Ameren Illinois (AILIN) 10-Q 10 May 21 2021 Q1 Quarterly report Financial data

Cover Page

Cover Page - shares	3 Months Ended
	Mar. 31, 2021	Apr. 30, 2021
Entity Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2021
Document Transition Report	false
Entity File Number	1-14756
Entity Registrant Name	Ameren Corporation
Entity Tax Identification Number	43-1723446
Entity Incorporation, State or Country Code	MO
Entity Address, Address Line One	1901 Chouteau Avenue
Entity Address, City or Town	St. Louis
Entity Address, State or Province	MO
Entity Address, Postal Zip Code	63103
City Area Code	(314)
Local Phone Number	621-3222
Title of 12(b) Security	Common Stock, $0.01 par value per share
Trading Symbol(s)	AEE
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		255,552,619
Entity Central Index Key	0001002910
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Amendment Flag	false
Union Electric Company
Entity Information [Line Items]
Entity File Number	1-2967
Entity Registrant Name	Union Electric Company
Entity Tax Identification Number	43-0559760
Entity Incorporation, State or Country Code	MO
Entity Address, Address Line One	1901 Chouteau Avenue
Entity Address, City or Town	St. Louis
Entity Address, State or Province	MO
Entity Address, Postal Zip Code	63103
City Area Code	(314)
Local Phone Number	621-3222
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		102,123,834
Entity Central Index Key	0000100826
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Amendment Flag	false
Ameren Illinois Company
Entity Information [Line Items]
Entity File Number	1-3672
Entity Registrant Name	Ameren Illinois Company
Entity Tax Identification Number	37-0211380
Entity Incorporation, State or Country Code	IL
Entity Address, Address Line One	10 Executive Drive
Entity Address, City or Town	Collinsville
Entity Address, State or Province	IL
Entity Address, Postal Zip Code	62234
City Area Code	(618)
Local Phone Number	343-8150
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		25,452,373
Entity Central Index Key	0000018654
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Amendment Flag	false

Consolidated Statement of Incom

Consolidated Statement of Income (Loss) and Comprehensive Income - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Operating Revenues:
Total operating revenues	$ 1,566	$ 1,440
Operating Expenses:
Fuel	65	140
Purchased power	191	134
Natural gas purchased for resale	165	107
Other operations and maintenance	420	438
Depreciation and amortization	281	255
Taxes other than income taxes	128	125
Total operating expenses	1,250	1,199
Operating Income	316	241
Other Income, Net	46	21
Interest Charges	100	93
Income Before Income Taxes	262	169
Income Taxes	27	21
Net Income	235	148
Less: Net Income Attributable to Noncontrolling Interests	2	2
Net Income Attributable to Ameren Common Shareholders	233	146
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	1	1
Comprehensive Income	236	149
Less: Comprehensive Income Attributable to Noncontrolling Interests	2	2
Comprehensive Income Attributable to Ameren Common Shareholders	$ 234	$ 147
Earnings Per Share, Basic and Diluted [Abstract]
Earnings per Common Share - Basic (in dollars per share)	$ 0.92	$ 0.59
Earnings per Common Share - Diluted (in dollars per share)	$ 0.91	$ 0.59
Weighted Average Common Shares Outstanding - Basic (in shares)	254.4	246.4
Weighted Average Number of Shares Outstanding - Diluted (in shares)	255.9	248.1
Electricity
Operating Revenues:
Total operating revenues	$ 1,156	$ 1,120
Natural Gas
Operating Revenues:
Total operating revenues	410	320
Union Electric Company
Operating Revenues:
Total operating revenues	704	680
Operating Expenses:
Fuel	65	140
Purchased power	88	39
Natural gas purchased for resale	31	18
Other operations and maintenance	225	239
Depreciation and amortization	156	139
Taxes other than income taxes	77	79
Total operating expenses	642	654
Operating Income	62	26
Other Income, Net	23	4
Interest Charges	39	40
Income Before Income Taxes	46	(10)
Income Taxes	(2)	(1)
Net Income	48	(9)
Preferred Stock Dividends	1	1
Net Income (Loss) Attributable to Parent	47	(10)
Union Electric Company | Electricity
Operating Revenues:
Total operating revenues	641	631
Union Electric Company | Natural Gas
Operating Revenues:
Total operating revenues	63	49
Ameren Illinois Company
Operating Revenues:
Total operating revenues	823	723
Operating Expenses:
Purchased power	106	98
Natural gas purchased for resale	134	89
Other operations and maintenance	194	199
Depreciation and amortization	115	107
Taxes other than income taxes	46	42
Total operating expenses	595	535
Operating Income	228	188
Other Income, Net	14	11
Interest Charges	42	39
Income Before Income Taxes	200	160
Income Taxes	50	39
Net Income	150	121
Preferred Stock Dividends	1	1
Net Income (Loss) Attributable to Parent	149	120
Ameren Illinois Company | Electricity
Operating Revenues:
Total operating revenues	476	452
Ameren Illinois Company | Natural Gas
Operating Revenues:
Total operating revenues	$ 347	$ 271

