other Operations and 2020 $ (b) $ 4 rent and facility services Maintenance 2019 2 5 2018 3 6 Ameren Services support services Other Operations and 2020 140 133 agreement Maintenance 2019 135 127 2018 136 126 Total Other Operations and 2020 $ 140 $ 137 Maintenance Expenses 2019 137 132 2018 139 132 Money pool borrowings (advances) (Interest Charges) 2020 $ (b) $ (b) Other Income, Net 2019 (b) (b) 2018 1 (b) (a) Not applicable. (b) Amount less than $1 million.

Commitments And Contingencies (

Commitments And Contingencies (Tables)	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Schedule Of Estimated Purchased Commitments	The table below presents our estimated minimum fuel, purchased power, and other commitments at December 31, 2020. Ameren’s and Ameren Illinois’ purchased power commitments include the Ameren Illinois agreements entered into as part of the IPA-administered power procurement process. Included in the Other column are minimum purchase commitments under contracts for equipment, design and construction, meter reading services, and service maintenance agreements related to Ameren Missouri’s High Prairie Renewable Energy Center, among other agreements, at December 31, 2020. Coal Natural Gas (a) Nuclear Purchased Power (b)(c)(d) Methane Other (e) Total Ameren: 2021 $ 321 $ 161 $ 56 $ 150 (f) $ 3 $ 97 $ 788 2022 194 111 12 64 3 29 413 2023 113 72 45 24 3 27 284 2024 94 31 26 7 3 25 186 2025 55 17 — — 3 28 103 Thereafter — 64 21 — 18 92 195 Total $ 777 $ 456 $ 160 $ 245 $ 33 $ 298 $ 1,969 Ameren Missouri: 2021 $ 321 $ 44 $ 56 $ — $ 3 $ 79 $ 503 2022 194 40 12 — 3 30 279 2023 113 35 45 — 3 28 224 2024 94 15 26 — 3 26 164 2025 55 7 — — 3 29 94 Thereafter — 17 21 — 18 70 126 Total $ 777 $ 158 $ 160 $ — $ 33 $ 262 $ 1,390 Ameren Illinois: 2021 $ — $ 117 $ — $ 160 (f) $ — $ 13 $ 290 2022 — 71 — 67 — — 138 2023 — 37 — 25 — — 62 2024 — 16 — 7 — — 23 2025 — 10 — — — — 10 Thereafter — 47 — — — — 47 Total $ — $ 298 $ — $ 259 $ — $ 13 $ 570 (a) Includes amounts for generation and for distribution. (b) The purchased power amounts for Ameren and Ameren Illinois exclude agreements for renewable energy credits through 2035 with various renewable energy suppliers due to the contingent nature of the payment amounts, with the exception of expected payments of $48 million through 2025. (c) The purchased power amounts for Ameren and Ameren Missouri exclude a 102-MW power purchase agreement with a wind farm operator, which expires in 2024, due to the contingent nature of the payment amounts. (d) The purchased power amounts for Ameren exclude obligations resulting from capacity supply and energy product agreements between Ameren Missouri and Ameren Illinois, as disclosed in Note 13 – Related-party Transactions. (e) The other amounts for Ameren exclude obligations resulting from interconnection agreements between Ameren Missouri and ATXI, as disclosed in Note 13 – Related-party Transactions. (f) In January 2018, as required by the FEJA, Ameren Illinois entered into agreements to acquire zero emission credits, through 2026. Annual zero emission credit commitment amounts will be published by the IPA each May prior to the start of the subsequent planning year, which begins each June. The amounts above reflect Ameren Illinois’ commitment to acquire approximately $26 million of zero emission credits through May 2021.

Supplemental Information (Table

Supplemental Information (Tables)	12 Months Ended
	Dec. 31, 2020
Supplemental Information [Abstract]
Schedule of Cash and Cash Equivalents Including Restricted Cash	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets and the statements of cash flows as of December 31, 2020 and 2019: December 31, 2020 December 31, 2019 Ameren Ameren Ameren Ameren Ameren Ameren Cash and cash equivalents $ 139 $ 136 $ — $ 16 $ 9 $ — Restricted cash included in “Other current assets” 17 5 6 14 4 5 Restricted cash included in “Other assets” 141 — 141 120 — 120 Restricted cash included in “Nuclear decommissioning trust fund” 4 4 — 26 26 — Total cash, cash equivalents, and restricted cash $ 301 $ 145 $ 147 $ 176 $ 39 $ 125 The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheet and the statement of cash flows as of December 31, 2020 and 2019: (In millions) 2020 2019 Cash and cash equivalents $ — $ — Restricted cash included in “Other current assets” 4 3 Total cash, cash equivalents, and restricted cash $ 4 $ 3
Accounts Receivable, Allowance for Credit Loss	The following table provides a reconciliation of the beginning and ending amount of the allowance for doubtful accounts for the years ended December 31, 2020 and 2019: December 31, 2020 December 31, 2019 Ameren Missouri Ameren Illinois (a) Ameren Ameren Missouri Ameren Illinois (a) Ameren Beginning of period $ 7 $ 10 $ 17 $ 7 $ 11 $ 18 Bad debt expense 15 33 48 9 21 30 Net write-offs (6) (9) (15) (9) (22) (31) End of period $ 16 $ 34 $ 50 $ 7 $ 10 $ 17 (a) Ameren Illinois has riders 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. In 2020, the rider for electric distribution allows for recovery of bad debt expense recognized under GAAP. See Note 2 – Rate and Regulatory Matters for additional information.
Schedule of Inventories	The following table presents the components of inventories for each of the Ameren Companies at December 31, 2020 and 2019: December 31, 2020 December 31, 2019 Ameren Ameren Ameren Ameren Ameren Ameren Fuel (a) $ 115 $ — $ 115 $ 126 $ — $ 126 Natural gas stored underground 5 52 57 6 57 63 Materials, supplies, and other 266 83 349 241 64 305 Total inventories $ 386 $ 135 $ 521 $ 373 $ 121 $ 494 (a) Consists of coal, oil, and propane.
Asset Retirement Obligation Disclosure	The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the years ended December 31, 2020 and 2019: December 31, 2020 December 31, 2019 Ameren Ameren Ameren Ameren Ameren Ameren Beginning balance at January 1 $ 687 $ 4 (a) $ 691 (b) $ 646 $ 4 $ 650 Liabilities incurred 36 (c) — 36 (c) — — — Liabilities settled (58) — (58) (20) — (20) Accretion (d) 29 1 30 28 — 28 Change in estimates 57 (e) — 57 (e) 33 (f) — 33 (f) Ending balance at December 31 $ 751 (g) $ 5 (a) $ 756 (b), (g) $ 687 $ 4 (a) $ 691 (b) (a) Included in “Other deferred credits and liabilities” on the balance sheet. (b) Balance included $60 million and $53 million in “Other current liabilities” on the balance sheet as of both December 31, 2020 and 2019, respectively. (c) In December 2020, Ameren Missouri recorded an ARO related to the decommissioning for the High Prairie Renewable Energy Center. (d) Accretion expense attributable to Ameren Missouri and Ameren Illinois was recorded as a decrease to regulatory liabilities and an increase to regulatory assets, respectively. (e) Ameren Missouri changed its fair value estimate primarily due to an update to the decommissioning of the Callaway Energy Center to reflect the cost study and funding analysis filed with the MoPSC in November 2020 and an increase in the cost estimate for closure of certain CCR storage facilities. (f) Ameren Missouri changed its fair value estimate primarily due to an increase in the cost estimate for closure of certain CCR storage facilities. (g) The balance as of December 31, 2020, includes an ARO related to the decommissioning of the Callaway Enter Center of $549 million.
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 years ended December 31, 2020, 2019, and 2018: 2020 2019 2018 Ameren Missouri $ 139 $ 147 $ 164 Ameren Illinois 115 117 118 Ameren $ 254 $ 264 $ 282
Schedule of Rates and Amounts For Allowance for Funds Used During Construction	The following table presents the average rate that was applied to eligible construction work in progress and the amounts of allowance for funds used during construction capitalized in 2020, 2019, and 2018: 2020 2019 2018 Average rate: Ameren Missouri 5 % 6 % 7 % Ameren Illinois 5 % 5 % 5 % Ameren: Allowance for equity funds used during construction $ 32 $ 28 $ 36 Allowance for borrowed funds used during construction 16 20 21 Total Ameren $ 48 $ 48 $ 57 Ameren Missouri: Allowance for equity funds used during construction $ 19 $ 19 $ 27 Allowance for borrowed funds used during construction 10 12 14 Total Ameren Missouri $ 29 $ 31 $ 41 Ameren Illinois: Allowance for equity funds used during construction $ 13 $ 9 $ 9 Allowance for borrowed funds used during construction 6 8 7 Total Ameren Illinois $ 19 $ 17 $ 16
Schedule of Earnings Per Share, Basic and Diluted	The following table reconciles the weighted-average number of common shares outstanding to the diluted weighted-average number of common shares outstanding for the years ended December 31, 2020, 2019, and 2018: 2020 2019 2018 Weighted-average Common Shares Outstanding – Basic 247.0 245.6 243.8 Assumed settlement of performance share units and restricted stock units 1.2 1.4 2.0 Dilutive effect of forward sale agreement 0.5 0.1 — Weighted-average Common Shares Outstanding – Diluted (a) 248.7 247.1 245.8 (a) There were no potentially dilutive securities excluded from the earnings per diluted share calculations for the years ended December 31, 2020, 2019, and 2018.
Schedule of Cash Flow, Supplemental Disclosures	The following table provides noncash financing and investing activity excluded from the statements of cash flows for the years ended December 31, 2020, 2019, and 2018: December 31, 2020 December 31, 2019 December 31, 2018 Ameren Ameren Ameren Ameren Ameren Ameren Ameren Ameren Ameren Investing Accrued capital expenditures $ 400 $ 183 $ 218 $ 333 $ 140 $ 163 $ 272 $ 121 $ 138 Accrued nuclear fuel expenditures — — — 19 19 — 20 20 — Accrued wind generation expenditures 46 46 — — — — — — — Net realized and unrealized gain (loss) – nuclear decommissioning trust fund 116 116 — 143 143 — (38) (38) — Exchange of bond investments for the extinguishment of senior unsecured notes (a) — — — 17 — 17 — — — Financing Issuance of common stock for stock-based compensation $ 38 $ — $ — $ 54 $ — $ — $ 35 $ — $ — Exchange of bond investments for the extinguishment of senior unsecured notes (a) — — — (17) — (17) — — — (a) See Note 5 – Long-term Debt and Equity Financings for additional information.

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2020
Segment Reporting [Abstract]
Schedule Of Segment Reporting Information, By Segment	The following tables present information about the reported revenue and specified items reflected in net income attributable to common shareholders and capital expenditures by segment at Ameren and Ameren Illinois for the years ended December 31, 2020, 2019, and 2018. Ameren, Ameren Missouri, and Ameren Illinois management review segment capital expenditure information rather than any individual or total asset amount. Ameren Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Other Intersegment Eliminations Ameren 2020 External revenues $ 3,069 $ 1,496 $ 760 $ 469 $ — $ — $ 5,794 Intersegment revenues 40 2 — 54 — (96) — Depreciation and amortization 604 288 81 98 4 — 1,075 Interest income 26 2 — 1 4 (4) 29 Interest charges 190 72 41 78 (a) 42 (4) 419 Income taxes (benefit) 34 42 36 78 (35) — 155 Net income (loss) attributable to Ameren common shareholders 436 143 99 216 (23) — 871 Capital expenditures 1,666 (b) 543 301 716 5 2 3,233 (b) 2019 External revenues $ 3,212 $ 1,487 $ 791 $ 401 $ — $ — $ 5,891 Intersegment revenues 31 17 6 63 — (98) 19 (c) Depreciation and amortization 556 273 78 84 4 — 995 Interest income 26 6 — 1 5 (5) 33 Interest charges 178 71 38 74 (a) 25 (5) 381 Income taxes (benefit) 68 45 30 64 (25) — 182 Net income (loss) attributable to Ameren common shareholders 426 146 84 185 (13) — 828 Capital expenditures 1,076 518 318 528 3 (32) (d) 2,411 2018 External revenues $ 3,555 $ 1,544 $ 814 $ 378 $ — $ — $ 6,291 Intersegment revenues 34 3 1 55 — (93) — Depreciation and amortization 550 259 65 77 4 — 955 Interest income 28 6 — — 4 (5) 33 Interest charges 200 73 38 75 (a) 19 (4) 401 Income taxes (benefit) 124 41 25 56 (9) — 237 Net income (loss) attributable to Ameren common shareholders 478 136 70 164 (33) — 815 Capital expenditures 914 503 311 562 5 (9) 2,286 (a) Ameren Transmission interest charges include an allocation of financing costs from Ameren (parent). (b) Includes $564 million at Ameren and Ameren Missouri for the acquisition of the High Prairie Renewable Energy Center for the year ended December 31, 2020. (c) Intersegment revenues at Ameren include $14 million and $5 million of revenue from Ameren Illinois Electric Distribution and Ameren Illinois Natural Gas, respectively, for the year ended December 31, 2019, for a software licensing agreement with Ameren Missouri. Under authoritative accounting guidance for rate-regulated entities, the revenue recognized by Ameren Illinois was not eliminated upon consolidation. See Note 13 – Related-party Transactions for additional information. (d) Intersegment capital expenditure eliminations include $24 million of eliminations for the year ended December 31, 2019, for a software licensing agreement between Ameren Illinois and Ameren Missouri. See Note 13 – Related-party Transactions for additional information. Ameren Illinois Ameren Illinois Electric Distribution Ameren Illinois Ameren Illinois Transmission Intersegment Eliminations Ameren Illinois 2020 External revenues $ 1,498 $ 760 $ 277 $ — $ 2,535 Intersegment revenues — — 52 (52) — Depreciation and amortization 288 81 65 — 434 Interest income 2 — 1 — 3 Interest charges 72 41 42 — 155 Income taxes 42 36 46 — 124 Net income available to common shareholder 143 99 137 — 379 Capital expenditures 543 301 603 — 1,447 2019 External revenues $ 1,504 $ 797 $ 226 $ — $ 2,527 Intersegment revenues — — 62 (62) — Depreciation and amortization 273 78 55 — 406 Interest income 6 — — — 6 Interest charges 71 38 38 — 147 Income taxes 45 30 35 — 110 Net income available to common shareholder 146 84 113 — 343 Capital expenditures 518 318 372 — 1,208 2018 External revenues $ 1,547 $ 815 $ 214 $ — $ 2,576 Intersegment revenues — — 53 (53) — Depreciation and amortization 259 65 50 — 374 Interest income 6 — — — 6 Interest charges 73 38 38 — 149 Income taxes 41 25 32 — 98 Net income available to common shareholder 136 70 98 — 304 Capital expenditures 503 311 444 — 1,258
Disaggregation of Revenue	The following tables present disaggregated revenues by segment at Ameren and Ameren Illinois for the years ended December 31, 2020, 2019, and 2018. Economic factors affect the nature, timing, amount, and uncertainty of revenues and cash flows in a similar manner across customer classes. Revenues from alternative revenue programs have a similar distribution among customer classes as revenues from contracts with customers. Other revenues not associated with contracts with customers are presented in the Other customer classification, along with electric transmission and off-system revenues. Ameren Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Intersegment Eliminations Ameren 2020 Residential $ 1,373 $ 867 $ — $ — $ — $ 2,240 Commercial 1,025 486 — — — 1,511 Industrial 261 124 — — — 385 Other 325 21 — 523 (94) 775 Total electric revenues $ 2,984 $ 1,498 $ — $ 523 $ (94) $ 4,911 Residential $ 76 $ — $ 541 $ — $ — $ 617 Commercial 29 — 136 — — 165 Industrial 4 — 14 — — 18 Other 16 — 69 — (2) 83 Total gas revenues $ 125 $ — $ 760 $ — $ (2) $ 883 Total revenues (a) $ 3,109 $ 1,498 $ 760 $ 523 $ (96) $ 5,794 2019 Residential $ 1,403 $ 848 $ — $ — $ — $ 2,251 Commercial 1,157 497 — — — 1,654 Industrial 278 127 — — — 405 Other 271 32 (b) — 464 (96) 671 Total electric revenues $ 3,109 $ 1,504 $ — $ 464 $ (96) $ 4,981 Residential $ 81 $ — $ 570 $ — $ — $ 651 Commercial 34 — 154 — — 188 Industrial 4 — 13 — — 17 Other 15 — 60 (b) — (2) 73 Total gas revenues $ 134 $ — $ 797 $ — $ (2) $ 929 Total revenues (a) $ 3,243 $ 1,504 $ 797 $ 464 $ (98) $ 5,910 2018 Residential $ 1,560 $ 867 $ — $ — $ — $ 2,427 Commercial 1,271 511 — — — 1,782 Industrial 312 130 — — — 442 Other 308 (c) 39 — 433 (92) 688 (c) Total electric revenues $ 3,451 $ 1,547 $ — $ 433 $ (92) $ 5,339 Residential $ 90 $ — $ 581 $ — $ — $ 671 Commercial 37 — 159 — — 196 Industrial 4 — 17 — — 21 Other 7 — 58 — (1) 64 Total gas revenues $ 138 $ — $ 815 $ — $ (1) $ 952 Total revenues (a) $ 3,589 $ 1,547 $ 815 $ 433 $ (93) $ 6,291 (a) The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the years ended December 31, 2020, 2019, and 2018: Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Ameren 2020 Revenues from alternative revenue programs $ (14) $ (20) $ 20 $ 50 $ 36 Other revenues not from contracts with customers 25 8 2 1 36 2019 Revenues from alternative revenue programs $ 35 $ (74) $ — $ (31) $ (70) Other revenues not from contracts with customers 19 7 2 — 28 2018 Revenues from alternative revenue programs $ (8) $ (3) $ (23) $ (25) $ (59) Other revenues not from contracts with customers 24 16 2 — 42 (b) Includes $14 million and $5 million for Ameren Illinois Electric Distribution and Ameren Illinois Natural Gas, respectively, for the year ended December 31, 2019, for a software licensing agreement with Ameren Missouri. See Note 13 – Related-party Transactions for additional information. (c) Includes $60 million for the year ended December 31, 2018, for the reduction to revenue for the excess amounts collected in rates to be refunded related to the TCJA from January 1, 2018, through July 31, 2018. See Note 2 – Rate and Regulatory Matters for additional information. Ameren Illinois Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Ameren Illinois 2020 Residential $ 867 $ 541 $ — $ — $ 1,408 Commercial 486 136 — — 622 Industrial 124 14 — — 138 Other 21 69 329 (52) 367 Total revenues (a) $ 1,498 $ 760 $ 329 $ (52) $ 2,535 2019 Residential $ 848 $ 570 $ — $ — $ 1,418 Commercial 497 154 — — 651 Industrial 127 13 — — 140 Other 32 (b) 60 (b) 288 (62) 318 Total revenues (a) $ 1,504 $ 797 $ 288 $ (62) $ 2,527 2018 Residential $ 867 $ 581 $ — $ — $ 1,448 Commercial 511 159 — — 670 Industrial 130 17 — — 147 Other 39 58 267 (53) 311 Total revenues (a) $ 1,547 $ 815 $ 267 $ (53) $ 2,576 (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 years ended December 31, 2020, 2019, and 2018: Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Ameren Illinois 2020 Revenues from alternative revenue programs $ (20) $ 20 $ 42 $ 42 Other revenues not from contracts with customers 8 2 — 10 2019 Revenues from alternative revenue programs $ (74) $ — $ (33) $ (107) Other revenues not from contracts with customers 7 2 — 9 2018 Revenues from alternative revenue programs $ (3) $ (23) $ (25) $ (51) Other revenues not from contracts with customers 16 2 — 18 (b) Includes $14 million and $5 million for Ameren Illinois Electric Distribution and Ameren Illinois Natural Gas, respectively, for the year ended December 31, 2019, for a software licensing agreement with Ameren Missouri. See Note 13 – Related-party Transactions for additional information.

Summary Of Significant Accoun_3

Summary Of Significant Accounting Policies (Narrative) (Details) customer in Millions	12 Months Ended
	Dec. 31, 2020USD ($)mi²customersegment	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Mar. 31, 2020
Accounting Policies [Line Items]
Workforce Utilizing Remote Work Arrangement	50.00%			65.00%
Percentage of accounts receivable balances that are 30 days past due or are part of a deferred payment arrangement	29.00%	18.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 133,000,000	$ 75,000,000
Goodwill	$ 411,000,000	411,000,000
Number of reportable segments | segment	4
Goodwill, Impairment Loss	$ 0
Noncontrolling Interest in Variable Interest Entity	37,000,000	28,000,000
Unrecorded Unconditional Purchase Obligation	1,969,000,000
Cash Surrender Value of Life Insurance	272,000,000	264,000,000
Loans, Gross, Insurance Policy	107,000,000	$ 114,000,000
MISO Resettlements	$ 0
Union Electric Company
Accounting Policies [Line Items]
Public Utilities, Area Serviced | mi²	24,000
Percentage of accounts receivable balances that are 30 days past due or are part of a deferred payment arrangement	22.00%	18.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 40,000,000	$ 30,000,000
Number of Months Between Callaway Maintenance Outages	18 months
Unrecorded Unconditional Purchase Obligation	$ 1,390,000,000
Ameren Illinois Company
Accounting Policies [Line Items]
Public Utilities, Area Serviced | mi²	43,700
Percentage of accounts receivable balances that are 30 days past due or are part of a deferred payment arrangement	35.00%	20.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 93,000,000	$ 45,000,000
Goodwill	$ 411,000,000	411,000,000
Number of reportable segments | segment	3
Goodwill, Impairment Loss	$ 0
Unrecorded Unconditional Purchase Obligation	570,000,000
Cash Surrender Value of Life Insurance	$ 115,000,000	$ 123,000,000
Minimum
Accounting Policies [Line Items]
Percent of average depreciable cost	3.00%	3.00%	3.00%
Maximum
Accounting Policies [Line Items]
Percent of average depreciable cost	4.00%	4.00%	4.00%
Electric | Union Electric Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers | customer	1.2
Power | Ameren Illinois Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers | customer	1.2
Natural gas | Union Electric Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers | customer	0.1
Natural gas | Ameren Illinois Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers | customer	0.8
Ameren Illinois Electric Distribution
Accounting Policies [Line Items]
Goodwill	$ 238,000,000
Ameren Illinois Gas
Accounting Policies [Line Items]
Goodwill	80,000,000
Ameren Illinois Transmission
Accounting Policies [Line Items]
Goodwill	93,000,000
Ameren Transmission
Accounting Policies [Line Items]
Goodwill	93,000,000
AROs | Union Electric Company
Accounting Policies [Line Items]
Noncash Depreciation related to ARO	28,000,000	$ 18,000,000	$ 14,000,000
Partnership Funding Commitment [Member]
Accounting Policies [Line Items]
Unrecorded Unconditional Purchase Obligation	$ 35,000,000

Rate and Regulatory Matters (Re

Rate and Regulatory Matters (Regulatory Framework-Missouri) (Details) - Union Electric Company	12 Months Ended
	Dec. 31, 2020
Public Utilities, General Disclosures [Line Items]
Number of months to complete a regulatory rate review	11 months
PISA Deferral Percentage	85.00%
Depreciation Percentage Not Included in PISA Deferral	15.00%
Sharing Level For Fac	95.00%
Percentage of variance not covered by FAC	5.00%
Frequency Rates Must be Reset to Use the FAC	four years
ISRS Return on Equity Percentage	9.725%
Maximum
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage	2.85%
PISA
Public Utilities, General Disclosures [Line Items]
Amortization Period	20 years

Rate and Regulatory Matters (_2

Rate and Regulatory Matters (Regulatory Framework-Illinois) (Details) - Ameren Illinois Company	12 Months Ended
	Dec. 31, 2020
FEJA
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Return on Equity, Percentage	5.80%
Return on equity adjustment	2.00%
QIP rider
Public Utilities, General Disclosures [Line Items]
QIP recovery begin date	two months
QIP rider reset	zero
Natural gas
Public Utilities, General Disclosures [Line Items]
Number of months to complete a regulatory rate review	11 months
Electric Distribution | FEJA
Public Utilities, General Disclosures [Line Items]
Amortization Period	two years
Maximum | QIP rider
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage	4.00%
Annual QIP rate cap	5.50%
Maximum | Electric Distribution | FEJA
Public Utilities, General Disclosures [Line Items]
Return on equity penalty	0.38%

Rate and Regulatory Matters (_3

Rate and Regulatory Matters (Regulatory Framework-Federal) (Details)	12 Months Ended
	Dec. 31, 2020
Midwest Independent Transmission System Operator, Inc
Public Utilities, General Disclosures [Line Items]
Incentive adder to FERC allowed base return on common equity	0.50%
FERC revenue requirement reconciliation adjustment
Public Utilities, General Disclosures [Line Items]
Amortization Period	two years

Rate and Regulatory Matters (Na

Rate and Regulatory Matters (Narrative-Missouri) (Details) $ in Millions	2 Months Ended	3 Months Ended	12 Months Ended
	Feb. 22, 2021USD ($)MWh	Mar. 31, 2021MWh	Dec. 31, 2020USD ($)MWh	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Public Utilities, General Disclosures [Line Items]
Purchased power			$ 513	$ 556	$ 581
Other operations and maintenance			1,661	1,745	1,772
Income tax benefit			155	182	237
Donations			25	12	33
Wind generation expenditures			564	0	0
Union Electric Company
Public Utilities, General Disclosures [Line Items]
Purchased power			171	193	164
Other operations and maintenance			886	960	972
Income tax benefit			34	68	124
Donations			12	3	14
Wind generation expenditures			564	0	0
Final Rate Order | Union Electric Company | Electric
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount			32
Reduction to annualized base level of net energy costs			115
Revenues			20
Purchased power			15
Other operations and maintenance			60
Income tax benefit			20
Revenue requirement change due to changes in MEEIA sales volumes			50
Amounts previously deferred under PISA			50
Donations			8
Final Rate Order | Union Electric Company | MEEIA 2019 [Domain] | Electric
Public Utilities, General Disclosures [Line Items]
Revenues			6
Incentive award if energy efficiency goals are achieved			11
Incentive award if energy efficiency goals are exceeded			1
MEEIA Energy efficiency investments for the 2022 program year			$ 70
Final Rate Order | Union Electric Company | MEEIA 2013 & 2016 | Electric
Public Utilities, General Disclosures [Line Items]
Revenues				$ 37	$ 11
Wind Generation Facility | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Amount of Megawatts | MWh			400
High Prairie Renewable Energy Center purchase price			$ 615
Minimum | Final Rate Order | Union Electric Company | Electric
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Return on Equity, Percentage			9.40%
Maximum | Final Rate Order | Union Electric Company | Electric
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Return on Equity, Percentage			9.80%
Subsequent Event | Wind Generation Facility | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Amount of Megawatts | MWh		300
Megawatts in service | MWh	120	150
Atchison Renewable Energy Center purchase price	$ 200

Rate And Regulatory Matters (_4

Rate And Regulatory Matters (Narrative-Illinois) (Details) - Ameren Illinois Company - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Public Utilities, General Disclosures [Line Items]
Deferred payment arrangement extension		24 months
Financial assistance program costs		$ 8
Final Rate Order
Public Utilities, General Disclosures [Line Items]
Electric Energy-Efficiency Revenue Requirement		$ 51
QIP rider
Public Utilities, General Disclosures [Line Items]
QIP rider reset		zero
IEIMA revenue requirement reconciliation adjustment | Final Rate Order | Electric Distribution
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 49
IEIMA revenue requirement reconciliation adjustment | Pending Rate Case | Electric Distribution
Public Utilities, General Disclosures [Line Items]
Requested electric distribution service rate reconciliation period		2 years
FEJA energy-efficiency rider | Final Rate Order
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 7
Subsequent Event | Pending Rate Case | Natural gas
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 76
Public Utilities, Requested Return on Equity, Percentage	9.67%
Public Utilities, Requested Equity Capital Structure, Percentage	52.00%
Rate Base	$ 2,100

Rate and Regulatory Matters (_5

Rate and Regulatory Matters (Narrative-Federal) (Details) - USD ($) $ in Millions	1 Months Ended		12 Months Ended
	May 31, 2020	Nov. 30, 2019	Dec. 31, 2020	Dec. 31, 2019
Public Utilities, General Disclosures [Line Items]
Lower revenue due to transmission formula rate revisions			$ 2
Income recognized due to higher FERC-allowed base ROE			13
Current regulatory liabilities			121	$ 164
Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Income recognized due to higher FERC-allowed base ROE			7
Current regulatory liabilities			$ 88	$ 84
Midwest Independent Transmission System Operator, Inc
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Return on Equity, Percentage			12.38%
Incentive adder to FERC allowed base return on common equity			0.50%
Final Rate Order | Midwest Independent Transmission System Operator, Inc
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved 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%
Current regulatory liabilities			$ 15
Final Rate Order | Midwest Independent Transmission System Operator, Inc | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities			$ 7
Final Rate Order | Midwest Independent Transmission System Operator, Inc | Maximum
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Return on Equity, Percentage			10.82%

Rate And Regulatory Matters (Sc

Rate And Regulatory Matters (Schedule Of Regulatory Assets And Liabilities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 1,209	$ 1,061
Current regulatory assets	(109)	(69)
Regulatory Assets, Noncurrent	1,100	992
Regulatory Liabilities	5,403	5,051
Current regulatory liabilities	(121)	(164)
Regulatory Liability, Noncurrent	5,282	4,887
Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	407	293
Current regulatory assets	(60)	(8)
Regulatory Assets, Noncurrent	347	285
Regulatory Liabilities	3,136	2,999
Current regulatory liabilities	(26)	(62)
Regulatory Liability, Noncurrent	$ 3,110	2,937
Period between refueling and maintenance outages	18 months
PISA Deferral Percentage	85.00%
Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 779	751
Current regulatory assets	(37)	(57)
Regulatory Assets, Noncurrent	742	694
Regulatory Liabilities	2,151	1,933
Current regulatory liabilities	(88)	(84)
Regulatory Liability, Noncurrent	2,063	1,849
Under-recovered FAC
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	48	0
Under-recovered FAC | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	48	0
Under-recovered FAC | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
Under-recovered Illinois electric power costs
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	4	4
Under-recovered Illinois electric power costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
Under-recovered Illinois electric power costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 4	4
Amortization Period	one year
MTM derivative losses
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 221	254
MTM derivative losses | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	21	12
MTM derivative losses | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	200	242
IEIMA revenue requirement reconciliation adjustment
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	17
Regulatory Liabilities	22	18
IEIMA revenue requirement reconciliation adjustment | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
Regulatory Liabilities	0	0
IEIMA revenue requirement reconciliation adjustment | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	17
Regulatory Liabilities	$ 22	18
Amortization Period	two years
FERC revenue requirement reconciliation adjustment
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 50	16
Regulatory Liabilities	$ 21	38
Amortization Period	two years
FERC revenue requirement reconciliation adjustment | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 0	0
Regulatory Liabilities	0	0
FERC revenue requirement reconciliation adjustment | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	28	1
Regulatory Liabilities	21	37
Under/Over-recovered VBA
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	11	0
Regulatory Liabilities	0	8
Under/Over-recovered VBA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
Regulatory Liabilities	0	0
Under/Over-recovered VBA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	11	0
Regulatory Liabilities	0	8
Pension and postretirement benefit costs
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	33
Regulatory Liabilities	375	0
Pension and postretirement benefit costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	7
Regulatory Liabilities	198	0
Pension and postretirement benefit costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	26
Regulatory Liabilities	177	0
Income taxes
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	183	177
Regulatory Liabilities	$ 2,192	2,326
Weighted-Average Amortization Period	34 years
Income taxes | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 117	114
Regulatory Liabilities	$ 1,317	1,428
Weighted-Average Amortization Period	26 years
Income taxes | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 65	61
Regulatory Liabilities	$ 790	813
Weighted-Average Amortization Period	42 years
Bad debt rider
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 11	0
Bad debt rider | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
Bad debt rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	11	0
Callaway costs
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	14	18
Callaway costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	14	18
Callaway costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
Callaway refueling and maintenance outage costs
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	39	0
Callaway refueling and maintenance outage costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	39	0
Callaway refueling and maintenance outage costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
Unamortized loss on reacquired debt
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	74	86
Unamortized loss on reacquired debt | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	52	55
Unamortized loss on reacquired debt | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	22	31
Environmental cost riders
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	93	127
Environmental cost riders | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
Environmental cost riders | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	93	127
Storm costs
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	7	7
Storm costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
Storm costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	7	7
Workers' compensation claims
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	9	11
Workers' compensation claims | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	4	4
Workers' compensation claims | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	5	7
Allowance for funds used during construction for pollution control equipment
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	15	15
Allowance for funds used during construction for pollution control equipment | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	15	15
Allowance for funds used during construction for pollution control equipment | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
Customer generation rebate program
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	17	1
Customer generation rebate program | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
Customer generation rebate program | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	17	1
Solar rebate program
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	5	5
Solar rebate program | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	5	5
Solar rebate program | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
PISA
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	78	41
PISA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 78	41
Amortization Period	20 years
PISA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 0	0
RESRAM
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	2	9
RESRAM | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	2	9
RESRAM | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
FEJA energy-efficiency rider
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	283	211
FEJA energy-efficiency rider | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
FEJA energy-efficiency rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	283	211
Other regulatory assets
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	45	29
Other regulatory assets | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	12	13
Other regulatory assets | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	33	16
Over-recovered FAC
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	10	39
Over-recovered FAC | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 10	39
Accumulation Period	four months
Amortization Period	eight months
Over-recovered FAC | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 0	0
Over-recovered Illinois electric power costs
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	15	11
Over-recovered Illinois electric power costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	0	0
Over-recovered Illinois electric power costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	15	11
Over-recovered PGA
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	22	22
Over-recovered PGA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	7	8
Over-recovered PGA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	15	14
MTM derivative gains
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	21	21
MTM derivative gains | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	11	18
MTM derivative gains | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	10	3
Estimated refund for FERC complaint cases
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	15	40
Estimated refund for FERC complaint cases | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	0	0
Estimated refund for FERC complaint cases | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	7	23
Cost of removal
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	1,923	1,884
Cost of removal | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	1,027	1,041
Cost of removal | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	873	827
AROs
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	436	303
AROs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	436	303
AROs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	0	0
Pension and postretirement benefit costs tracker
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	55	72
Pension and postretirement benefit costs tracker | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 55	72
Regulatory Liability, Amortization Period	4 years
Pension and postretirement benefit costs tracker | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 0	0
Renewable energy credits and zero emission credits
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	200	155
Renewable energy credits and zero emission credits | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	0	0
Renewable energy credits and zero emission credits | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	200	155
Excess income taxes collected in 2018
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	45	60
Excess income taxes collected in 2018 | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	45	60
Excess income taxes collected in 2018 | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	0	0
Other regulatory liabilities
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	51	54
Other regulatory liabilities | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	30	30
Other regulatory liabilities | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 21	$ 24
FAC adjustments | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Accumulation Period	four months
Asset Retirement Obligation - Nuclear decommissioning trust fund balance | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 982
Asset Retirement Obligation - removal costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 546
Minimum | FEJA energy-efficiency rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Asset, Amortization Period	6 years
Maximum | FEJA energy-efficiency rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Asset, Amortization Period	13 years

Property And Plant, Net (Schedu

Property And Plant, Net (Schedule Of Property And Plant, Net) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)equipment	Dec. 31, 2019USD ($)
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	$ 39,719	$ 36,652
Accumulated depreciation and amortization	13,773	13,006
Property and plant, before construction work in progress	25,946	23,646
Property, Plant and Equipment, Net	$ 26,807	24,376
Number of combustion turbine electric generation equipment with related financing obligations | equipment	2
Number of financing obligations | equipment	2
Gross asset value, financing obligations	$ 240	236
Total accumulated depreciation, financing obligations	99	95
Coal Fired Electric Generation Equipment
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	4,875	4,730
Natural Gas and Oil Fired Electric Generation Equipment
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	1,097	1,090
Nuclear Powered Electric Generation Equipment
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	5,608	5,414
Renewable Electric Generation Equipment
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	1,301	646
Electric distribution
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	13,433	12,670
Electric transmission
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	6,831	6,148
Natural gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	3,869	3,552
Other
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	2,705	2,402
Construction work in progress	786	595
Nuclear fuel in process
Property, Plant and Equipment [Line Items]
Construction work in progress	75	135
Oil Fired Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost		36
Union Electric Company
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	23,098	21,357
Accumulated depreciation and amortization	9,689	9,195
Property and plant, before construction work in progress	13,409	12,162
Property, Plant and Equipment, Net	13,879	12,635
Union Electric Company | Coal Fired Electric Generation Equipment
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	4,875	4,730
Union Electric Company | Natural Gas and Oil Fired Electric Generation Equipment
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	1,097	1,090
Union Electric Company | Nuclear Powered Electric Generation Equipment
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	5,608	5,414
Union Electric Company | Renewable Electric Generation Equipment
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	1,301	646
Union Electric Company | Electric distribution
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	6,784	6,371
Union Electric Company | Electric transmission
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	1,482	1,405
Union Electric Company | Natural gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	561	528
Union Electric Company | Other
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	1,390	1,173
Construction work in progress	395	338
Union Electric Company | Nuclear fuel in process
Property, Plant and Equipment [Line Items]
Construction work in progress	75	135
Ameren Illinois Company
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	14,602	13,417
Accumulated depreciation and amortization	3,780	3,536
Property and plant, before construction work in progress	10,822	9,881
Property, Plant and Equipment, Net	11,201	10,083
Ameren Illinois Company | Coal Fired Electric Generation Equipment
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	0	0
Ameren Illinois Company | Natural Gas and Oil Fired Electric Generation Equipment
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	0	0
Ameren Illinois Company | Nuclear Powered Electric Generation Equipment
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	0	0
Ameren Illinois Company | Renewable Electric Generation Equipment
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	0	0
Ameren Illinois Company | Electric distribution
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	6,649	6,299
Ameren Illinois Company | Electric transmission
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	3,575	3,101
Ameren Illinois Company | Natural gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	3,308	3,024
Ameren Illinois Company | Other
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	1,070	993
Construction work in progress	379	202
Ameren Illinois Company | Nuclear fuel in process
Property, Plant and Equipment [Line Items]
Construction work in progress	0	0
Other
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	2,019	1,878
Accumulated depreciation and amortization	304	275
Property and plant, before construction work in progress	1,715	1,603
Property, Plant and Equipment, Net	1,727	1,658
Other | Coal Fired Electric Generation Equipment
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	0	0
Other | Natural Gas and Oil Fired Electric Generation Equipment
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	0	0
Other | Nuclear Powered Electric Generation Equipment
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	0	0
Other | Renewable Electric Generation Equipment
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	0	0
Other | Electric distribution
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	0	0
Other | Electric transmission
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	1,774	1,642
Other | Natural gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	0	0
Other | Other
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	245	236
Construction work in progress	12	55
Other | Nuclear fuel in process
Property, Plant and Equipment [Line Items]
Construction work in progress	$ 0	$ 0
Minimum | Electric generation
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Minimum | Electric distribution
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	20 years
Minimum | Electric transmission
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	50 years
Minimum | Natural gas
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	20 years
Minimum | Other
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Maximum | Electric generation
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	72 years
Maximum | Electric distribution
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	80 years
Maximum | Electric transmission
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	75 years
Maximum | Natural gas
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	80 years
Maximum | Other
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	55 years
Maximum | Union Electric Company | Electric generation
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	150 years