Consolidated Statement of Inc_2

Consolidated Statement of Income (Loss) and Comprehensive Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Statement [Abstract]
Pension and other postretirement benefit plan activity, tax	$ 0	$ 0

Consolidated Balance Sheet

Consolidated Balance Sheet - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Current Assets:
Cash and cash equivalents	$ 6	$ 139
Accounts receivable - trade (less allowance for doubtful accounts)	464	415
Unbilled revenue	210	269
Miscellaneous accounts receivable	61	65
Inventories	467	521
Restricted cash	134	17
Current regulatory assets	367	109
Other current assets	114	118
Total current assets	1,823	1,653
Property, Plant, and Equipment, Net	27,307	26,807
Investments and Other Assets:
Nuclear decommissioning trust fund	1,010	982
Goodwill	411	411
Regulatory assets	1,249	1,100
Other assets	989	1,077
Total investments and other assets	3,659	3,570
TOTAL ASSETS	32,789	32,030
Current Liabilities:
Current maturities of long-term debt	8	8
Short-term debt	889	490
Accounts and wages payable	581	958
Taxes accrued	128	82
Interest accrued	84	114
Current regulatory liabilities	225	121
Other current liabilities	392	407
Total current liabilities	2,307	2,180
Long-term Debt, Net	11,527	11,078
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes and tax credits, net	3,253	3,211
Regulatory liabilities	5,230	5,282
Asset retirement obligations	705	696
Pension and other postretirement benefits	38	37
Other deferred credits and liabilities	452	466
Total deferred credits and other liabilities	9,678	9,692
Commitments and Contingencies
Shareholders’ Equity:
Common Stock	3	3
Other paid-in capital, principally premium on common stock	6,295	6,179
Retained earnings	2,850	2,757
Accumulated other comprehensive loss	0	(1)
Total shareholders’ equity	9,148	8,938
Noncontrolling Interests	129	142
Total equity	9,277	9,080
TOTAL LIABILITIES AND EQUITY	32,789	32,030
Union Electric Company
Current Assets:
Cash and cash equivalents	1	136
Advances to money pool	0	139
Accounts receivable - trade (less allowance for doubtful accounts)	156	166
Accounts receivable – affiliates	69	57
Unbilled revenue	109	133
Miscellaneous accounts receivable	43	36
Inventories	362	386
Restricted cash	3	5
Current regulatory assets	141	60
Other current assets	70	79
Total current assets	951	1,192
Property, Plant, and Equipment, Net	14,221	13,879
Investments and Other Assets:
Nuclear decommissioning trust fund	1,010	982
Regulatory assets	413	347
Other assets	375	383
Total investments and other assets	1,798	1,712
TOTAL ASSETS	16,970	16,783
Current Liabilities:
Current maturities of long-term debt	8	8
Short-term debt	204	0
Accounts and wages payable	267	501
Accounts payable – affiliates	40	46
Taxes accrued	85	42
Interest accrued	46	53
Current asset retirement obligations	59	60
Other current liabilities	116	123
Total current liabilities	825	833
Long-term Debt, Net	5,096	5,096
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes and tax credits, net	1,756	1,742
Regulatory liabilities	3,130	3,110
Asset retirement obligations	700	691
Pension and other postretirement benefits	34	35
Other deferred credits and liabilities	59	66
Total deferred credits and other liabilities	5,679	5,644
Commitments and Contingencies
Shareholders’ Equity:
Common Stock	511	511
Other paid-in capital, principally premium on common stock	2,631	2,518
Preferred stock	80	80
Retained earnings	2,148	2,101
Total shareholders’ equity	5,370	5,210
TOTAL LIABILITIES AND EQUITY	16,970	16,783
Ameren Illinois Company
Current Assets:
Cash and cash equivalents	0	0
Accounts receivable - trade (less allowance for doubtful accounts)	293	234
Accounts receivable – affiliates	71	64
Unbilled revenue	101	136
Miscellaneous accounts receivable	5	12
Inventories	105	135
Restricted cash	125	6
Current regulatory assets	217	37
Other current assets	24	23
Total current assets	941	647
Property, Plant, and Equipment, Net	11,358	11,201
Investments and Other Assets:
Goodwill	411	411
Regulatory assets	820	742
Other assets	442	534
Total investments and other assets	1,673	1,687
TOTAL ASSETS	13,972	13,535
Current Liabilities:
Short-term debt	323	0
Borrowings from money pool	0	19
Accounts and wages payable	255	363
Accounts payable – affiliates	43	51
Customer deposits	70	74
Current regulatory liabilities	200	88
Other current liabilities	207	221
Total current liabilities	1,098	816
Long-term Debt, Net	3,947	3,946
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes and tax credits, net	1,430	1,367
Regulatory liabilities	1,988	2,063
Pension and other postretirement benefits	67	69
Environmental remediation	51	57
Other deferred credits and liabilities	249	251
Total deferred credits and other liabilities	3,785	3,807
Commitments and Contingencies
Shareholders’ Equity:
Common Stock	0	0
Other paid-in capital, principally premium on common stock	2,692	2,652
Preferred stock	49	62
Retained earnings	2,401	2,252
Total shareholders’ equity	5,142	4,966
TOTAL LIABILITIES AND EQUITY	$ 13,972	$ 13,535