Property and Plant, Net (Sche_2

Property and Plant, Net (Schedule of Capitalized Software) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Capitalized software costs
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ 93	$ 78	$ 71
Gross carrying value	1,021	901
Accumulated amortization	(640)	(584)
Union Electric Company | Capitalized software costs
Finite-Lived Intangible Assets [Line Items]
Amortization expense	44	30	24
Gross carrying value	398	303
Accumulated amortization	(189)	(153)
Ameren Illinois Company | Capitalized software costs
Finite-Lived Intangible Assets [Line Items]
Amortization expense	45	45	$ 44
Gross carrying value	397	377
Accumulated amortization	$ (238)	$ (221)
Minimum
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	3 years
Maximum
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	10 years

Property and Plant, Net (Sche_3

Property and Plant, Net (Schedule of Capitalized Software, Future Amortization Expense) (Details) - Capitalized software costs $ in Millions	Dec. 31, 2020USD ($)
Finite-Lived Intangible Assets [Line Items]
2021	$ 105
2022	94
2023	81
2024	55
2025	23
Union Electric Company
Finite-Lived Intangible Assets [Line Items]
2021	55
2022	49
2023	45
2024	33
2025	15
Ameren Illinois Company
Finite-Lived Intangible Assets [Line Items]
2021	46
2022	42
2023	34
2024	21
2025	$ 7

Short-Term Debt And Liquidity_2

Short-Term Debt And Liquidity (Narrative) (Details)	12 Months Ended
	Dec. 31, 2020USD ($)lender	Dec. 31, 2019
Missouri Credit Agreement 2012
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,200,000,000
Illinois Credit Agreement 2012
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	1,100,000,000
Multiyear Credit Facility
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	2,300,000,000
Line of Credit Facility, Commitment Fee Amount	$ 100,000,000
Actual debt-to-capital ratio	0.56
Multiyear Credit Facility | Maximum
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.65
Credit Agreements
Short-term Debt [Line Items]
Net Liquidity Available	$ 1,900,000,000
Covenant terms, default provisions, maximum indebtedness	100,000,000
Parent Company | Missouri Credit Agreement 2012
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	900,000,000
Parent Company | Illinois Credit Agreement 2012
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	500,000,000
Union Electric Company | Missouri Credit Agreement 2012
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 850,000,000
Actual debt-to-capital ratio	0.48
Ameren Illinois Company | Illinois Credit Agreement 2012
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 800,000,000
Actual debt-to-capital ratio	0.45
Utilities [Member]
Short-term Debt [Line Items]
Short Term Debt, Weighted Average Interest Rate During Period	0.64%	2.48%
Multiyear Credit Facility
Short-term Debt [Line Items]
Number of lenders | lender	22
Line of credit facility, maximum borrowing capacity, per lender	$ 130,000,000
Multiyear Credit Facility | Missouri Credit Agreement 2012
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	1,400,000,000
Multiyear Credit Facility | Illinois Credit Agreement 2012
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,300,000,000

Short-Term Debt And Liquidity_3

Short-Term Debt And Liquidity (Schedule Of Maximum Aggregate Amount Available On Credit Agreements) (Details) $ in Millions	Dec. 31, 2020USD ($)
Missouri Credit Agreement 2012
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,200
Missouri Credit Agreement 2012 | Parent Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	900
Missouri Credit Agreement 2012 | Union Electric Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	850
Illinois Credit Agreement 2012
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	1,100
Illinois Credit Agreement 2012 | Parent Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	500
Illinois Credit Agreement 2012 | Ameren Illinois Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 800

Short-Term Debt And Liquidity_4

Short-Term Debt And Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Short-term Debt [Line Items]
Average daily amount outstanding	$ 263	$ 700
Commercial paper issuances outstanding at period-end	$ 490	$ 440
Weighted-average interest rate	1.31%	2.60%
Peak amount outstanding during period(a)	$ 908	$ 1,113
Peak interest rate	5.05%	5.00%
Parent Company
Short-term Debt [Line Items]
Average daily amount outstanding	$ 108	$ 421
Commercial paper issuances outstanding at period-end	$ 490	$ 153
Weighted-average interest rate	1.04%	2.66%
Peak amount outstanding during period(a)	$ 490	$ 651
Peak interest rate	3.30%	3.80%
Union Electric Company
Short-term Debt [Line Items]
Average daily amount outstanding	$ 109	$ 122
Commercial paper issuances outstanding at period-end	$ 0	$ 234
Weighted-average interest rate	1.73%	2.62%
Peak amount outstanding during period(a)	$ 573	$ 549
Peak interest rate	5.05%	2.97%
Ameren Illinois Company
Short-term Debt [Line Items]
Average daily amount outstanding	$ 46	$ 157
Commercial paper issuances outstanding at period-end	$ 0	$ 53
Weighted-average interest rate	0.97%	2.43%
Peak amount outstanding during period(a)	$ 250	$ 356
Peak interest rate	3.40%	5.00%

Long-Term Debt And Equity Fin_3

Long-Term Debt And Equity Financings (Narrative) (Details)	1 Months Ended	12 Months Ended
	Oct. 31, 2020USD ($)	Dec. 31, 2020USD ($)numberOfWindGenerationFacilities$ / sharesshares	Dec. 31, 2019USD ($)shares	Dec. 31, 2018USD ($)shares	Feb. 11, 2021USD ($)shares	Dec. 29, 2020USD ($)shares	Oct. 30, 2020USD ($)	Feb. 03, 2020USD ($)	Nov. 26, 2019USD ($)	Oct. 01, 2019USD ($)Rate	Aug. 05, 2019shares
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized (in shares) | shares		100,000,000
Preferred stock, par value (in dollars per share) | $ / shares		$ 0.01
Preferred stock, shares outstanding (in shares) | shares		0
Stock Issued During Period, Shares, New Issues | shares		700,000	900,000	1,200,000
Issuances of common stock		$ 476,000,000	$ 68,000,000	$ 74,000,000
Stock Issued During Period, Shares, Other | shares		500,000	800,000	700,000
Stock Issued		$ 38,000,000	$ 54,000,000	$ 35,000,000
Common Stock, Shares Authorized Under DRPlus Plan | shares		4,000,000
Common Stock, Shares Authorized Under 401(k) Plan | shares		4,000,000
Forward Contract Indexed to Issuer's Equity, Indexed Shares | shares											7,500,000
Repayments of Other Long-term Debt		$ 442,000,000	$ 580,000,000	$ 841,000,000
Debt Default Provision Excess		$ 25,000,000
Shares issued under forward sale agreement | shares		5,900,000	0	0		5,900,000
Common Stock Value Issued Through Forward Sale Agreement						$ 425,000,000
Dividend reinvestment and 401 (k) plans [Member]
Long-Term Debt And Equity Financings [Line Items]
Issuances of common stock		$ 51,000,000	$ 68,000,000	$ 74,000,000
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized (in shares) | shares		7,500,000
Preferred stock, par value (in dollars per share) | $ / shares		$ 1
Preferred stock, shares outstanding (in shares) | shares		0
Stock Issued		$ 0	0	0
Repayments of Other Long-term Debt		$ 92,000,000	580,000,000	384,000,000
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized (in shares) | shares		2,600,000
Preferred stock, par value (in dollars per share) | $ / shares		$ 0
Preferred stock, shares outstanding (in shares) | shares		0
Stock Issued		$ 0	0	0
Repayments of Other Long-term Debt		$ 0	0	$ 457,000,000
Common stock equity to capitalization ratio		54.00%
Ameren Missouri and Ameren Illinois
Long-Term Debt And Equity Financings [Line Items]
Bonds interest rate assumption		5.00%
Dividend rate on preferred shares, percentage		6.00%
Senior Unsecured Notes 2.50% Due 2024 | Unsecured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 450,000,000
Proceeds from Issuance of Unsecured Debt			447,000,000
Senior Unsecured Notes 2.50% Due 2024 | Ameren (parent) | Unsecured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 450,000,000	450,000,000
Long-term debt interest rate		2.50%
5.00% Senior secured notes due 2020 | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount								$ 85,000,000
5.00% Senior secured notes due 2020 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 0	85,000,000
Long-term debt interest rate		5.00%
First Mortgage Bonds, 3.50%, Due 2029 - $450 Issuance | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 450,000,000	450,000,000
Long-term debt interest rate		3.50%
Senior Secured Notes 6.70% due 2019 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate		6.70%
Repayments of Other Long-term Debt		$ 329,000,000
1992 Series Bonds, 1998 Series A Bonds, 1998 Series B Bonds, 1998 Series C Bonds [Domain] | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		207,000,000
First Mortgage Bonds, 3.25%, Due 2049 - $330 Issuance | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 330,000,000	330,000,000							$ 330,000,000
Long-term debt interest rate		3.25%
Proceeds from issuance of secured debt		$ 326,000,000
Senior Unsecured Notes 5.10% Due 2019 [Domain] | Union Electric Company | Unsecured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate | Rate										5.10%
Repayments of Other Long-term Debt		244,000,000
First Mortgage Bonds, 5.45%, Due 2028 - Less Than $1 Million Outstanding [Domain] | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount										$ 1,000,000
Long-term debt interest rate | Rate										5.45%
First Mortgage Bonds, 4.00%, Due 2048 - $425 Issuance | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 425,000,000	425,000,000
Long-term debt interest rate		4.00%
Proceeds from issuance of secured debt			447,000,000
First Mortgage Bonds, 3.25%, Due 2050 - $300 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 300,000,000	300,000,000						$ 300,000,000
Long-term debt interest rate		3.25%
Proceeds from issuance of secured debt			296,000,000
First Mortgage Bonds, 4.50%, Due 2049 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 500,000,000	500,000,000
Long-term debt interest rate		4.50%
Senior Unsecured Notes, 3.43%, Due 2050 | Ameren Transmission Company of Illinois | Unsecured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 450,000,000	450,000,000
Ratio of Indebtedness to Net Capital		0.70
Ratio of Indebtedness to Total Assets		0.10
Senior Unsecured Notes, 3.43%, Due 2050 | Ameren Transmission Company of Illinois | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate		3.43%
Senior Unsecured Notes 2.70% due 2020 | Ameren (parent) | Unsecured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 0	350,000,000				$ 350,000,000
Long-term debt interest rate		2.70%
Senior Secured Notes 3.50% Due 2031 | Ameren (parent) | Unsecured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 800,000,000	0
Long-term debt interest rate		3.50%
Proceeds from Issuance of Unsecured Debt		$ 793,000,000
First Mortgage Bonds, 2.95%, Due 2030 - $465 Issuance | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 465,000,000	0
Long-term debt interest rate		2.95%
Proceeds from issuance of secured debt		$ 462,000,000
First Mortgage Bonds, 2.625%, Due 2051 - $550 Issuance | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 550,000,000	0
Long-term debt interest rate		2.625%
Proceeds from issuance of secured debt	$ 543,000,000
Number Of Acquired Wind Generation Facilities | numberOfWindGenerationFacilities		2
First Mortgage Bonds, 1.55%, Due 2030 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 375,000,000	$ 0
Long-term debt interest rate		1.55%
Proceeds from issuance of secured debt		$ 371,000,000
Minimum | Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Common stock equity to capitalization ratio		30.00%
Subsequent Event
Long-Term Debt And Equity Financings [Line Items]
Shares issued under forward sale agreement | shares					1,600,000
Common Stock Value Issued Through Forward Sale Agreement					$ 113,000,000

Long-Term Debt And Equity Fin_4

Long-Term Debt And Equity Financings (Schedule Of Long-Term Debt Outstanding) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Oct. 30, 2020	Feb. 03, 2020	Dec. 31, 2019	Nov. 26, 2019	Oct. 01, 2019
Debt Instrument [Line Items]
Less: Maturities due within one year	$ (8,000,000)			$ (442,000,000)
Long-term Debt, Net	11,078,000,000			8,915,000,000
August 2022	505,000,000
August 2024	850,000,000
Thereafter	9,287,000,000
Ameren (parent)
Debt Instrument [Line Items]
Long-term debt, gross	1,600,000,000			1,150,000,000
Less: Unamortized discount and premium	(2,000,000)			0
Debt Issuance Costs, Net	(10,000,000)			(6,000,000)
Long-term Debt, Net	1,588,000,000			794,000,000
August 2022	0
August 2024	450,000,000
Thereafter	1,150,000,000
Union Electric Company
Debt Instrument [Line Items]
Long-term debt, gross	5,152,000,000			4,229,000,000
Less: Unamortized discount and premium	(12,000,000)			(9,000,000)
Debt Issuance Costs, Net	(36,000,000)			(30,000,000)
Less: Maturities due within one year	(8,000,000)			(92,000,000)
Long-term Debt, Net	5,096,000,000			4,098,000,000
Debt Securities, Held-to-maturity	256,000,000			263,000,000
August 2022	55,000,000
August 2024	350,000,000
Thereafter	4,499,000,000
Union Electric Company | City Of Bowling Green Agreement - Peno Creek Ct
Debt Instrument [Line Items]
Debt instrument face amount	16,000,000			23,000,000
Union Electric Company | Audrain County Agreement - Audrain County Ct
Debt Instrument [Line Items]
Debt instrument face amount	240,000,000			240,000,000
Ameren Illinois Company
Debt Instrument [Line Items]
Long-term debt, gross	3,988,000,000			3,613,000,000
Less: Unamortized discount and premium	(6,000,000)			(4,000,000)
Debt Issuance Costs, Net	(36,000,000)			(34,000,000)
Long-term Debt, Net	3,946,000,000			3,575,000,000
August 2022	400,000,000
August 2024	0
Thereafter	3,288,000,000
Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Long-term debt, gross	450,000,000			450,000,000
Debt Issuance Costs, Net	(2,000,000)			(2,000,000)
Long-term Debt, Net	448,000,000			448,000,000
August 2022	50,000,000
August 2024	50,000,000
Thereafter	350,000,000
Unsecured Debt | Senior Unsecured Notes 2.50% Due 2024
Debt Instrument [Line Items]
Debt instrument face amount	450,000,000
Unsecured Debt | Ameren (parent) | Senior Unsecured Notes 2.70% due 2020
Debt Instrument [Line Items]
Debt instrument face amount	$ 0	$ 350,000,000		350,000,000
Long-term debt interest rate	2.70%
Unsecured Debt | Ameren (parent) | Senior Unsecured Notes 2.50% Due 2024
Debt Instrument [Line Items]
Debt instrument face amount	$ 450,000,000			450,000,000
Long-term debt interest rate	2.50%
Unsecured Debt | Ameren (parent) | Senior Unsecured Notes365 due 2026
Debt Instrument [Line Items]
Debt instrument face amount	$ 350,000,000			350,000,000
Long-term debt interest rate	3.65%
Unsecured Debt | Ameren (parent) | Senior Secured Notes 3.50% Due 2031
Debt Instrument [Line Items]
Debt instrument face amount	$ 800,000,000			0
Long-term debt interest rate	3.50%
Unsecured Debt | Ameren Transmission Company of Illinois | Senior Unsecured Notes, 3.43%, Due 2050
Debt Instrument [Line Items]
Debt instrument face amount	$ 450,000,000			450,000,000
Secured Debt | 5.00% Senior secured notes due 2020
Debt Instrument [Line Items]
Debt instrument face amount			$ 85,000,000
Secured Debt | Union Electric Company | 5.00% Senior secured notes due 2020
Debt Instrument [Line Items]
Debt instrument face amount	$ 0			85,000,000
Long-term debt interest rate	5.00%
Secured Debt | Union Electric Company | Senior Secured Notes350 Due2024
Debt Instrument [Line Items]
Debt instrument face amount	$ 350,000,000			350,000,000
Long-term debt interest rate	3.50%
Secured Debt | Union Electric Company | Senior Secured Notes, 2.95%, Due 2027
Debt Instrument [Line Items]
Debt instrument face amount	$ 400,000,000			400,000,000
Long-term debt interest rate	2.95%
Secured Debt | Union Electric Company | First Mortgage Bonds, 3.50%, Due 2029 - $450 Issuance
Debt Instrument [Line Items]
Debt instrument face amount	$ 450,000,000			450,000,000
Long-term debt interest rate	3.50%
Secured Debt | Union Electric Company | First Mortgage Bonds, 2.95%, Due 2030 - $465 Issuance
Debt Instrument [Line Items]
Debt instrument face amount	$ 465,000,000			0
Long-term debt interest rate	2.95%
Secured Debt | Union Electric Company | 5.50% Senior secured notes due 2034
Debt Instrument [Line Items]
Debt instrument face amount	$ 184,000,000			184,000,000
Long-term debt interest rate	5.50%
Secured Debt | Union Electric Company | 5.30% Senior secured notes due 2037
Debt Instrument [Line Items]
Debt instrument face amount	$ 300,000,000			300,000,000
Long-term debt interest rate	5.30%
Secured Debt | Union Electric Company | 8.45% Senior secured notes due 2039
Debt Instrument [Line Items]
Debt instrument face amount	$ 350,000,000			350,000,000
Long-term debt interest rate	8.45%
Secured Debt | Union Electric Company | 3.90% Senior secured notes due 2042
Debt Instrument [Line Items]
Debt instrument face amount	$ 485,000,000			485,000,000
Long-term debt interest rate	3.90%
Secured Debt | Union Electric Company | Senior Secured Notes, 3.65%, Due 2045
Debt Instrument [Line Items]
Debt instrument face amount	$ 400,000,000			400,000,000
Long-term debt interest rate	3.65%
Secured Debt | Union Electric Company | First Mortgage Bonds, 4.00%, Due 2048 - $425 Issuance
Debt Instrument [Line Items]
Debt instrument face amount	$ 425,000,000			425,000,000
Long-term debt interest rate	4.00%
Secured Debt | Union Electric Company | First Mortgage Bonds, 3.25%, Due 2049 - $330 Issuance
Debt Instrument [Line Items]
Debt instrument face amount	$ 330,000,000			330,000,000		$ 330,000,000
Long-term debt interest rate	3.25%
Secured Debt | Union Electric Company | First Mortgage Bonds, 2.625%, Due 2051 - $550 Issuance
Debt Instrument [Line Items]
Debt instrument face amount	$ 550,000,000			0
Long-term debt interest rate	2.625%
Secured Debt | Ameren Illinois Company | Senior Secured Notes, 2.70%, Due 2022
Debt Instrument [Line Items]
Debt instrument face amount	$ 400,000,000			400,000,000
Long-term debt interest rate	2.70%
Secured Debt | Ameren Illinois Company | Senior Secured Notes, 3.25%, Due 2025
Debt Instrument [Line Items]
Debt instrument face amount	$ 300,000,000			300,000,000
Long-term debt interest rate	3.25%
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.125% Due 2028
Debt Instrument [Line Items]
Debt instrument face amount	$ 60,000,000			60,000,000
Long-term debt interest rate	6.125%
Secured Debt | Ameren Illinois Company | First Mortgage Bonds, 3.80%, Due 2028
Debt Instrument [Line Items]
Debt instrument face amount	$ 430,000,000			430,000,000
Long-term debt interest rate	3.80%
Secured Debt | Ameren Illinois Company | First Mortgage Bonds, 1.55%, Due 2030
Debt Instrument [Line Items]
Debt instrument face amount	$ 375,000,000			0
Long-term debt interest rate	1.55%
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.70% Due 2036
Debt Instrument [Line Items]
Debt instrument face amount	$ 61,000,000			61,000,000
Long-term debt interest rate	6.70%
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.70% Due 2036
Debt Instrument [Line Items]
Debt instrument face amount	$ 42,000,000			42,000,000
Long-term debt interest rate	6.70%
Secured Debt | Ameren Illinois Company | Senior Secured Notes 4.80% Due 2043
Debt Instrument [Line Items]
Debt instrument face amount	$ 280,000,000			280,000,000
Long-term debt interest rate	4.80%
Secured Debt | Ameren Illinois Company | Senior Secured Notes 4.30% Due 2044
Debt Instrument [Line Items]
Debt instrument face amount	$ 250,000,000			250,000,000
Long-term debt interest rate	4.30%
Secured Debt | Ameren Illinois Company | Senior Secured Notes, 4.15%, Due 2046
Debt Instrument [Line Items]
Debt instrument face amount	$ 490,000,000			490,000,000
Long-term debt interest rate	4.15%
Secured Debt | Ameren Illinois Company | Senior Secured Notes, 3.70%, Due 2047
Debt Instrument [Line Items]
Debt instrument face amount	$ 500,000,000			500,000,000
Long-term debt interest rate	3.70%
Secured Debt | Ameren Illinois Company | First Mortgage Bonds, 4.50%, Due 2049
Debt Instrument [Line Items]
Debt instrument face amount	$ 500,000,000			500,000,000
Long-term debt interest rate	4.50%
Secured Debt | Ameren Illinois Company | First Mortgage Bonds, 3.25%, Due 2050 - $300
Debt Instrument [Line Items]
Debt instrument face amount	$ 300,000,000			300,000,000	$ 300,000,000
Long-term debt interest rate	3.25%
Secured Debt | Ameren Transmission Company of Illinois | Senior Unsecured Notes, 3.43%, Due 2050
Debt Instrument [Line Items]
Long-term debt interest rate	3.43%
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1992 Series due 2022
Debt Instrument [Line Items]
Debt instrument face amount	$ 47,000,000			47,000,000
Long-term debt interest rate	1.60%
Debt Instrument, Interest Rate During Period	2.58%
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1998 Series A due 2033
Debt Instrument [Line Items]
Debt instrument face amount	$ 60,000,000			60,000,000
Long-term debt interest rate	2.90%
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1998 Series B due 2033
Debt Instrument [Line Items]
Debt instrument face amount	$ 50,000,000			50,000,000
Long-term debt interest rate	2.90%
Debt Instrument, Interest Rate During Period	3.57%
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1998 Series C due 2033
Debt Instrument [Line Items]
Debt instrument face amount	$ 50,000,000			$ 50,000,000
Long-term debt interest rate	2.75%
Debt Instrument, Interest Rate During Period	3.43%
Senior Notes | Ameren Transmission | Senior Unsecured Notes, 3.43%, Due 2050
Debt Instrument [Line Items]
Long-term debt interest rate	3.43%
Debt Instrument, Redemption, Period One | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
August 2022	$ 49,500,000
Debt Instrument, Redemption, Period Two | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
August 2024	49,500,000
Debt Instrument, Redemption, Period Three | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Thereafter	49,500,000
Debt Instrument, Redemption, Period Four | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Thereafter	49,500,000
Debt Instrument, Redemption, Period Five | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Thereafter	49,500,000
Debt Instrument, Redemption, Period Six | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Thereafter	49,500,000
Debt Instrument, Redemption, Period Seven | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Thereafter	76,500,000
Debt Instrument, Redemption, Period Eight | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Thereafter	$ 76,500,000

Long-Term Debt And Equity Fin_5

Long-Term Debt And Equity Financings (Schedule Of Maturities Of Long-Term Debt) (Details) $ in Millions	Dec. 31, 2020USD ($)
Debt Instrument [Line Items]
2021	$ 8
2022	505
2023	240
August 2024	850
2025	300
Thereafter	9,287
Total	11,190
Ameren (parent)
Debt Instrument [Line Items]
2021	0
2022	0
2023	0
August 2024	450
2025	0
Thereafter	1,150
Total	1,600
Debt Instrument, Unamortized Discount or Premium and Debt Issuance Costs	12
Union Electric Company
Debt Instrument [Line Items]
2021	8
2022	55
2023	240
August 2024	350
2025	0
Thereafter	4,499
Total	5,152
Debt Instrument, Unamortized Discount or Premium and Debt Issuance Costs	48
Ameren Illinois Company
Debt Instrument [Line Items]
2021	0
2022	400
2023	0
August 2024	0
2025	300
Thereafter	3,288
Total	3,988
Debt Instrument, Unamortized Discount or Premium and Debt Issuance Costs	42
Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
2021	0
2022	50
2023	0
August 2024	50
2025	0
Thereafter	350
Total	450
Debt Instrument, Unamortized Discount or Premium and Debt Issuance Costs	$ 2

Long-Term Debt And Equity Fin_6

Long-Term Debt And Equity Financings (Schedule Of Outstanding Preferred Stock) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	0
Preferred stock, voluntary liquidation (in dollars per share)	$ 105.50
Preferred stock, par value (in dollars per share)	$ 0.01
Preferred stock, authorized (in shares)	100,000,000
Union Electric Company and Ameren Illinois
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, issued (in shares)	$ 142	$ 142
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	0
Preferred stock, issued (in shares)	$ 80	80
Preferred stock, par value (in dollars per share)	$ 1
Preferred stock, authorized (in shares)	7,500,000
Union Electric Company | $3.50 Series
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	130,000
Preferred stock, redemption price per share (in dollars per share)	$ 110
Preferred stock, issued (in shares)	$ 13	13
Dividend rate on preferred shares, per-dollar amount (in dollars per share)	$ 3.50
Union Electric Company | $3.70 Series
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	40,000
Preferred stock, redemption price per share (in dollars per share)	$ 104.75
Preferred stock, issued (in shares)	$ 4	4
Dividend rate on preferred shares, per-dollar amount (in dollars per share)	$ 3.70
Union Electric Company | $4.00 Series
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	150,000
Preferred stock, redemption price per share (in dollars per share)	$ 105.625
Preferred stock, issued (in shares)	$ 15	15
Dividend rate on preferred shares, per-dollar amount (in dollars per share)	$ 4
Union Electric Company | $4.30 Series
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	40,000
Preferred stock, redemption price per share (in dollars per share)	$ 105
Preferred stock, issued (in shares)	$ 4	4
Dividend rate on preferred shares, per-dollar amount (in dollars per share)	$ 4.30
Union Electric Company | $4.50 Series
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	213,595
Preferred stock, redemption price per share (in dollars per share)	$ 110
Preferred stock, issued (in shares)	$ 21	21
Dividend rate on preferred shares, per-dollar amount (in dollars per share)	$ 4.50
Union Electric Company | $4.56 Series
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	200,000
Preferred stock, redemption price per share (in dollars per share)	$ 102.47
Preferred stock, issued (in shares)	$ 20	20
Dividend rate on preferred shares, per-dollar amount (in dollars per share)	$ 4.56
Union Electric Company | $4.75 Series
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	20,000
Preferred stock, redemption price per share (in dollars per share)	$ 102.176
Preferred stock, issued (in shares)	$ 2	2
Dividend rate on preferred shares, per-dollar amount (in dollars per share)	$ 4.75
Union Electric Company | $5.50 Series A
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	14,000
Preferred stock, redemption price per share (in dollars per share)	$ 110
Preferred stock, issued (in shares)	$ 1	1
Dividend rate on preferred shares, per-dollar amount (in dollars per share)	$ 5.50
Union Electric Company | Par Value $100
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, par value (in dollars per share)	$ 100
Preferred stock, authorized (in shares)	25,000,000
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	0
Preferred stock, issued (in shares)	$ 62	62
Preferred stock, par value (in dollars per share)	$ 0
Preferred stock, authorized (in shares)	2,600,000
Ameren Illinois Company | 4.00% Series
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	144,275
Preferred stock, redemption price per share (in dollars per share)	$ 101
Preferred stock, issued (in shares)	$ 14	14
Dividend rate on preferred shares, percentage	0.04%
Ameren Illinois Company | 4.08% Series
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	45,224
Preferred stock, redemption price per share (in dollars per share)	$ 103
Preferred stock, issued (in shares)	$ 5	5
Dividend rate on preferred shares, percentage	0.0408%
Ameren Illinois Company | 4.20% Series
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	23,655
Preferred stock, redemption price per share (in dollars per share)	$ 104
Preferred stock, issued (in shares)	$ 2	2
Dividend rate on preferred shares, percentage	0.042%
Ameren Illinois Company | 4.25% Series
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	50,000
Preferred stock, redemption price per share (in dollars per share)	$ 102
Preferred stock, issued (in shares)	$ 5	5
Dividend rate on preferred shares, percentage	0.0425%
Ameren Illinois Company | 4.26% Series
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	16,621
Preferred stock, redemption price per share (in dollars per share)	$ 103
Preferred stock, issued (in shares)	$ 2	2
Dividend rate on preferred shares, percentage	0.0426%
Ameren Illinois Company | 4.42% Series
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	16,190
Preferred stock, redemption price per share (in dollars per share)	$ 103
Preferred stock, issued (in shares)	$ 2	2
Dividend rate on preferred shares, percentage	0.0442%
Ameren Illinois Company | 4.70% Series
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	18,429
Preferred stock, redemption price per share (in dollars per share)	$ 104.30
Preferred stock, issued (in shares)	$ 2	2
Dividend rate on preferred shares, percentage	0.047%
Ameren Illinois Company | 4.90% Series
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	73,825
Preferred stock, redemption price per share (in dollars per share)	$ 102
Preferred stock, issued (in shares)	$ 7	7
Dividend rate on preferred shares, percentage	0.049%
Ameren Illinois Company | 4.92% Series
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	49,289
Preferred stock, redemption price per share (in dollars per share)	$ 103.50
Preferred stock, issued (in shares)	$ 5	5
Dividend rate on preferred shares, percentage	0.0492%
Ameren Illinois Company | 5.16% Series
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	50,000
Preferred stock, redemption price per share (in dollars per share)	$ 102
Preferred stock, issued (in shares)	$ 5	5
Dividend rate on preferred shares, percentage	0.0516%
Ameren Illinois Company | 6.625% Series
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	124,274
Preferred stock, redemption price per share (in dollars per share)	$ 100
Preferred stock, issued (in shares)	$ 12	12
Dividend rate on preferred shares, percentage	0.00066%
Ameren Illinois Company | 7.75% Series
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, shares outstanding (in shares)	4,542
Preferred stock, redemption price per share (in dollars per share)	$ 100
Preferred stock, issued (in shares)	$ 1	$ 1
Dividend rate on preferred shares, percentage	0.0775%
Ameren Illinois Company | Par Value $100
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, par value (in dollars per share)	$ 100
Preferred stock, authorized (in shares)	2,000,000

Long-Term Debt And Equity Fin_7

Long-Term Debt And Equity Financings Long-Term Debt And Equity Financings (Schedule Of Environmental Bonds) (Details) - Union Electric Company - Environmental Improvement And Pollution Control Revenue Bonds - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
1992 Series due 2022
Debt Instrument [Line Items]
Debt instrument face amount	$ 47,000,000	$ 47,000,000
Long-term debt interest rate	1.60%
Debt Instrument, Interest Rate During Period	2.58%
Series A 1998 Due 2033
Debt Instrument [Line Items]
Debt instrument face amount	$ 60,000,000
Debt Instrument, Interest Rate During Period	3.43%
1998 Series B due 2033
Debt Instrument [Line Items]
Debt instrument face amount	$ 50,000,000	50,000,000
Long-term debt interest rate	2.90%
Debt Instrument, Interest Rate During Period	3.57%
1998 Series C due 2033
Debt Instrument [Line Items]
Debt instrument face amount	$ 50,000,000	50,000,000
Long-term debt interest rate	2.75%
Debt Instrument, Interest Rate During Period	3.43%
1998 Series A due 2033
Debt Instrument [Line Items]
Debt instrument face amount	$ 60,000,000	$ 60,000,000
Long-term debt interest rate	2.90%

Long-Term Debt and Equity Fin_8

Long-Term Debt and Equity Financings (Schedule of Required and Actual Debt Ratios) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)
Union Electric Company
Debt Instrument [Line Items]
Bonds Issuable Based On Coverage Ratio	$ 5,025
Preferred Stock Issuable Based On Coverage Ratio	2,872
Retired Bond Capacity	$ 2,442
Union Electric Company | Actual Interest Coverage Ratio
Debt Instrument [Line Items]
Interest Coverage Ratio	3.4
Dividend Coverage Ratio	128.5
Ameren Illinois Company
Debt Instrument [Line Items]
Bonds Issuable Based On Coverage Ratio	$ 7,229
Preferred Stock Issuable Based On Coverage Ratio	203
Retired Bond Capacity	$ 643
Ameren Illinois Company | Actual Interest Coverage Ratio
Debt Instrument [Line Items]
Interest Coverage Ratio	7.1
Dividend Coverage Ratio	3.3
Minimum | Union Electric Company | Required Dividend Coverage Ratio
Debt Instrument [Line Items]
Interest Coverage Ratio	2
Dividend Coverage Ratio	2.5
Minimum | Ameren Illinois Company | Required Dividend Coverage Ratio
Debt Instrument [Line Items]
Interest Coverage Ratio	2
Dividend Coverage Ratio	1.5

Other Income, Net (Other Income

Other Income, Net (Other Income And Expenses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 32	$ 28	$ 36
Interest income on industrial development revenue bonds	25	25	26
Other interest income	4	8	7
Non-service cost components of net periodic benefit income	116	90	70
Miscellaneous income	13	6	8
Donations	(25)	(12)	(33)
Miscellaneous expense	(14)	(15)	(12)
Total other income, net	151	130	102
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	19	19	27
Interest income on industrial development revenue bonds	25	25	26
Other interest income	1	1	2
Non-service cost components of net periodic benefit income	46	18	17
Miscellaneous income	4	5	4
Donations	(12)	(3)	(14)
Miscellaneous expense	(7)	(7)	(6)
Total other income, net	76	58	56
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	(4)	29	17
Union Electric Company | Final Rate Order | Electric
Other Nonoperating Income (Expense) [Line Items]
Donations	(8)
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	13	9	9
Other interest income	3	6	6
Non-service cost components of net periodic benefit income	48	47	34
Miscellaneous income	6	3	3
Donations	(5)	(5)	(6)
Miscellaneous expense	(6)	(7)	(4)
Total other income, net	$ 59	$ 53	$ 42

Derivative Financial Instrume_3

Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Details) lb in Thousands, gal in Millions, MWh in Millions, MMBTU in Millions	12 Months Ended
	Dec. 31, 2020MMBTUMWhlbgal	Dec. 31, 2019MWhMMBTUlbgal
Fuel Oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | gal	43	58
Natural gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	147	156
Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	13	12
Uranium
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass | lb	365	565
Union Electric Company | Fuel Oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | gal	43	58
Union Electric Company | Natural gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	33	20
Union Electric Company | Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	6	5
Union Electric Company | Uranium
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass | lb	365	565
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	114	136
Ameren Illinois Company | Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	7	7
Ameren Illinois Company | Uranium
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass | lb	0	0

Derivative Financial Instrume_4

Derivative Financial Instruments (Derivative Instruments Carrying Value) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Derivative assets	$ 22	$ 26
Derivative liabilities	221	255
Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	2	4
Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets	0	2
Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities	7	4
Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	2	3
Natural gas | Other current assets
Derivative [Line Items]
Derivative assets	9	3
Natural gas | Other assets
Derivative [Line Items]
Derivative assets	4	1
Natural gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	2	13
Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	1	7
Power | Other current assets
Derivative [Line Items]
Derivative assets	7	14
Power | Other assets
Derivative [Line Items]
Derivative assets	0	2
Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	20	19
Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	189	208
Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Union Electric Company
Derivative [Line Items]
Derivative assets	12	22
Derivative liabilities	21	13
Union Electric Company | Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	2	4
Union Electric Company | Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets	0	2
Union Electric Company | Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities	7	4
Union Electric Company | Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	2	3
Union Electric Company | Natural gas | Other current assets
Derivative [Line Items]
Derivative assets	1	0
Union Electric Company | Natural gas | Other assets
Derivative [Line Items]
Derivative assets	2	0
Union Electric Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	1	1
Union Electric Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Union Electric Company | Power | Other current assets
Derivative [Line Items]
Derivative assets	7	14
Union Electric Company | Power | Other assets
Derivative [Line Items]
Derivative assets	0	2
Union Electric Company | Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	3	2
Union Electric Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	8	1
Union Electric Company | Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	10	4
Derivative liabilities	200	242
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	8	3
Ameren Illinois Company | Natural gas | Other assets
Derivative [Line Items]
Derivative assets	2	1
Ameren Illinois Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	1	12
Ameren Illinois Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	1	6
Ameren Illinois Company | Power | Other current assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Power | Other assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	17	17
Ameren Illinois Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	181	207
Ameren Illinois Company | Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	$ 0	$ 0

Fair Value Measurements (Schedu

Fair Value Measurements (Schedule of Valuation Process and Unobservable Inputs) (Details) - Power $ in Millions	Dec. 31, 2020USD ($)$ / MMBTU$ / MWh	Dec. 31, 2019USD ($)$ / MWh$ / MMBTU
Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative assets | $	$ 5	$ 14
Derivative Liability | $	$ (201)	$ (225)
Measurement Input, Commodity Forward Price | Minimum | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	23	22
Measurement Input, Commodity Forward Price | Maximum | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	37	34
Measurement Input, Commodity Forward Price | Weighted Average | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	29	25
Measurement Input, Nodal Basis | Minimum | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	(6)	(6)
Measurement Input, Nodal Basis | Maximum | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	0	0
Measurement Input, Nodal Basis | Weighted Average | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	(2)	(2)
Measurement Input, Commodity Future Price | Minimum | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input | $ / MMBTU	2	(1)
Measurement Input, Commodity Future Price | Maximum | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input | $ / MMBTU	6	0
Measurement Input, Commodity Future Price | Weighted Average | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input | $ / MMBTU	3	0

Fair Value Measurements (Sche_2

Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	$ 977	$ 842
Assets	999	868
Excluded receivables, payables, and accrued income, net	5	5
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	680	569
Assets	682	569
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	297	273
Assets	306	276
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Assets	11	23
Power | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	5	14
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	22	26
Derivative liabilities	221	255
Commodity Contract | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	0
Derivative liabilities	14	4
Commodity Contract | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	9	3
Derivative liabilities	2	16
Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	11	23
Derivative liabilities	205	235
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	989	864
Union Electric Company | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	682	569
Union Electric Company | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	300	275
Union Electric Company | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	7	20
Union Electric Company | Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	6
Derivative liabilities	9	7
Union Electric Company | Fuel Oils | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	6	1
Union Electric Company | Fuel Oils | 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 | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	6
Derivative liabilities	3	6
Union Electric Company | Natural gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	0
Derivative liabilities	1	2
Union Electric Company | Natural gas | 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 | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	0
Derivative liabilities	1	2
Union Electric Company | Natural gas | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Union Electric Company | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	7	16
Derivative liabilities	11	3
Union Electric Company | Power | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	0
Derivative liabilities	8	0
Union Electric Company | Power | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	2
Derivative liabilities	0	2
Union Electric Company | Power | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	5	14
Derivative liabilities	3	1
Union Electric Company | Uranium
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	1
Union Electric Company | Uranium | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Union Electric Company | Uranium | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Union Electric Company | Uranium | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	1
Union Electric Company | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	12	22
Derivative liabilities	21	13
Union Electric Company | Commodity Contract | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	0
Derivative liabilities	14	1
Union Electric Company | Commodity Contract | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	2
Derivative liabilities	1	4
Union Electric Company | Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	7	20
Derivative liabilities	6	8
Union Electric Company | Equity securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	680	569
Union Electric Company | Equity securities | U.S. large capitalization | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	680	569
Union Electric Company | Equity securities | U.S. large capitalization | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Equity securities | U.S. large capitalization | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt securities | U.S. Treasury and agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	115	107
Union Electric Company | Debt securities | U.S. Treasury and agency securities | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt securities | U.S. Treasury and agency securities | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	115	107
Union Electric Company | Debt securities | U.S. Treasury and agency securities | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt securities | Corporate Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	115	93
Union Electric Company | Debt securities | Corporate Bonds | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt securities | Corporate Bonds | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	115	93
Union Electric Company | Debt securities | Corporate Bonds | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt securities | Other debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	67	73
Union Electric Company | Debt securities | Other debt securities | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt securities | Other debt securities | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	67	73
Union Electric Company | Debt securities | Other debt securities | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Ameren Illinois Company | Natural gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	10	4
Derivative liabilities	2	18
Ameren Illinois Company | Natural gas | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	3
Ameren Illinois Company | Natural gas | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	6	1
Derivative liabilities	1	12
Ameren Illinois Company | Natural gas | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	3
Derivative liabilities	1	3
Ameren Illinois Company | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	198	224
Ameren Illinois Company | Power | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Power | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Power | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	198	224
Ameren Illinois Company | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	200	242
Ameren Illinois Company | Commodity Contract | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	3
Ameren Illinois Company | Commodity Contract | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	1	12
Ameren Illinois Company | Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	$ 199	$ 227