Consolidated Balance Sheet (Par

Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Accounts Receivable, Allowance for Credit Loss, Current	$ 47	$ 50
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	400,000,000	400,000,000
Common Stock, Shares, Outstanding	255,500,000	253,300,000
Union Electric Company
Accounts Receivable, Allowance for Credit Loss, Current	$ 15	$ 16
Common stock, par value (in dollars per share)	$ 5	$ 5
Common stock, shares authorized (in shares)	150,000,000	150,000,000
Common Stock, Shares, Outstanding	102,100,000	102,100,000
Ameren Illinois Company
Accounts Receivable, Allowance for Credit Loss, Current	$ 32	$ 34
Common stock, no par value (in dollars per share)	$ 0	$ 0
Common stock, shares authorized (in shares)	45,000,000	45,000,000
Common Stock, Shares, Outstanding	25,500,000	25,500,000

Consolidated Statement of Cash

Consolidated Statement of Cash Flows - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash Flows From Operating Activities:
Net income	$ 235	$ 148
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	295	263
Amortization of nuclear fuel	0	23
Amortization of debt issuance costs and premium/discounts	5	5
Deferred income taxes and investment tax credits, net	26	23
Allowance for equity funds used during construction	(7)	(4)
Stock-based compensation costs	6	6
Other	8	17
Changes in assets and liabilities:
Receivables	(5)	(5)
Inventories	54	23
Accounts and wages payable	(252)	(221)
Taxes accrued	60	47
Regulatory assets and liabilities	(421)	(14)
Assets, other	(9)	(3)
Liabilities, other	(34)	(18)
Pension and other postretirement benefits	4	0
Net cash provided by (used in) operating activities	(35)	290
Cash Flows From Investing Activities:
Capital expenditures	(694)	(636)
Wind generation expenditures	(193)	0
Nuclear fuel expenditures	(1)	(35)
Purchases of securities – nuclear decommissioning trust fund	(152)	(96)
Sales and maturities of securities – nuclear decommissioning trust fund	150	81
Other	1	2
Net cash used in investing activities	(889)	(684)
Cash Flows From Financing Activities:
Dividends on common stock	(140)	(122)
Dividends paid to noncontrolling interest holders	(2)	(2)
Short-term debt, net	399	175
Maturities of long-term debt	0	(85)
Issuances of long-term debt	450	465
Issuances of common stock	125	13
Redemptions of Ameren Illinois preferred stock	(13)	0
Employee payroll taxes related to stock-based compensation	(17)	(20)
Debt issuance costs	(3)	(3)
Other	(4)	0
Net cash provided by financing activities	795	421
Net change in cash, cash equivalents, and restricted cash	(129)	27
Cash, cash equivalents, and restricted cash at beginning of year	301	176
Cash, cash equivalents, and restricted cash at end of period	172	203
Union Electric Company
Cash Flows From Operating Activities:
Net income	48	(9)