Fair Value Measurements (Sche_3

Fair Value Measurements (Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level 3 In The Fair Value Hierarchy) (Details) - Power - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	$ (196)	$ (211)	$ (183)
Included in regulatory assets/liabilities	23	(33)
Settlements	(8)	8
Transfers out of Level 3	0	(3)
Change in unrealized gains (losses) related to assets/liabilities held at December 31	10	(42)
Union Electric Company
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	2	13	0
Included in regulatory assets/liabilities	15	23
Settlements	(26)	(7)
Transfers out of Level 3	0	(3)
Change in unrealized gains (losses) related to assets/liabilities held at December 31	1	12
Ameren Illinois Company
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	(198)	(224)	$ (183)
Included in regulatory assets/liabilities	8	(56)
Settlements	18	15
Transfers out of Level 3	0	0
Change in unrealized gains (losses) related to assets/liabilities held at December 31	$ 9	$ (54)

Fair Value Measurements (Sche_4

Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Financial Assets and Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	$ 301	$ 176	$ 107	$ 68
Short-term debt	490	440
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	145	39	8	7
Short-term debt	0	234
Advances to money pool	139	0
Debt Issuance Costs, Net	36	30
Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	147	125	$ 80	$ 41
Short-term debt	0	53
Due to Related Parties, Current	19	0
Debt Issuance Costs, Net	36	34
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	301	176
Available-for-sale Securities and Held-to-maturity Securities	256	263
Short-term debt	490	440
Long-term debt (including current portion)	11,086	9,357
Debt Issuance Costs, Net	84	72
Carrying Amount | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	145	39
Available-for-sale Securities and Held-to-maturity Securities	256	263
Short-term debt		234
Long-term debt (including current portion)	5,104	4,190
Advances to money pool	139
Due to Related Parties, Current	19
Debt Issuance Costs, Net	36	30
Carrying Amount | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	147	125
Short-term debt		53
Long-term debt (including current portion)	3,946	3,575
Debt Issuance Costs, Net	36	34
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	301	176
Investments, Fair Value Disclosure	256	263
Short-term Debt, Fair Value	490	440
Long-term Debt, Fair Value	13,315	10,441
Fair Value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	301	176
Investments, Fair Value Disclosure	0	0
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	0	0
Fair Value | 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	263
Short-term Debt, Fair Value	490	440
Long-term Debt, Fair Value	12,778	9,957
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	537	484
Fair Value | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	145	39
Investments, Fair Value Disclosure	256	263
Short-term Debt, Fair Value		234
Long-term Debt, Fair Value	6,160	4,772
Advances to money pool	139
Fair Value | Union Electric Company | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	145	39
Investments, Fair Value Disclosure	0	0
Short-term Debt, Fair Value		0
Long-term Debt, Fair Value	0	0
Advances to money pool	0
Fair Value | Union Electric Company | 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	263
Short-term Debt, Fair Value		234
Long-term Debt, Fair Value	6,160	4,772
Advances to money pool	139
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
Investments, Fair Value Disclosure	0	0
Short-term Debt, Fair Value		0
Long-term Debt, Fair Value	0	0
Advances to money pool	0
Fair Value | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	147	125
Short-term Debt, Fair Value		53
Long-term Debt, Fair Value	4,822	4,019
Due to Related Parties, Current	19
Fair Value | Ameren Illinois Company | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	147	125
Short-term Debt, Fair Value		0
Long-term Debt, Fair Value	0	0
Due to Related Parties, Current	0
Fair Value | Ameren Illinois Company | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	0	0
Short-term Debt, Fair Value		53
Long-term Debt, Fair Value	4,822	4,019
Due to Related Parties, Current	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
Short-term Debt, Fair Value		0
Long-term Debt, Fair Value	0	$ 0
Due to Related Parties, Current	$ 0

Callaway Energy Center (Narrati

Callaway Energy Center (Narrative) (Details) - Union Electric Company $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)
Nuclear Waste Matters [Line Items]
Frequency of Decommissioning Cost Study	3 years
Nuclear Plant
Nuclear Waste Matters [Line Items]
Estimated Nuclear Generator Repairs	$ 65
Annual decommissioning costs included in costs of service	$ 7
Minimum | Nuclear Decommissioning Trust Fund
Nuclear Waste Matters [Line Items]
Trust Fund Investments, Target Allocation Percentage	60.00%
Maximum | Nuclear Decommissioning Trust Fund
Nuclear Waste Matters [Line Items]
Trust Fund Investments, Target Allocation Percentage	70.00%

Callaway Energy Center (Proceed

Callaway Energy Center (Proceeds From The Sale Of Investments And Related Gross Realized Gains And Losses In Nuclear Decommissioning Trust Fund) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Nuclear Waste Matters [Line Items]
Proceeds from sales and maturities	$ 183	$ 260	$ 299
Union Electric Company
Nuclear Waste Matters [Line Items]
Proceeds from sales and maturities	183	260	299
Gross realized gains	10	10	18
Gross realized losses	$ 3	$ 2	$ 5

Callaway Energy Center (Fair Va

Callaway Energy Center (Fair Values Of Investments In Debt And Equity Securities In Nuclear Decommissioning Trust Fund) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Nuclear Waste Matters [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 272
Cash, cash equivalents, and restricted cash	301	$ 176	$ 107	$ 68
Fair Value	982	847
Cash and cash equivalents	139	16
Union Electric Company
Nuclear Waste Matters [Line Items]
Cost	475	450
Cash, cash equivalents, and restricted cash	145	39	$ 8	$ 7
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	516	404
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	9	7
Fair Value	982	847
Cash and cash equivalents	136	9
Cost	982	847
Union Electric Company | Debt securities
Nuclear Waste Matters [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	272	262
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	25	11
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Fair Value	297	273
Union Electric Company | Equity securities
Nuclear Waste Matters [Line Items]
Available-for-sale Equity Securities, Amortized Cost Basis	198	183
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax	491	393
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax	9	7
Equity Securities, FV-NI	680	569
Union Electric Company | Cash and cash equivalents
Nuclear Waste Matters [Line Items]
Marketable Securities	4	26
Cash, cash equivalents, and restricted cash	0	0
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	0	0
Cash and cash equivalents	4	26
Union Electric Company | Other Debt And Equity Securities
Nuclear Waste Matters [Line Items]
Net Receivables (Payables) From Pending Securities Sales, Interest, and Securities Purchases, Cost Basis	1	(21)
Net Receivables (Payables) From Pending Securities Sales, Interest, And Securities Purchases, Available-For-sale Securities, Accumulated Gross Unrealized Loss, Before Tax	0	0
Net Receivables (Payables) From Pending Securities Sales, Interest, and Securities Purchases, Fair Value	1	(21)
Net Receivables (Payables) From Pending Securities Sales, Interest, And Securities Purchases, Available-For-sale Securities, Accumulated Gross Unrealized Gain, Before Tax	$ 0	$ 0

Callaway Energy Center (Cost an

Callaway Energy Center (Cost and Fair Values of Investments In Debt Securities in Nuclear Decommissioning Trust Fund According to Contractual Maturities) (Details) $ in Millions	Dec. 31, 2020USD ($)
Nuclear Waste Matters [Abstract]
Cost, Less than 5 years	$ 120
Cost, 5 years to 10 years	69
Cost, Due after 10 years	83
Cost, Total	272
Fair Value, Less than 5 years	123
Fair Value, 5 years to 10 years	75
Fair Value, Due after 10 years	99
Fair Value, Total	$ 297

Callaway Energy Center (Insuran

Callaway Energy Center (Insurance Disclosure) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)
Nuclear Waste Matters [Line Items]
Number Of Years The Limit Of Liability And The Maximum Potential Annual Payments Are Adjusted	five years
Number of weeks of coverage after the first twelve weeks of an outage	1
Number Of Additional Weeks After Initial Indemnity Coverage For Power Outage	1.365
Public Liability And Nuclear Worker Liability - American Nuclear Insurers
Nuclear Waste Matters [Line Items]
Maximum coverages	$ 450
Maximum assessments for single incidents	0
Public Liability And Nuclear Worker Liability - Pool Participation
Nuclear Waste Matters [Line Items]
Maximum coverages	13,348
Maximum assessments for single incidents	138
Threshold Amount For which a Retrospective Assessment For a Covered loss is necessary	450
Maximum Annual Payment Per Incident At Licensed Commercial Nuclear Reactor	21
Public Liability And Nuclear Worker Liability
Nuclear Waste Matters [Line Items]
Maximum coverages	13,798
Maximum assessments for single incidents	138
Property Damage - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Maximum coverages	3,200
Maximum assessments for single incidents	25
Replacement Power - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Maximum coverages	490
Maximum assessments for single incidents	7
Amount of Weekly Indemnity Coverage Commencing Twelve Weeks After Power Outage	4.5
Amount of additional weekly indemnity coverage commencing after initial indemnity coverage	3.6
Amount Of Weekly Indemnity Coverage Thereafter Not Exceeding Policy Limit	490
Sub-limit of for Non-Nuclear Events	328
Radiation Event
Nuclear Waste Matters [Line Items]
Maximum coverages	2,700
Aggregate nuclear power industry insurance policy limit for losses from terrorist attacks within twelve month period	3,200
Non-radiation event
Nuclear Waste Matters [Line Items]
Maximum coverages	2,300
Aggregate nuclear power industry insurance policy limit for losses from terrorist attacks within twelve month period	1,800
Property Damage European Mutual Association for Nuclear Insurance
Nuclear Waste Matters [Line Items]
Maximum coverages	$ 490

Retirement Benefits (Narrative)

Retirement Benefits (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020USD ($)bond	Dec. 31, 2019USD ($)	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan		$ 249	$ 216
Defined Benefit Plan Assumptions Used Calculating Benefit Obligation, Change in Discount Rate		0.75%
Number of high-quality corporate bonds | bond		830
Defined benefit plan, estimated future employer contributions during the next five years		$ 60
Collateralized loan percentage compared to asset's market value		102.00%
Securities Loaned		$ 365	246
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan		$ 0	$ (403)
Expected return on plan assets		7.00%	7.00%	7.00%
Pension Plan | Forecast
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets	6.50%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan		$ 249	$ 187
Expected return on plan assets		7.00%	7.00%	7.00%
Union Electric Company | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Future funding requirement, percentage		30.00%
Ameren Illinois Company | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Future funding requirement, percentage		60.00%

Retirement Benefits (Summary Of

Retirement Benefits (Summary Of Benefit Liability Recorded) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	$ (249)	$ (216)
Union Electric Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	(25)	(142)
Ameren Illinois Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	$ (210)	$ (16)

Retirement Benefits (Funded Sta

Retirement Benefits (Funded Status Of Benefit Plans And Amounts Included In Regulatory Assets And OCI) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in plan assets:
Funded status – deficiency (surplus)	$ (249)	$ (216)
Amounts recognized in the balance sheet consist of:
Noncurrent liability	37	401
Net liability (asset) recognized	(249)	(216)
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation at end of year	5,213	4,735
Change in benefit obligation:
Net benefit obligation at beginning of year	4,967	4,459
Service cost	110	88	$ 100
Interest cost	174	187	169
Plan amendments	0	0
Participant contributions	0	0
Actuarial loss	508	469
Benefits paid	(249)	(236)
Net benefit obligation at end of year	5,510	4,967	4,459
Change in plan assets:
Fair value of plan assets at beginning of year	4,564	3,899
Actual return on plan assets	1,143	878
Employer contributions	52	23	60
Participant contributions	0	0
Benefits paid	(249)	(236)
Fair value of plan assets at end of year	5,510	4,564	3,899
Funded status – deficiency (surplus)	0	403
Accrued benefit cost (asset) at December 31	0	403
Amounts recognized in the balance sheet consist of:
Noncurrent asset	(39)	0
Current liability	2	2
Noncurrent liability	37	401
Net liability (asset) recognized	0	(403)
Amounts recognized in regulatory assets consist of:
Net actuarial (gain) loss	(138)	244
Prior service credit	0	0
Amounts recognized in accumulated OCI (pretax) consist of:
Net actuarial loss	5	26
Total	(133)	270
Postretirement Benefits
Change in benefit obligation:
Net benefit obligation at beginning of year	1,110	1,034
Service cost	19	18	21
Interest cost	39	43	40
Plan amendments	0	2
Participant contributions	8	8
Actuarial loss	91	69
Benefits paid	(63)	(64)
Net benefit obligation at end of year	1,204	1,110	1,034
Change in plan assets:
Fair value of plan assets at beginning of year	1,297	1,113
Actual return on plan assets	209	237
Employer contributions	2	3	2
Participant contributions	8	8
Benefits paid	(63)	(64)
Fair value of plan assets at end of year	1,453	1,297	$ 1,113
Funded status – deficiency (surplus)	(249)	(187)
Accrued benefit cost (asset) at December 31	(249)	(187)
Amounts recognized in the balance sheet consist of:
Noncurrent asset	(249)	(187)
Current liability	0	0
Noncurrent liability	0	0
Amounts recognized in regulatory assets consist of:
Net actuarial (gain) loss	(200)	(170)
Prior service credit	(37)	(41)
Amounts recognized in accumulated OCI (pretax) consist of:
Net actuarial loss	6	4
Total	$ (231)	$ (207)

Retirement Benefits (Assumption

Retirement Benefits (Assumptions Used To Determine Benefit Obligations) (Details)	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Postretirement Health Coverage
Defined Benefit Plan Disclosure [Line Items]
Medical cost trend rate (initial)	3.00%
Medical cost trend rate (ultimate)	3.00%
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate at measurement date	2.75%	3.50%
Increase in future compensation	3.50%	3.50%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Weighted-Average Interest Crediting Rate	5.00%	5.00%	5.00%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate at measurement date	2.75%	3.50%
Increase in future compensation	3.50%	3.50%
Medical cost trend rate (initial)	5.00%	5.00%	5.00%
Medical cost trend rate (ultimate)	5.00%	5.00%	5.00%

Retirement Benefits (Cash Contr

Retirement Benefits (Cash Contributions Made To Benefit Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	$ 52	$ 23	$ 60
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	2	3	2
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	17	3	18
Union Electric Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	1	1	1
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	27	19	35
Ameren Illinois Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	1	1	1
Other | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	8	1	7
Other | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	$ 0	$ 1	$ 0

Retirement Benefits (Target All

Retirement Benefits (Target Allocation Of The Plans' Asset Categories) (Details)	Dec. 31, 2020	Dec. 31, 2019
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	100.00%	100.00%
Pension Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	1.00%	3.00%
Pension Benefits | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	59.00%	57.00%
Pension Benefits | U.S. large-capitalization
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	26.00%	27.00%
Pension Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	9.00%	7.00%
Pension Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	15.00%	14.00%
Pension Benefits | Global
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	9.00%	9.00%
Pension Benefits | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	36.00%	36.00%
Pension Benefits | Real estate
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	4.00%	4.00%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	100.00%	100.00%
Postretirement Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	3.00%	1.00%
Postretirement Benefits | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	64.00%	65.00%
Postretirement Benefits | U.S. large-capitalization
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	31.00%	31.00%
Postretirement Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	8.00%	9.00%
Postretirement Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	15.00%	14.00%
Postretirement Benefits | Global
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	10.00%	11.00%
Postretirement Benefits | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	33.00%	34.00%
Minimum | Pension Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%
Minimum | Pension Benefits | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	51.00%
Minimum | Pension Benefits | U.S. large-capitalization
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	21.00%
Minimum | Pension Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	3.00%
Minimum | Pension Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	9.00%
Minimum | Pension Benefits | Global
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	3.00%
Minimum | Pension Benefits | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	35.00%
Minimum | Pension Benefits | Real estate
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%
Minimum | Pension Benefits | Private equity
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%
Minimum | Postretirement Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%
Minimum | Postretirement Benefits | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	55.00%
Minimum | Postretirement Benefits | U.S. large-capitalization
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	23.00%
Minimum | Postretirement Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	3.00%
Minimum | Postretirement Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	9.00%
Minimum | Postretirement Benefits | Global
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	5.00%
Minimum | Postretirement Benefits | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	33.00%
Maximum | Pension Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	5.00%
Maximum | Pension Benefits | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	61.00%
Maximum | Pension Benefits | U.S. large-capitalization
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	31.00%
Maximum | Pension Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	13.00%
Maximum | Pension Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	19.00%
Maximum | Pension Benefits | Global
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	13.00%
Maximum | Pension Benefits | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	45.00%
Maximum | Pension Benefits | Real estate
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	9.00%
Maximum | Pension Benefits | Private equity
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	5.00%
Maximum | Postretirement Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	7.00%
Maximum | Postretirement Benefits | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	65.00%
Maximum | Postretirement Benefits | U.S. large-capitalization
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	33.00%
Maximum | Postretirement Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	13.00%
Maximum | Postretirement Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	19.00%
Maximum | Postretirement Benefits | Global
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	15.00%
Maximum | Postretirement Benefits | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	43.00%

Retirement Benefits (Fair Value

Retirement Benefits (Fair Value Of Plan Assets Utilizing Fair Value Hierarchy) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 5,510	$ 4,564	$ 3,899
Pension Benefits | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	886	645
Pension Benefits | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,889	1,690
Pension Benefits | NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,921	2,388
Pension Benefits | Includes Medical Benefit Component Under Section401 H And Excludes Receivables Related To Pending Security Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,696	4,723
Pension Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	145	139
Pension Benefits | Cash And Cash Equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Cash And Cash Equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Cash And Cash Equivalents | NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	145	139
Pension Benefits | U.S. large-capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,511	1,253
Pension Benefits | U.S. large-capitalization | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | U.S. large-capitalization | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | U.S. large-capitalization | NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,511	1,253
Pension Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	513	344
Pension Benefits | U.S. small and mid-capitalization | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	513	344
Pension Benefits | U.S. small and mid-capitalization | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | U.S. small and mid-capitalization | NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	867	659
Pension Benefits | International | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	375	296
Pension Benefits | International | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | International | NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	492	363
Pension Benefits | Global
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	546	407
Pension Benefits | Global | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Global | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Global | NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	546	407
Pension Benefits | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	523	610
Pension Benefits | Corporate Bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Corporate Bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	506	597
Pension Benefits | Corporate Bonds | NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	17	13
Pension Benefits | Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	50	75
Pension Benefits | Municipal Bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Municipal Bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	50	75
Pension Benefits | Municipal Bonds | NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | U.S. Treasury and agency securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,328	1,015
Pension Benefits | U.S. Treasury and agency securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	5
Pension Benefits | U.S. Treasury and agency securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,325	1,010
Pension Benefits | U.S. Treasury and agency securities | NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	8
Pension Benefits | Other | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(5)	0
Pension Benefits | Other | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8	8
Pension Benefits | Other | NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Real estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	208	211
Pension Benefits | Real estate | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Real estate | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Real estate | NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	208	211
Pension Benefits | Private equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2	2
Pension Benefits | Private equity | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Private equity | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Private equity | NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2	2
Pension Benefits | Medical benefit assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(219)	(176)
Pension Benefits | Net receivables
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	33	17
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,453	1,297	$ 1,113
Postretirement Benefits | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	496	402
Postretirement Benefits | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	106	107
Postretirement Benefits | NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	629	611
Postretirement Benefits | Includes Medical Benefit Component Under Section401 H And Excludes Receivables Related To Pending Security Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,231	1,120
Postretirement Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	38	12
Postretirement Benefits | Cash And Cash Equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	38	12
Postretirement Benefits | Cash And Cash Equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | Cash And Cash Equivalents | NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | U.S. large-capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	386	350
Postretirement Benefits | U.S. large-capitalization | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	279	238
Postretirement Benefits | U.S. large-capitalization | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | U.S. large-capitalization | NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	107	112
Postretirement Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	104	93
Postretirement Benefits | U.S. small and mid-capitalization | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	104	93
Postretirement Benefits | U.S. small and mid-capitalization | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | U.S. small and mid-capitalization | NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	182	161
Postretirement Benefits | International | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	75	59
Postretirement Benefits | International | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | International | NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	107	102
Postretirement Benefits | Global
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	120	120
Postretirement Benefits | Global | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | Global | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | Global | NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	120	120
Postretirement Benefits | Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	106	107
Postretirement Benefits | Municipal Bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | Municipal Bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	106	107
Postretirement Benefits | Municipal Bonds | NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	295	277
Postretirement Benefits | Other | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | Other | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | Other | NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	295	277
Postretirement Benefits | Medical benefit assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	219	176
Postretirement Benefits | Net receivables
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 3	$ 1

Retirement Benefits (Components

Retirement Benefits (Components Of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Non-service Cost or Income Components	$ 116	$ 90	$ 70
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	110	88	100
Interest cost	174	187	169
Expected return on plan assets	(291)	(276)	(276)
Prior service credit	(1)	(1)	(1)
Actuarial (gain) loss	60	25	68
Defined Benefit Plan, Non-service Cost or Income Components	(58)	(65)	(40)
Net periodic benefit cost (income)	52	23	60
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	19	18	21
Interest cost	39	43	40
Expected return on plan assets	(80)	(77)	(77)
Prior service credit	(4)	(5)	(4)
Actuarial (gain) loss	(9)	(15)	(6)
Defined Benefit Plan, Non-service Cost or Income Components	(54)	(54)	(47)
Net periodic benefit cost (income)	$ (35)	$ (36)	$ (26)

Retirement Benefits (Summary _2

Retirement Benefits (Summary Of Benefit Plan Costs Incurred) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 52	$ 23	$ 60
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	(35)	(36)	(26)
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	22	5	22
Union Electric Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	(5)	(6)	(1)
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	32	20	39
Ameren Illinois Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	(31)	(30)	(25)
Other | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	(2)	(2)	(1)
Other | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 1	$ 0	$ 0

Retirement Benefits (Schedule O

Retirement Benefits (Schedule Of Expected Payments From Qualified Trust And Company Funds) (Details) $ in Millions	Dec. 31, 2020USD ($)
Pension Benefits | Paid From Qualified Trust
Defined Benefit Plan Disclosure [Line Items]
2021	$ 260
2022	267
2023	274
2024	278
2025	283
2026 – 2030	1,454
Pension Benefits | Paid From Company Funds
Defined Benefit Plan Disclosure [Line Items]
2021	2
2022	2
2023	3
2024	3
2025	3
2026 – 2030	12
Postretirement Benefits | Paid From Qualified Trust
Defined Benefit Plan Disclosure [Line Items]
2021	58
2022	60
2023	61
2024	62
2025	61
2026 – 2030	303
Postretirement Benefits | Paid From Company Funds
Defined Benefit Plan Disclosure [Line Items]
2021	2
2022	2
2023	2
2024	2
2025	2
2026 – 2030	$ 11

Retirement Benefits (Assumpti_2

Retirement Benefits (Assumptions Used To Determine Net Periodic Benefit Cost) (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate at measurement date	3.50%	4.25%	3.50%
Expected return on plan assets	7.00%	7.00%	7.00%
Increase in future compensation	3.50%	3.50%	3.50%
Cash balance pension plan interest crediting rate	5.00%	5.00%	5.00%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate at measurement date	3.50%	4.25%	3.50%
Expected return on plan assets	7.00%	7.00%	7.00%
Increase in future compensation	3.50%	3.50%	3.50%
Medical cost trend rate (initial)	5.00%	5.00%	5.00%
Medical cost trend rate (ultimate)	5.00%	5.00%	5.00%
Postretirement Health Coverage
Defined Benefit Plan Disclosure [Line Items]
Medical cost trend rate (initial)	3.00%
Medical cost trend rate (ultimate)	3.00%

Retirement Benefits (Schedule_2

Retirement Benefits (Schedule Of Potential Changes In Key Assumptions) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service Cost and Interest Cost, .25% decrease in discount rate	$ (1)
Defined Benefit Plan Effect Of a Quarter-Percentage Point Decrease of the Discount Rate On the Expected Return on Assets	0
Benefit Obligation, .25% decrease in discount rate	195
Defined Benefit Plan Effect of a Quarter-Percentage Decrease of Return On Assets On the Service and Interest Cost Components	0
Defined Benefit Plan Effect Of a Quarter-Percentage Point Decrease of Return on Assets on the Expected Return on Assets	10
Defined Benefit Plan Effect of a Quarter-Percentage Decrease of Return on Assets on the Projected Benefit Obligation	0
Service Cost and Interest Cost, .25% increase in salary rate	2
Defined Benefit Plan Effect Of a Quarter-Percentage Point Increase of the assumed annual salary increase Rate On the Expected Return on Assets	0
Benefit Obligation, .25% increase in salary rate	20
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service Cost and Interest Cost, .25% decrease in discount rate	0
Defined Benefit Plan Effect Of a Quarter-Percentage Point Decrease of the Discount Rate On the Expected Return on Assets	0
Benefit Obligation, .25% decrease in discount rate	42
Defined Benefit Plan Effect of a Quarter-Percentage Decrease of Return On Assets On the Service and Interest Cost Components	0
Defined Benefit Plan Effect Of a Quarter-Percentage Point Decrease of Return on Assets on the Expected Return on Assets	3
Defined Benefit Plan Effect of a Quarter-Percentage Decrease of Return on Assets on the Projected Benefit Obligation	0
Service Cost and Interest Cost, .25% increase in salary rate	0
Defined Benefit Plan Effect Of a Quarter-Percentage Point Increase of the assumed annual salary increase Rate On the Expected Return on Assets	0
Benefit Obligation, .25% increase in salary rate	$ 0

Retirement Benefits (Schedule_3

Retirement Benefits (Schedule Of Matching Contributions) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 38	$ 35	$ 33
Union Electric Company
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	20	19	17
Ameren Illinois Company
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	17	16	15
Other
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 1	$ 0	$ 1

Stock-Based Compensation (Narra

Stock-Based Compensation (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)	8,000,000
Maximum shares available for grants (in shares)	2,300,000
Settled performance share units and restricted stock units	$ 58	$ 83	$ 54
Compensation cost not yet recognized	$ 34
Expected weighted average recognition period for share-based compensation expense, in months	22 months
Performance Share Units | Market Condition PSUs
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
Award Requisite Service Period	5 years
Percentage of shares issued per share unit, minimum	0.00%
Percentage of shares issued per share unit, maximum	200.00%
Performance Share Units | Performance Condition PSUs
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
Award Requisite Service Period	5 years
Percentage of shares issued per share unit, minimum	0.00%
Percentage of shares issued per share unit, maximum	200.00%
Restricted Stock Units
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
Award Requisite Service Period	5 years
Income taxes
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee Service Share-based Compensation, Tax Benefit from Exercise of Stock Options	$ 8	$ 15	$ 6

Stock-Based Compensation (Summa

Stock-Based Compensation (Summary Of Nonvested Shares) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Performance Share Units | Market Condition PSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares issued per share unit, minimum	0.00%
Percentage of shares issued per share unit, maximum	200.00%
Fully Vested Undistributed Retirement-eligible units (in shares)	366,243	503,283
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	38 months
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Share Units, Nonvested at beginning of year (in shares)	539,877
Share Units, Granted (in shares)	267,154
Share Units, Unearned or forfeited (in shares)	(16,303)
Share Units, Undistributed Vested Units (in shares)	(178,802)
Share Units, Vested and distributed (in shares)	(147,787)
Share Units, Performance Adjustment (in shares)	0
Share Units, Nonvested at end of year (in shares)	464,139	539,877
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Weighted-average Fair Value per Unit, Nonvested at beginning of year (in dollars per share)	$ 63.79
Fair value of share units awarded	82.49	$ 67.42	$ 62.88
Weighted-average Fair Value per Unit, Unearned or forfeited (in dollars per share)	73.42
Weighted-average Fair Value per Unit, Vested and undistributed (in dollars per share)	69.87
Weighted-average Fair Value per Unit, Vested and distributed (in dollars per share)	59.16
Weighted-average Fair Value per Unit, Performance Adjustment (in dollars per share)	0
Weighted-average Fair Value per Unit, Nonvested at end of year (in dollars per share)	$ 73.34	$ 63.79
Performance Share Units | Performance Condition PSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares issued per share unit, minimum	0.00%
Percentage of shares issued per share unit, maximum	200.00%
Fully Vested Undistributed Retirement-eligible units (in shares)	7,607
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	38 months
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Share Units, Nonvested at beginning of year (in shares)	0
Share Units, Granted (in shares)	43,052
Share Units, Unearned or forfeited (in shares)	(1,155)
Share Units, Undistributed Vested Units (in shares)	(7,607)
Share Units, Vested and distributed (in shares)	0
Share Units, Performance Adjustment (in shares)	(2,394)
Share Units, Nonvested at end of year (in shares)	31,896	0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Weighted-average Fair Value per Unit, Nonvested at beginning of year (in dollars per share)	$ 0
Fair value of share units awarded	76.69
Weighted-average Fair Value per Unit, Unearned or forfeited (in dollars per share)	76.80
Weighted-average Fair Value per Unit, Vested and undistributed (in dollars per share)	76.78
Weighted-average Fair Value per Unit, Vested and distributed (in dollars per share)	0
Weighted-average Fair Value per Unit, Performance Adjustment (in dollars per share)	76.66
Weighted-average Fair Value per Unit, Nonvested at end of year (in dollars per share)	$ 76.66	$ 0
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fully Vested Undistributed Retirement-eligible units (in shares)	160,034	79,854
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	38 months
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Share Units, Nonvested at beginning of year (in shares)	220,277
Share Units, Granted (in shares)	173,571
Share Units, Unearned or forfeited (in shares)	(7,507)
Share Units, Undistributed Vested Units (in shares)	(80,180)
Share Units, Vested and distributed (in shares)	(2,466)
Share Units, Performance Adjustment (in shares)	0
Share Units, Nonvested at end of year (in shares)	303,695	220,277
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Weighted-average Fair Value per Unit, Nonvested at beginning of year (in dollars per share)	$ 61.13
Fair value of share units awarded	76.91
Weighted-average Fair Value per Unit, Unearned or forfeited (in dollars per share)	69.89
Weighted-average Fair Value per Unit, Vested and undistributed (in dollars per share)	66.72
Weighted-average Fair Value per Unit, Vested and distributed (in dollars per share)	54.30
Weighted-average Fair Value per Unit, Performance Adjustment (in dollars per share)	0
Weighted-average Fair Value per Unit, Nonvested at end of year (in dollars per share)	$ 68.52	$ 61.13
Restricted Stock Units | September 18, 2020 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Granted in Period, Fair Value	$ 3
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	3 years
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Share Units, Granted (in shares)	37,104

Stock-Based Compensation (Sum_2

Stock-Based Compensation (Summary of Expense) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 21	$ 20	$ 20
Employee service share-based compensation, tax benefit from compensation expense	6	5	6
Share-based Compensation Expense, Net of Tax	15	15	14
Ameren Missouri [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	5	4	4
Ameren Illinois Company
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	3	3	3
Other
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 13	$ 13	$ 13
Performance Share Units | Market Condition PSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of share units awarded	$ 82.49	$ 67.42	$ 62.88
Three-year risk-free rate	1.62%	2.46%	1.98%
Ameren’s common stock volatility	15.00%	17.00%	17.00%
Volatility range for peer group, minimum	14.00%	15.00%	15.00%
Volatility range for peer group, maximum	28.00%	25.00%	23.00%

Income Taxes (Narrative) (Detai

Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Taxes [Line Items]
Deferred Tax Liabilities, Net	$ 3,177	$ 2,880
Union Electric Company
Income Taxes [Line Items]
Deferred Tax Liabilities, Net	1,708	1,573
Ameren Illinois Company
Income Taxes [Line Items]
Deferred Tax Liabilities, Net	1,367	1,224
Other
Income Taxes [Line Items]
Deferred Tax Liabilities, Net	$ 102	$ 83
State | Union Electric Company
Income Taxes [Line Items]
State Income Tax Statutory Rate	4.00%
Deferred Tax Liabilities, Net	$ 122
State | Union Electric Company | Maximum
Income Taxes [Line Items]
State Income Tax Statutory Rate	6.25%

Income Taxes (Schedule Of Effec

Income Taxes (Schedule Of Effective Income Tax Rate Reconciliation) (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes [Line Items]
Federal statutory corporate income tax rate	21.00%	21.00%	21.00%
Amortization of excess deferred income taxes	(9.00%)	(7.00%)	(4.00%)
Amortization of deferred investment tax credit	(1.00%)	(1.00%)	(1.00%)
State tax	5.00%	6.00%	6.00%
Stock-based compensation	(1.00%)	(1.00%)
Other depreciation differences			0.00%
TCJA			1.00%
Tax Credits			0.00%
Other permanent items			(1.00%)
Effective income tax rate	15.00%	18.00%	22.00%
Union Electric Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate	21.00%	21.00%	21.00%
Amortization of excess deferred income taxes	(16.00%)	(11.00%)	(4.00%)
Amortization of deferred investment tax credit	(1.00%)	(1.00%)	(1.00%)
State tax	3.00%	5.00%	4.00%
Stock-based compensation	0.00%	0.00%
Other depreciation differences			0.00%
TCJA			1.00%
Tax Credits			(1.00%)
Other permanent items			0.00%
Effective income tax rate	7.00%	14.00%	20.00%
Ameren Illinois Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate	21.00%	21.00%	21.00%
Amortization of excess deferred income taxes	(3.00%)	(4.00%)	(4.00%)
Amortization of deferred investment tax credit	(1.00%)	0.00%	0.00%
State tax	7.00%	7.00%	7.00%
Stock-based compensation	0.00%	0.00%
Other depreciation differences			(1.00%)
TCJA			1.00%
Tax Credits			0.00%
Other permanent items			0.00%
Effective income tax rate	24.00%	24.00%	24.00%

Income Taxes (Schedule Of Compo

Income Taxes (Schedule Of Components Of Income Tax Expense (Benefit)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes [Line Items]
Current Federal taxes	$ 2	$ (4)	$ (10)
Current State taxes	5	19	23
Deferred Federal Income Tax Expense (Benefit)	187	185	220
Deferred State taxes	57	59	49
Amortization of Excess Deferred Taxes	(91)	(72)	(40)
Deferred investment tax credits, amortization	(5)	(5)	(5)
Income tax benefit	155	182	237
Union Electric Company
Income Taxes [Line Items]
Current Federal taxes	14	65	104
Current State taxes	3	22	29
Deferred Federal Income Tax Expense (Benefit)	82	37	22
Deferred State taxes	15	5	(2)
Amortization of Excess Deferred Taxes	(75)	(56)	(24)
Deferred investment tax credits, amortization	(5)	(5)	(5)
Income tax benefit	34	68	124
Ameren Illinois Company
Income Taxes [Line Items]
Current Federal taxes	12	19	4
Current State taxes	(6)	11	6
Deferred Federal Income Tax Expense (Benefit)	81	66	75
Deferred State taxes	52	29	28
Amortization of Excess Deferred Taxes	(15)	(15)	(15)
Deferred investment tax credits, amortization	0	0	0
Income tax benefit	124	110	98
Other
Income Taxes [Line Items]
Current Federal taxes	(24)	(88)	(118)
Current State taxes	8	(14)	(12)
Deferred Federal Income Tax Expense (Benefit)	24	82	123
Deferred State taxes	(10)	25	23
Amortization of Excess Deferred Taxes	(1)	(1)	(1)
Deferred investment tax credits, amortization	0	0	0
Income tax benefit	$ (3)	$ 4	$ 15

Income Taxes (Schedule Of Defer

Income Taxes (Schedule Of Deferred Tax Assets And Liabilities Resulting From Temporary Differences) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Income Taxes [Line Items]
Plant related	$ 3,876	$ 3,616
Regulatory assets and liabilities, net	(515)	(548)
Deferred employee benefit costs - asset	(104)	(111)
Tax carryforwards	(97)	(98)
Other - liabilities	17	21
Total net accumulated deferred income tax liabilities	3,177	2,880
Accumulated Deferred Investment Tax Credit	34	39
Accumulated deferred income taxes and investment tax credits, net	3,211	2,919
Union Electric Company
Income Taxes [Line Items]
Plant related	2,112	2,000
Regulatory assets and liabilities, net	(285)	(310)
Deferred employee benefit costs - asset	(58)	(59)
Tax carryforwards	(26)	(25)
Other - assets	(35)	(33)
Total net accumulated deferred income tax liabilities	1,708	1,573
Accumulated Deferred Investment Tax Credit	34	39
Accumulated deferred income taxes and investment tax credits, net	1,742	1,612
Ameren Illinois Company
Income Taxes [Line Items]
Plant related	1,559	1,423
Regulatory assets and liabilities, net	(207)	(214)
Deferred employee benefit costs - liability	8	7
Tax carryforwards	(6)	(3)
Other - liabilities	13	11
Total net accumulated deferred income tax liabilities	1,367	1,224
Accumulated Deferred Investment Tax Credit	0	0
Accumulated deferred income taxes and investment tax credits, net	1,367	1,224
Other
Income Taxes [Line Items]
Plant related	205	193
Regulatory assets and liabilities, net	(23)	(24)
Deferred employee benefit costs - asset	(54)	(59)
Tax carryforwards	(65)	(70)
Other - liabilities	39	43
Total net accumulated deferred income tax liabilities	102	83
Accumulated Deferred Investment Tax Credit	0	0
Accumulated deferred income taxes and investment tax credits, net	$ 102	$ 83

Income Taxes (Schedule Of Net O

Income Taxes (Schedule Of Net Operating Loss Carryforwards And Tax Credit Carryforwards) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 7
Tax credit carryforwards	90	$ 98
Deferred Tax Assets, Charitable Contribution Carryforwards	3	3
Deferred Tax Assets, Valuation Allowance, Noncurrent	(3)	(3)
Deferred Tax Assets, Charitable Contribution Carryforwards, Net Of Valuation Allowance	0	0
Federal
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	83	95
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	7
Tax credit carryforwards	7	3
Union Electric Company
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0
Tax credit carryforwards	26	25
Deferred Tax Assets, Charitable Contribution Carryforwards	0	0
Deferred Tax Assets, Valuation Allowance, Noncurrent	0	0
Deferred Tax Assets, Charitable Contribution Carryforwards, Net Of Valuation Allowance	0	0
Union Electric Company | Federal
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	26	25
Union Electric Company | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0
Tax credit carryforwards	0	0
Ameren Illinois Company
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	3
Tax credit carryforwards	3	3
Deferred Tax Assets, Charitable Contribution Carryforwards	0	0
Deferred Tax Assets, Valuation Allowance, Noncurrent	0	0
Deferred Tax Assets, Charitable Contribution Carryforwards, Net Of Valuation Allowance	0	0
Ameren Illinois Company | Federal
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	3	3
Ameren Illinois Company | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	3
Tax credit carryforwards	0	0
Other
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	4
Tax credit carryforwards	61	70
Deferred Tax Assets, Charitable Contribution Carryforwards	3	3
Deferred Tax Assets, Valuation Allowance, Noncurrent	(3)	(3)
Deferred Tax Assets, Charitable Contribution Carryforwards, Net Of Valuation Allowance	0	0
Other | Federal
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	54	67
Other | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	4
Tax credit carryforwards	$ 7	$ 3