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	171	148
Amortization of nuclear fuel	0	23
Amortization of debt issuance costs and premium/discounts	1	2
Deferred income taxes and investment tax credits, net	4	(5)
Allowance for equity funds used during construction	(4)	(2)
Other	3	2
Changes in assets and liabilities:
Receivables	20	(3)
Inventories	24	(18)
Accounts and wages payable	(201)	(172)
Taxes accrued	39	55
Regulatory assets and liabilities	(164)	16
Assets, other	13	2
Liabilities, other	(9)	0
Pension and other postretirement benefits	4	2
Net cash provided by (used in) operating activities	(51)	41
Cash Flows From Investing Activities:
Capital expenditures	(341)	(278)
Wind generation expenditures	(193)	0
Nuclear fuel expenditures	(1)	(35)
Purchases of securities – nuclear decommissioning trust fund	(152)	(96)
Sales and maturities of securities – nuclear decommissioning trust fund	150	81
Money pool advances, net	139	0
Net cash used in investing activities	(398)	(328)
Cash Flows From Financing Activities:
Dividends on preferred stock	(1)	(1)
Short-term debt, net	204	(104)
Maturities of long-term debt	0	(85)
Issuances of long-term debt	0	465
Capital contributions from parent	113	0
Debt issuance costs	0	(3)
Net cash provided by financing activities	316	272
Net change in cash, cash equivalents, and restricted cash	(133)	(15)
Cash, cash equivalents, and restricted cash at beginning of year	145	39
Cash, cash equivalents, and restricted cash at end of period	12	24
Ameren Illinois Company
Cash Flows From Operating Activities:
Net income	150	121
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	115	106
Amortization of debt issuance costs and premium/discounts	3	2
Deferred income taxes and investment tax credits, net	53	22
Allowance for equity funds used during construction	(3)	(2)
Other	3	(2)
Changes in assets and liabilities:
Receivables	(24)	(6)
Inventories	30	41
Accounts and wages payable	(40)	(20)
Taxes accrued	3	16
Regulatory assets and liabilities	(255)	(28)
Assets, other	(15)	(4)
Liabilities, other	(9)	(14)
Pension and other postretirement benefits	(1)	(2)
Net cash provided by (used in) operating activities	13	232
Cash Flows From Investing Activities:
Capital expenditures	(337)	(324)
Other	0	1
Net cash used in investing activities	(337)	(323)
Cash Flows From Financing Activities:
Dividends on preferred stock	(1)	(1)
Short-term debt, net	323	7
Money pool borrowings, net	(19)	0
Capital contributions from parent	40	100
Redemption of preferred stock	(13)	0
Other	(4)	0
Net cash provided by financing activities	326	106
Net change in cash, cash equivalents, and restricted cash	2	15
Cash, cash equivalents, and restricted cash at beginning of year	147	125
Cash, cash equivalents, and restricted cash at end of period	$ 149	$ 140