Related Party Transactions (Nar

Related Party Transactions (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Ameren Illinois Capacity Supply Agreements with Ameren Missouri | Ameren Illinois Company | 2020 Procurement
Related Party Transaction [Line Items]
Energy Supply Agreements Amount	$ 3
Ameren Missouri Software Licensing with Ameren Illinois | Ameren Illinois Electric Distribution
Related Party Transaction [Line Items]
Revenue from Related Parties		$ 14
Ameren Missouri Software Licensing with Ameren Illinois | Ameren Illinois Gas
Related Party Transaction [Line Items]
Revenue from Related Parties		5
Ameren Missouri Software Licensing with Ameren Illinois | Ameren Illinois Company
Related Party Transaction [Line Items]
Related Party Transaction, Amounts of Transaction		24
Capitalized Computer Software, Net		5
Revenue from Related Parties		19
Ameren Missouri Software Licensing with Ameren Illinois | Union Electric Company
Related Party Transaction [Line Items]
Capitalized Computer Software, Gross		$ 24

Related Party Transactions (Sch

Related Party Transactions (Schedule of Related Party Electric Power Supply Agreements) (Details) - Ameren Illinois Company - Ameren Illinois Power Supply Agreements with Ameren Missouri	12 Months Ended
	Dec. 31, 2020MWh$ / MWh
April 2017 Procurement
Schedule of Related Party Electric Power Supply Agreements [Line Items]
Related Party Long Term Contract For Purchase Of Electric Power | MWh	85,600
Related Party Long Term Contract For Purchase Of Electric Power Rate | $ / MWh	34
April 2018 Procurement
Schedule of Related Party Electric Power Supply Agreements [Line Items]
Related Party Long Term Contract For Purchase Of Electric Power | MWh	110,000
Related Party Long Term Contract For Purchase Of Electric Power Rate | $ / MWh	32
April 2019 Procurement
Schedule of Related Party Electric Power Supply Agreements [Line Items]
Related Party Long Term Contract For Purchase Of Electric Power | MWh	288,000
Related Party Long Term Contract For Purchase Of Electric Power Rate | $ / MWh	35
September 2019 Procurement
Schedule of Related Party Electric Power Supply Agreements [Line Items]
Related Party Long Term Contract For Purchase Of Electric Power | MWh	170,800
Related Party Long Term Contract For Purchase Of Electric Power Rate | $ / MWh	29
September 2020 Procurement
Schedule of Related Party Electric Power Supply Agreements [Line Items]
Related Party Long Term Contract For Purchase Of Electric Power | MWh	204,800
Related Party Long Term Contract For Purchase Of Electric Power Rate | $ / MWh	31

Related Party Transactions (S_2

Related Party Transactions (Schedule of Affiliate Receivables and Payables) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Union Electric Company
Related Party Transaction [Line Items]
Accounts payable – affiliates	$ 46	$ 52
Accounts Receivable, Related Parties, Current	57	30
Union Electric Company | Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts payable – affiliates	0	15
Union Electric Company | Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	9	15
Ameren Illinois Company
Related Party Transaction [Line Items]
Accounts payable – affiliates	51	82
Accounts Receivable, Related Parties, Current	64	28
Ameren Illinois Company | Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts payable – affiliates	6	43
Ameren Illinois Company | Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	$ 15	$ 17

Related Party Transactions (S_3

Related Party Transactions (Schedule of Capital Contributions) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Union Electric Company
Related Party Transaction [Line Items]
Capital contribution from parent	$ 491	$ 124	$ 45
Ameren Illinois Company
Related Party Transaction [Line Items]
Capital contribution from parent	$ 464	$ 15	$ 160

Related Party Transactions (Eff

Related Party Transactions (Effects of Related-party Transactions on the Statement of Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Union Electric Company | Ameren Missouri Power Supply Agreements with Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues	$ 11	$ 3	$ 11
Union Electric Company | Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	26	27	22
Operating Expenses	1	2	3
Union Electric Company | Ameren Missouri and Ameren Illinois miscellaneous support services and services provided to ATXI
Related Party Transaction [Line Items]
Operating Revenues	3	1	1
Union Electric Company | Total Operating Revenues
Related Party Transaction [Line Items]
Operating Revenues	40	31	34
Union Electric Company | Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	140	135	136
Union Electric Company | Total Related Party Other Operations and Maintenance
Related Party Transaction [Line Items]
Operating Expenses	140	137	139
Union Electric Company | Money Pool Borrowings (Advances)
Related Party Transaction [Line Items]
Interest (Charges) Income	1	1	1
Ameren Illinois Company | Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	1	2	3
Operating Expenses	4	5	6
Ameren Illinois Company | Ameren Missouri and Ameren Illinois miscellaneous support services and services provided to ATXI
Related Party Transaction [Line Items]
Operating Revenues	1	2	1
Ameren Illinois Company | Ameren Missouri Software Licensing with Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues		19
Ameren Illinois Company | Total Operating Revenues
Related Party Transaction [Line Items]
Operating Revenues	2	23	4
Ameren Illinois Company | Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Operating Expenses	11	3	11
Ameren Illinois Company | Ameren Illinois Transmission Services From ATXI
Related Party Transaction [Line Items]
Operating Expenses	2	2	1
Ameren Illinois Company | Total Purchased Power
Related Party Transaction [Line Items]
Operating Expenses	13	5	12
Ameren Illinois Company | Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	133	127	126
Ameren Illinois Company | Total Related Party Other Operations and Maintenance
Related Party Transaction [Line Items]
Operating Expenses	137	132	132
Ameren Illinois Company | Money Pool Borrowings (Advances)
Related Party Transaction [Line Items]
Interest (Charges) Income	$ 1	$ 1	$ 1

Commitments And Contingencies_2

Commitments And Contingencies (Schedule Of Estimated Purchased Commitments) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)MWh
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	$ 788
2022	413
2023	284
2024	186
2025	103
Thereafter	195
Total unrecorded unconditional purchase obligation	1,969
Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	321
2022	194
2023	113
2024	94
2025	55
Thereafter	0
Total unrecorded unconditional purchase obligation	777
Natural gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	161
2022	111
2023	72
2024	31
2025	17
Thereafter	64
Total unrecorded unconditional purchase obligation	456
Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	56
2022	12
2023	45
2024	26
2025	0
Thereafter	21
Total unrecorded unconditional purchase obligation	160
Total Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	150
2022	64
2023	24
2024	7
2025	0
Thereafter	0
Total unrecorded unconditional purchase obligation	$ 245
Amount of Megawatts | MWh	102
Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	$ 3
2022	3
2023	3
2024	3
2025	3
Thereafter	18
Total unrecorded unconditional purchase obligation	33
Other(e)
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	97
2022	29
2023	27
2024	25
2025	28
Thereafter	92
Total unrecorded unconditional purchase obligation	298
Renewable Energy Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Total unrecorded unconditional purchase obligation	48
Zero Emission Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Total unrecorded unconditional purchase obligation	26
Union Electric Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	503
2022	279
2023	224
2024	164
2025	94
Thereafter	126
Total unrecorded unconditional purchase obligation	1,390
Union Electric Company | Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	321
2022	194
2023	113
2024	94
2025	55
Thereafter	0
Total unrecorded unconditional purchase obligation	777
Union Electric Company | Natural gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	44
2022	40
2023	35
2024	15
2025	7
Thereafter	17
Total unrecorded unconditional purchase obligation	158
Union Electric Company | Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	56
2022	12
2023	45
2024	26
2025	0
Thereafter	21
Total unrecorded unconditional purchase obligation	160
Union Electric Company | Total Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	0
2022	0
2023	0
2024	0
2025	0
Thereafter	0
Total unrecorded unconditional purchase obligation	$ 0
Amount of Megawatts | MWh	102
Union Electric Company | Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	$ 3
2022	3
2023	3
2024	3
2025	3
Thereafter	18
Total unrecorded unconditional purchase obligation	33
Union Electric Company | Other(e)
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	79
2022	30
2023	28
2024	26
2025	29
Thereafter	70
Total unrecorded unconditional purchase obligation	262
Ameren Illinois Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	290
2022	138
2023	62
2024	23
2025	10
Thereafter	47
Total unrecorded unconditional purchase obligation	570
Ameren Illinois Company | Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	0
2022	0
2023	0
2024	0
2025	0
Thereafter	0
Total unrecorded unconditional purchase obligation	0
Ameren Illinois Company | Natural gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	117
2022	71
2023	37
2024	16
2025	10
Thereafter	47
Total unrecorded unconditional purchase obligation	298
Ameren Illinois Company | Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	0
2022	0
2023	0
2024	0
2025	0
Thereafter	0
Total unrecorded unconditional purchase obligation	0
Ameren Illinois Company | Total Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	160
2022	67
2023	25
2024	7
2025	0
Thereafter	0
Total unrecorded unconditional purchase obligation	259
Ameren Illinois Company | Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	0
2022	0
2023	0
2024	0
2025	0
Thereafter	0
Total unrecorded unconditional purchase obligation	0
Ameren Illinois Company | Other(e)
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	13
2022	0
2023	0
2024	0
2025	0
Thereafter	0
Total unrecorded unconditional purchase obligation	13
Ameren Illinois Company | Renewable Energy Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Total unrecorded unconditional purchase obligation	48
Ameren Illinois Company | Zero Emission Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Total unrecorded unconditional purchase obligation	$ 26

Commitments And Contingencies_3

Commitments And Contingencies (Environmental Matters) (Details) $ in Millions	Dec. 31, 2020USD ($)scrubbercentersite	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Asset Retirement Obligation	$ 756	$ 691	$ 650
Union Electric Company
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	$ 751	687	646
Ameren Illinois Company
Asset Retirement Obligation	5	$ 4	$ 4
Manufactured Gas Plant
Accrual for environmental loss contingencies	$ 94
Manufactured Gas Plant | Ameren Illinois Company
Number of remediation sites | site	44
Accrual for environmental loss contingencies	$ 94
Minimum
Estimated capital costs to comply with existing and known federal and state air emissions regulations	175
Minimum | Union Electric Company
Estimated capital costs to comply with existing and known federal and state air emissions regulations	175
Minimum | Coal Combustion Residuals Estimate | Union Electric Company
Estimated capital costs to comply with existing and known federal and state air emissions regulations	75
Minimum | Manufactured Gas Plant | Ameren Illinois Company
Estimate of possible loss	94
Maximum
Estimated capital costs to comply with existing and known federal and state air emissions regulations	225
Maximum | Union Electric Company
Estimated capital costs to comply with existing and known federal and state air emissions regulations	225
Maximum | Coal Combustion Residuals Estimate | Union Electric Company
Estimated capital costs to comply with existing and known federal and state air emissions regulations	100
Maximum | Manufactured Gas Plant | Ameren Illinois Company
Estimate of possible loss	157
Rush Island Energy Center | Union Electric Company
Estimated capital costs to comply with preliminary court order	1,000
Rush Island Energy Center | Minimum | Union Electric Company
Estimated operations and maintenance costs to comply with preliminary court order	30
Rush Island Energy Center | Maximum | Union Electric Company
Estimated operations and maintenance costs to comply with preliminary court order	50
New CCR Rules Estimate
Asset Retirement Obligation	111
New CCR Rules Estimate | Union Electric Company
Asset Retirement Obligation	$ 111

Supplemental Information (Narra

Supplemental Information (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Supplemental Information [Abstract]
Payables for purchased receivables	$ 28	$ 32
Deferred Compensation Liability, Classified, Noncurrent	$ 90	$ 86

Supplemental Information (Cash

Supplemental Information (Cash and Cash Equivalents) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	$ 139	$ 16
Restricted Cash and Cash Equivalents, Current	17	14
Restricted Cash and Cash Equivalents, Noncurrent	141	120
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	4	26
Cash, cash equivalents, and restricted cash	301	176	$ 107	$ 68
Union Electric Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	136	9
Restricted Cash and Cash Equivalents, Current	5	4
Restricted Cash and Cash Equivalents, Noncurrent	0	0
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	4	26
Cash, cash equivalents, and restricted cash	145	39	8	7
Ameren Illinois Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	0	0
Restricted Cash and Cash Equivalents, Current	6	5
Restricted Cash and Cash Equivalents, Noncurrent	141	120
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	0	0
Cash, cash equivalents, and restricted cash	$ 147	$ 125	$ 80	$ 41

Supplemental Information (Allow

Supplemental Information (Allowance for Doubtful Accounts) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning of period	$ 17	$ 18
Bad debt expense	48	30
Net write-offs	(15)	(31)
End of period	50	17
Union Electric Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning of period	7	7
Bad debt expense	15	9
Net write-offs	(6)	(9)
End of period	16	7
Ameren Illinois Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning of period	10	11
Bad debt expense	33	21
Net write-offs	(9)	(22)
End of period	$ 34	$ 10

Supplemental Information (Inven

Supplemental Information (Inventories) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Public Utilities, Inventory [Line Items]
Fuel	$ 115	$ 126
Gas stored underground	57	63
Other materials and supplies	349	305
Total Inventories	521	494
Union Electric Company
Public Utilities, Inventory [Line Items]
Fuel	115	126
Gas stored underground	5	6
Other materials and supplies	266	241
Total Inventories	386	373
Ameren Illinois Company
Public Utilities, Inventory [Line Items]
Fuel	0	0
Gas stored underground	52	57
Other materials and supplies	83	64
Total Inventories	$ 135	$ 121

Supplemental Information (Sched

Supplemental Information (Schedule of Asset Retirement Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	$ 756	$ 691	$ 650
Asset Retirement Obligation, Liabilities Incurred	36	0
Liabilities settled	(58)	(20)
Accretion	30	28
Asset Retirement Obligation, Revision of Estimate	57	33
Current Asset Retirement Obligation	60	53
Nuclear Plant
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	549
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	751	687	646
Asset Retirement Obligation, Liabilities Incurred	36	0
Liabilities settled	(58)	(20)
Accretion	29	28
Asset Retirement Obligation, Revision of Estimate	57	33
Current Asset Retirement Obligation	60	53
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	5	4	$ 4
Asset Retirement Obligation, Liabilities Incurred	0	0
Liabilities settled	0	0
Accretion	1	0
Asset Retirement Obligation, Revision of Estimate	$ 0	$ 0

Supplemental Information (Sch_2

Supplemental Information (Schedule of Excise Taxes) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Line Items]
Excise Tax Expense	$ 254	$ 264	$ 282
Union Electric Company
Accounting Policies [Line Items]
Excise Tax Expense	139	147	164
Ameren Illinois Company
Accounting Policies [Line Items]
Excise Tax Expense	$ 115	$ 117	$ 118

Supplemental Information (All_2

Supplemental Information (Allowance For Funds Used During Construction) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Allowance for Funds Used During Construction, Rate [Line Items]
Allowance for equity funds used during construction	$ 32	$ 28	$ 36
Allowance for borrowed funds used during construction	16	20	21
Total	$ 48	$ 48	$ 57
Union Electric Company
Allowance for Funds Used During Construction, Rate [Line Items]
Public Utilities, Allowance for Funds Used During Construction, Rate	5.00%	6.00%	7.00%
Allowance for equity funds used during construction	$ 19	$ 19	$ 27
Allowance for borrowed funds used during construction	10	12	14
Total	$ 29	$ 31	$ 41
Ameren Illinois Company
Allowance for Funds Used During Construction, Rate [Line Items]
Public Utilities, Allowance for Funds Used During Construction, Rate	5.00%	5.00%	5.00%
Allowance for equity funds used during construction	$ 13	$ 9	$ 9
Allowance for borrowed funds used during construction	6	8	7
Total	$ 19	$ 17	$ 16

Supplemental Information (Earni

Supplemental Information (Earnings Per Share) (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share Reconciliation [Abstract]
Weighted-average Common Shares Outstanding – Basic	247,000,000	245,600,000	243,800,000
Assumed settlement of performance share units and restricted stock units (in shares)	1,200,000	1,400,000	2,000,000
Dilutive effect of forward sale agreement (in shares)	500,000	100,000	0
Weighted Average Number of Shares Outstanding, Diluted (in shares)	248,700,000	247,100,000	245,800,000
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount (in shares)	0	0	0

Supplemental Information (Suppl

Supplemental Information (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	$ 400	$ 333	$ 272
Net Realized and Unrealized Gain (Loss) - nuclear trust fund	116	143	(38)
Exchange of bond investments for extinguishment of senior unsecured notes	0	(17)	0
Noncash financing activity - Issuance of common stock for stock-based compensation	38	54	35
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	183	140	121
Net Realized and Unrealized Gain (Loss) - nuclear trust fund	116	143	(38)
Exchange of bond investments for extinguishment of senior unsecured notes	0	0	0
Noncash financing activity - Issuance of common stock for stock-based compensation	0	0	0
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	218	163	138
Net Realized and Unrealized Gain (Loss) - nuclear trust fund	0	0	0
Exchange of bond investments for extinguishment of senior unsecured notes	0	(17)	0
Noncash financing activity - Issuance of common stock for stock-based compensation	0	0	0
Nuclear Fuel
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	0	19	20
Nuclear Fuel | Union Electric Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	0	19	20
Nuclear Fuel | Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	0	0	0
Alternative Energy
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	46	0	0
Alternative Energy | Union Electric Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	46	0	0
Alternative Energy | Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	$ 0	$ 0	$ 0

Segment Information (Schedule O

Segment Information (Schedule Of Segment Reporting Information By Segment) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)segment	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment	4
External revenues	$ 5,794	$ 5,910	$ 6,291
Depreciation and amortization	1,075	995	955
Interest income	29	33	33
Interest Charges	419	381	401
Income tax benefit	155	182	237
Net Income (Loss) Available to Common Stockholders, Basic	871	828	815
Capital expenditures	3,233	2,411	2,286
Wind generation expenditures	564	0	0
Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	5,794	5,891	6,291
Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	$ 0	19	0
Union Electric Company
Segment Reporting Information [Line Items]
Number of reportable segments | segment	1
Ameren Illinois Electric Distribution | Ameren Missouri Software Licensing with Ameren Illinois
Segment Reporting Information [Line Items]
Intersegment revenues		14
Ameren Illinois Gas | Ameren Missouri Software Licensing with Ameren Illinois
Segment Reporting Information [Line Items]
Intersegment revenues		5
Ameren Illinois Company
Segment Reporting Information [Line Items]
Number of reportable segments | segment	3
External revenues	$ 2,535	2,527	2,576
Depreciation and amortization	434	406	374
Interest income	3	6	6
Interest Charges	155	147	149
Income tax benefit	124	110	98
Net Income Available to Common Shareholder	379	343	304
Capital expenditures	1,447	1,208	1,258
Ameren Illinois Company | Ameren Missouri Software Licensing with Ameren Illinois
Segment Reporting Information [Line Items]
Intersegment revenues		19
Ameren Illinois Company | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	2,535	2,527	2,576
Ameren Illinois Company | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0
Operating Segments | Union Electric Company
Segment Reporting Information [Line Items]
External revenues	3,109	3,243	3,589
Depreciation and amortization	604	556	550
Interest income	26	26	28
Interest Charges	190	178	200
Income tax benefit	34	68	124
Net Income Available to Common Shareholder	436	426	478
Capital expenditures	1,666	1,076	914
Operating Segments | Union Electric Company | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	3,069	3,212	3,555
Operating Segments | Union Electric Company | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	40	31	34
Operating Segments | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	1,498	1,504	1,547
Depreciation and amortization	288	273	259
Interest income	2	6	6
Interest Charges	72	71	73
Income tax benefit	42	45	41
Net Income Available to Common Shareholder	143	146	136
Capital expenditures	543	518	503
Operating Segments | Ameren Illinois Electric Distribution | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	1,496	1,487	1,544
Operating Segments | Ameren Illinois Electric Distribution | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	2	17	3
Operating Segments | Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues	760	797	815
Depreciation and amortization	81	78	65
Interest income	0	0	0
Interest Charges	41	38	38
Income tax benefit	36	30	25
Net Income Available to Common Shareholder	99	84	70
Capital expenditures	301	318	311
Operating Segments | Ameren Illinois Gas | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	760	791	814
Operating Segments | Ameren Illinois Gas | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	6	1
Operating Segments | Ameren Transmission
Segment Reporting Information [Line Items]
External revenues	523	464	433
Depreciation and amortization	98	84	77
Interest income	1	1	0
Interest Charges	78	74	75
Income tax benefit	78	64	56
Net Income Available to Common Shareholder	216	185	164
Capital expenditures	716	528	562
Operating Segments | Ameren Transmission | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	469	401	378
Operating Segments | Ameren Transmission | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	54	63	55
Operating Segments | Other Segment
Segment Reporting Information [Line Items]
Depreciation and amortization	4	4	4
Interest income	4	5	4
Interest Charges	42	25	19
Income tax benefit	(35)	(25)	(9)
Net Income Available to Common Shareholder	(23)	(13)	(33)
Capital expenditures	5	3	5
Operating Segments | Other Segment | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	0	0	0
Operating Segments | Other Segment | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Depreciation and amortization	288	273	259
Interest income	2	6	6
Interest Charges	72	71	73
Income tax benefit	42	45	41
Net Income Available to Common Shareholder	143	146	136
Capital expenditures	543	518	503
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	1,498	1,504	1,547
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas
Segment Reporting Information [Line Items]
Depreciation and amortization	81	78	65
Interest income	0	0	0
Interest Charges	41	38	38
Income tax benefit	36	30	25
Net Income Available to Common Shareholder	99	84	70
Capital expenditures	301	318	311
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	760	797	815
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Segment Reporting Information [Line Items]
Depreciation and amortization	65	55	50
Interest income	1	0	0
Interest Charges	42	38	38
Income tax benefit	46	35	32
Net Income Available to Common Shareholder	137	113	98
Capital expenditures	603	372	444
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	277	226	214
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	52	62	53
Intersegment Elimination
Segment Reporting Information [Line Items]
External revenues	(96)	(98)	(93)
Depreciation and amortization	0	0	0
Interest income	(4)	(5)	(5)
Interest Charges	(4)	(5)	(4)
Income tax benefit	0	0	0
Net Income Available to Common Shareholder	0	0	0
Capital expenditures	2	(32)	(9)
Intersegment Elimination | Ameren Missouri Software Licensing with Ameren Illinois
Segment Reporting Information [Line Items]
Capital expenditures		24
Intersegment Elimination | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	0	0	0
Intersegment Elimination | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	(96)	(98)	(93)
Intersegment Elimination | Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	(52)	(62)	(53)
Depreciation and amortization	0	0	0
Interest income	0	0	0
Interest Charges	0	0	0
Income tax benefit	0	0	0
Net Income Available to Common Shareholder	0	0	0
Capital expenditures	0	0	0
Intersegment Elimination | Ameren Illinois Company | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	0	0	0
Intersegment Elimination | Ameren Illinois Company | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	$ (52)	$ (62)	$ (53)

Segment Information (Disaggrega

Segment Information (Disaggregation of Revenues) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
External revenues	$ 5,794	$ 5,910	$ 6,291
Revenues from alternative revenue programs	36	(70)	(59)
Other revenues not from contracts with customers	36	28	42
TCJA - Excess Amounts Collected in Rates
Disaggregation of Revenue [Line Items]
TCJA Revenue Reduction			60
Ameren Illinois Electric Distribution | Ameren Missouri Software Licensing with Ameren Illinois
Disaggregation of Revenue [Line Items]
Revenue from Related Parties		14
Ameren Illinois Gas | Ameren Missouri Software Licensing with Ameren Illinois
Disaggregation of Revenue [Line Items]
Revenue from Related Parties		5
Electric
Disaggregation of Revenue [Line Items]
External revenues	4,911	4,981	5,339
Electric | Residential
Disaggregation of Revenue [Line Items]
External revenues	2,240	2,251	2,427
Electric | Commercial
Disaggregation of Revenue [Line Items]
External revenues	1,511	1,654	1,782
Electric | Industrial
Disaggregation of Revenue [Line Items]
External revenues	385	405	442
Electric | Other
Disaggregation of Revenue [Line Items]
External revenues	775	671	688
Natural gas
Disaggregation of Revenue [Line Items]
External revenues	883	929	952
Natural gas | Residential
Disaggregation of Revenue [Line Items]
External revenues	617	651	671
Natural gas | Commercial
Disaggregation of Revenue [Line Items]
External revenues	165	188	196
Natural gas | Industrial
Disaggregation of Revenue [Line Items]
External revenues	18	17	21
Natural gas | Other
Disaggregation of Revenue [Line Items]
External revenues	83	73	64
Ameren Illinois Company
Disaggregation of Revenue [Line Items]
External revenues	2,535	2,527	2,576
Revenues from alternative revenue programs	42	(107)	(51)
Other revenues not from contracts with customers	10	9	18
Ameren Illinois Company | Ameren Missouri Software Licensing with Ameren Illinois
Disaggregation of Revenue [Line Items]
Revenue from Related Parties		19
Ameren Illinois Company | Residential
Disaggregation of Revenue [Line Items]
External revenues	1,408	1,418	1,448
Ameren Illinois Company | Commercial
Disaggregation of Revenue [Line Items]
External revenues	622	651	670
Ameren Illinois Company | Industrial
Disaggregation of Revenue [Line Items]
External revenues	138	140	147
Ameren Illinois Company | Other
Disaggregation of Revenue [Line Items]
External revenues	367	318	311
Ameren Illinois Company | Electric
Disaggregation of Revenue [Line Items]
External revenues	1,775	1,730	1,761
Ameren Illinois Company | Natural gas
Disaggregation of Revenue [Line Items]
External revenues	760	797	815
Operating Segments | Union Electric Company
Disaggregation of Revenue [Line Items]
External revenues	3,109	3,243	3,589
Revenues from alternative revenue programs	(14)	35	(8)
Other revenues not from contracts with customers	25	19	24
Operating Segments | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External revenues	1,498	1,504	1,547
Revenues from alternative revenue programs	(20)	(74)	(3)
Other revenues not from contracts with customers	8	7	16
Operating Segments | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
External revenues	760	797	815
Revenues from alternative revenue programs	20	0	(23)
Other revenues not from contracts with customers	2	2	2
Operating Segments | Ameren Transmission
Disaggregation of Revenue [Line Items]
External revenues	523	464	433
Revenues from alternative revenue programs	50	(31)	(25)
Other revenues not from contracts with customers	1	0	0
Operating Segments | Electric | Union Electric Company
Disaggregation of Revenue [Line Items]
External revenues	2,984	3,109	3,451
Operating Segments | Electric | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
External revenues	1,373	1,403	1,560
Operating Segments | Electric | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
External revenues	1,025	1,157	1,271
Operating Segments | Electric | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
External revenues	261	278	312
Operating Segments | Electric | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
External revenues	325	271	308
Operating Segments | Electric | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External revenues	1,498	1,504	1,547
Operating Segments | Electric | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
External revenues	867	848	867
Operating Segments | Electric | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
External revenues	486	497	511
Operating Segments | Electric | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
External revenues	124	127	130
Operating Segments | Electric | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
External revenues	21	32	39
Operating Segments | Electric | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Electric | Ameren Illinois Gas | Residential
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Electric | Ameren Illinois Gas | Commercial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Electric | Ameren Illinois Gas | Industrial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Electric | Ameren Illinois Gas | Other
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Electric | Ameren Transmission
Disaggregation of Revenue [Line Items]
External revenues	523	464	433
Operating Segments | Electric | Ameren Transmission | Residential
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Electric | Ameren Transmission | Commercial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Electric | Ameren Transmission | Industrial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Electric | Ameren Transmission | Other
Disaggregation of Revenue [Line Items]
External revenues	523	464	433
Operating Segments | Natural gas | Union Electric Company
Disaggregation of Revenue [Line Items]
External revenues	125	134	138
Operating Segments | Natural gas | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
External revenues	76	81	90
Operating Segments | Natural gas | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
External revenues	29	34	37
Operating Segments | Natural gas | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
External revenues	4	4	4
Operating Segments | Natural gas | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
External revenues	16	15	7
Operating Segments | Natural gas | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Natural gas | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Natural gas | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Natural gas | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Natural gas | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Natural gas | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
External revenues	760	797	815
Operating Segments | Natural gas | Ameren Illinois Gas | Residential
Disaggregation of Revenue [Line Items]
External revenues	541	570	581
Operating Segments | Natural gas | Ameren Illinois Gas | Commercial
Disaggregation of Revenue [Line Items]
External revenues	136	154	159
Operating Segments | Natural gas | Ameren Illinois Gas | Industrial
Disaggregation of Revenue [Line Items]
External revenues	14	13	17
Operating Segments | Natural gas | Ameren Illinois Gas | Other
Disaggregation of Revenue [Line Items]
External revenues	69	60	58
Operating Segments | Natural gas | Ameren Transmission
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Natural gas | Ameren Transmission | Residential
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Natural gas | Ameren Transmission | Commercial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Natural gas | Ameren Transmission | Industrial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Natural gas | Ameren Transmission | Other
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(20)	(74)	(3)
Other revenues not from contracts with customers	8	7	16
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	20	0	(23)
Other revenues not from contracts with customers	2	2	2
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	42	(33)	(25)
Other revenues not from contracts with customers	0	0	0
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External revenues	1,498	1,504	1,547
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
External revenues	867	848	867
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
External revenues	486	497	511
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
External revenues	124	127	130
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
External revenues	21	32	39
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
External revenues	329	288	267
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Transmission | Residential
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Transmission | Commercial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Transmission | Industrial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Transmission | Other
Disaggregation of Revenue [Line Items]
External revenues	329	288	267
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
External revenues	760	797	815
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas | Residential
Disaggregation of Revenue [Line Items]
External revenues	541	570	581
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas | Commercial
Disaggregation of Revenue [Line Items]
External revenues	136	154	159
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas | Industrial
Disaggregation of Revenue [Line Items]
External revenues	14	13	17
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas | Other
Disaggregation of Revenue [Line Items]
External revenues	69	60	58
Intersegment Elimination
Disaggregation of Revenue [Line Items]
External revenues	(96)	(98)	(93)
Intersegment Elimination | Electric
Disaggregation of Revenue [Line Items]
External revenues	(94)	(96)	(92)
Intersegment Elimination | Electric | Residential
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Intersegment Elimination | Electric | Commercial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Intersegment Elimination | Electric | Industrial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Intersegment Elimination | Electric | Other
Disaggregation of Revenue [Line Items]
External revenues	(94)	(96)	(92)
Intersegment Elimination | Natural gas
Disaggregation of Revenue [Line Items]
External revenues	(2)	(2)	(1)
Intersegment Elimination | Natural gas | Residential
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Intersegment Elimination | Natural gas | Commercial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Intersegment Elimination | Natural gas | Industrial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Intersegment Elimination | Natural gas | Other
Disaggregation of Revenue [Line Items]
External revenues	(2)	(2)	(1)
Intersegment Elimination | Ameren Illinois Company
Disaggregation of Revenue [Line Items]
External revenues	(52)	(62)	(53)
Intersegment Elimination | Ameren Illinois Company | Residential
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Intersegment Elimination | Ameren Illinois Company | Commercial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Intersegment Elimination | Ameren Illinois Company | Industrial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Intersegment Elimination | Ameren Illinois Company | Other
Disaggregation of Revenue [Line Items]
External revenues	$ (52)	$ (62)	$ (53)

Schedule I - Condensed Financ_2

Schedule I - Condensed Financial Information Of Parent (Statement of Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Financial Statements, Captions [Line Items]
Total operating revenues	$ 5,794	$ 5,910	$ 6,291
Operating expenses	4,494	4,643	4,934
Operating Income	1,300	1,267	1,357
Interest income from affiliates	4	8	7
Total other expense, net	151	130	102
Interest Charges	(419)	(381)	(401)
Income tax benefit	155	182	237
Comprehensive Income from Continuing Operations
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	16	5	(4)
Comprehensive Income Attributable to Ameren Common Shareholders	887	833	811
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement benefit plan activity, tax (benefit)	5	1	(1)
Parent Company
Condensed Financial Statements, Captions [Line Items]
Total operating revenues	0	0	0
Operating expenses	12	15	11
Operating Income	(12)	(15)	(11)
Equity in earnings of subsidiaries	908	850	857
Interest income from affiliates	4	5	3
Total other expense, net	(8)	(2)	(12)
Interest Charges	(57)	(39)	(34)
Income tax benefit	36	29	12
Net Income Available to Common Shareholder	871	828	815
Comprehensive Income from Continuing Operations
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	16	5	(4)
Comprehensive Income Attributable to Ameren Common Shareholders	887	833	811
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement benefit plan activity, tax (benefit)	$ 5	$ 1	$ (1)

Schedule I - Condensed Financ_3

Schedule I - Condensed Financial Information Of Parent (Balance Sheet) (Details) - USD ($) $ / shares in Units, $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets:
Cash and cash equivalents	$ 139	$ 16
Miscellaneous accounts and notes receivable	65	63
Other current assets	135	118
Total current assets	1,653	1,431
Other assets	1,077	876
Total assets	32,030	28,933
Liabilities and Shareholders' Equity:
Current maturities of long-term debt	8	442
Short-term debt	490	440
Other current liabilities	489	491
Total current liabilities	2,180	2,505
Long-term debt	11,078	8,915
Pension and other postretirement benefits	37	401
Other deferred credits and liabilities	466	467
Commitments and Contingencies (Note 5)
Common stock, $.01 par value, 400.0 shares authorized – shares outstanding of 253.3 and 246.2, respectively	3	2
Other paid-in capital, principally premium on common stock	6,179	5,694
Retained earnings	2,757	2,380
Accumulated other comprehensive loss	(1)	(17)
Total shareholders' equity	9,080	8,201	$ 7,773
TOTAL LIABILITIES AND EQUITY	$ 32,030	$ 28,933
Common Stock, Par or Stated Value Per Share (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	400,000,000	400,000,000
Common Stock, Shares, Outstanding	253,300,000	246,200,000	244,500,000	242,600,000
Parent Company
Assets:
Cash and cash equivalents	$ 0	$ 0
Advances to money pool	16	102
Accounts receivable – affiliates	12	73
Miscellaneous accounts and notes receivable	15	4
Other current assets	4	3
Total current assets	47	182
Investments in subsidiaries	10,872	9,108
Note receivable – ATXI	75	75
Accumulated deferred income taxes, net	42	49
Other assets	167	145
Total assets	11,203	9,559
Liabilities and Shareholders' Equity:
Current maturities of long-term debt	0	350
Short-term debt	490	153
Borrowings from money pool	0	24
Accounts payable – affiliates	41	39
Other current liabilities	34	23
Total current liabilities	565	589
Long-term debt	1,588	794
Pension and other postretirement benefits	27	37
Other deferred credits and liabilities	85	80
Total liabilities	2,265	1,500
Commitments and Contingencies (Note 5)
Common stock, $.01 par value, 400.0 shares authorized – shares outstanding of 253.3 and 246.2, respectively	3	2
Other paid-in capital, principally premium on common stock	6,179	5,694
Retained earnings	2,757	2,380
Accumulated other comprehensive loss	(1)	(17)
Total shareholders' equity	8,938	8,059
TOTAL LIABILITIES AND EQUITY	$ 11,203	$ 9,559
Common Stock, Par or Stated Value Per Share (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	400,000,000	400,000,000
Common Stock, Shares, Outstanding	253,300,000	246,200,000

Schedule I - Condensed Financ_4

Schedule I - Condensed Financial Information Of Parent (Statement of Cash Flows) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Financial Statements, Captions [Line Items]
Net cash flows provided by operating activities	$ 1,727	$ 2,170	$ 2,170
Cash Flows From Investing Activities:
Other	11	3	18
Net cash flows provided by (used in) investing activities	(3,329)	(2,435)	(2,336)
Cash flows from financing activities:
Dividends on common stock	(494)	(472)	(451)
Short-term debt, net	50	(157)	112
Maturities of long-term debt	(442)	(580)	(841)
Issuances of long-term debt	2,183	1,527	1,352
Issuances of common stock	476	68	74
Employee payroll taxes related to stock-based compensation	(20)	(29)	(19)
Debt issuance costs	(20)	(17)	(14)
Net cash flows used in financing activities	1,727	334	205
Net change in cash, cash equivalents, and restricted cash	125	69	39
Cash, cash equivalents, and restricted cash at beginning of year	176	107	68
Cash, cash equivalents, and restricted cash at end of year	301	176	107
Noncash Investing and Financing Items [Abstract]
Noncash financing activity - Issuance of common stock for stock-based compensation	38	54	35
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net cash flows provided by operating activities	147	491	550
Cash Flows From Investing Activities:
Money pool advances, net	86
Money pool advances, net		(26)	(63)
Investments in subsidiaries	(956)	(142)	(208)
Other	8	5	5
Net cash flows provided by (used in) investing activities	(862)	(163)	(266)
Cash flows from financing activities:
Dividends on common stock	(494)	(472)	(451)
Short-term debt, net	337	(317)	87
Money pool borrowings, net	(24)	(22)	18
Maturities of long-term debt	(350)	0	0
Issuances of long-term debt	798	450	0
Issuances of common stock	476	68	74
Employee payroll taxes related to stock-based compensation	(20)	(29)	(19)
Debt issuance costs	(7)	(4)	0
Net cash flows used in financing activities	716	(326)	(291)
Net change in cash, cash equivalents, and restricted cash	1	2	(7)
Cash, cash equivalents, and restricted cash at beginning of year	3	1	8
Cash, cash equivalents, and restricted cash at end of year	4	3	1
Cash dividends received from consolidated subsidiaries	105	445	450
Noncash Investing and Financing Items [Abstract]
Noncash financing activity - Issuance of common stock for stock-based compensation	$ 38	$ 54	$ 35

Schedule I - Condensed Financ_5

Schedule I - Condensed Financial Information Of Parent Cash and Cash Equivalents (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	$ 139	$ 16
Restricted cash included in Other current assets	141	120
Cash, cash equivalents, and restricted cash	301	176	$ 107	$ 68
Parent 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	4	3
Cash, cash equivalents, and restricted cash	$ 4	$ 3	$ 1	$ 8

Schedule I - Condensed Financ_6

Schedule I - Condensed Financial Information Of Parent Guarantees (Details) $ in Millions	Dec. 31, 2020USD ($)
Parent Company
Other Commitments [Line Items]
Guarantees Outstanding	$ 3

Schedule I - Condensed Financ_7

Schedule I - Condensed Financial Information Of Parent Other Income (Expense), Net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other Nonoperating Income (Expense) [Line Items]
Non-service cost components of net periodic benefit income	$ 116	$ 90	$ 70
Donations	(25)	(12)	(33)
Total other expense, net	151	130	102
Parent Company
Other Nonoperating Income (Expense) [Line Items]
Non-service cost components of net periodic benefit income	1	2	2
Donations	(8)	(3)	(13)
Other expenses, net	(1)	(1)	(1)
Total other expense, net	$ (8)	$ (2)	$ (12)

Schedule II - Valuation And Q_2

Schedule II - Valuation And Qualifying Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Allowance For Doubtful Accounts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	$ 17	$ 18	$ 19
Charged to Costs and Expenses	42	26	27
Charged to Other Accounts	6	4	4
Deductions	15	31	32
Balance at End of Period	50	17	18
Valuation Allowance of Deferred Tax Assets
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	3	5	11
Charged to Costs and Expenses - Tax Valuation Adjustments	0	(2)	(6)
Charged to Other Accounts	0	0	0
Deductions	0	0	0
Balance at End of Period	3	3	5
Union Electric Company | Allowance For Doubtful Accounts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	7	7	7
Charged to Costs and Expenses	15	9	9
Charged to Other Accounts	0	0	0
Deductions	6	9	9
Balance at End of Period	16	7	7
Ameren Illinois Company | Allowance For Doubtful Accounts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	10	11	12
Charged to Costs and Expenses	27	17	18
Charged to Other Accounts	6	4	4
Deductions	9	22	23
Balance at End of Period	$ 34	$ 10	$ 11