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 Interests	Total Ameren Corporation Shareholders' Equity	Union Electric Company	Union Electric CompanyCommon Stock	Union Electric CompanyOther Paid-in Capital	Union Electric CompanyPreferred Stock	Union Electric CompanyRetained Earnings	Ameren Illinois Company	Ameren Illinois CompanyCommon Stock	Ameren Illinois CompanyOther Paid-in Capital	Ameren Illinois CompanyPreferred Stock	Ameren Illinois CompanyRetained Earnings
Beginning of year at Dec. 31, 2019			$ 5,694	$ 2,380	$ (17)		$ 142				$ 2,027		$ 1,731			$ 2,188	$ 62	$ 1,882
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Settlement of forward sale agreement through common shares issuance			0
Shares issued under the DRPlus and 401(k) plan			13
Stock-based compensation activity			(12)
Capital contribution from parent									$ 0		0			$ (100)		100
Net income	$ 148								(9)				(9)	121				121
Net income attributable to Ameren common shareholders	146			146
Dividends on common stock				(122)
Preferred stock dividends													(1)					(1)
Change in deferred retirement benefit costs	1				1
Net income attributable to noncontrolling interest holders	$ (2)						2
Dividends paid to noncontrolling interest holders							(2)
Redemptions of preferred stock			0													0
Common stock shares outstanding at beginning of year (in shares) at Dec. 31, 2019	246,200,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under the DRPlus and 401(k) plan (in shares)	200,000
Shares issued for stock-based compensation (in shares)	500,000
Common stock shares outstanding at end of period (in shares) at Mar. 31, 2020	246,900,000
End of period at Mar. 31, 2020	$ 8,227	$ 2	5,695	2,404	(16)	$ (16)	142			$ 511	2,027	$ 80	1,721		$ 0	2,288	62	2,002
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per common share (in dollars per share)	$ 0.495
Shareholders' equity, end of year at Mar. 31, 2020								$ 8,085	4,339					4,352
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under forward sale agreement (in shares)	0
Beginning of year at Dec. 31, 2020	$ 9,080		6,179	2,757	(1)		142				2,518		2,101			2,652	62	2,252
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Settlement of forward sale agreement through common shares issuance			113
Shares issued under the DRPlus and 401(k) plan			12
Stock-based compensation activity			(9)
Capital contribution from parent									(113)		113			(40)		40
Net income	235								$ 48				48	$ 150				150
Net income attributable to Ameren common shareholders	233			233
Dividends on common stock				(140)
Preferred stock dividends													(1)					(1)
Change in deferred retirement benefit costs	1				1
Net income attributable to noncontrolling interest holders	$ (2)						2
Dividends paid to noncontrolling interest holders							(2)
Redemptions of preferred stock			13													13
Common stock shares outstanding at beginning of year (in shares) at Dec. 31, 2020	253,300,000								102,100,000					25,500,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under the DRPlus and 401(k) plan (in shares)	100,000
Shares issued for stock-based compensation (in shares)	500,000
Common stock shares outstanding at end of period (in shares) at Mar. 31, 2021	255,500,000								102,100,000					25,500,000
End of period at Mar. 31, 2021	$ 9,277	$ 3	$ 6,295	$ 2,850	$ 0	$ 0	$ 129			$ 511	$ 2,631	$ 80	$ 2,148		$ 0	$ 2,692	$ 49	$ 2,401
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per common share (in dollars per share)	$ 0.550
Shareholders' equity, end of year at Mar. 31, 2021	$ 9,148							$ 9,148	$ 5,370					$ 5,142
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under forward sale agreement (in shares)	1,600,000

Summary Of Significant Accounti

Summary Of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES General Ameren, headquartered in St. Louis, Missouri, is a public utility holding company whose primary assets are its equity interests in its subsidiaries. Ameren’s subsidiaries are separate, independent legal entities with separate businesses, assets, and liabilities. Dividends on Ameren’s common stock and the payment of expenses by Ameren depend on distributions made to it by its subsidiaries. Ameren’s principal subsidiaries are listed below. Ameren has other subsidiaries that conduct other activities, such as providing shared services. • Union Electric Company, doing business as Ameren Missouri, operates a rate-regulated electric generation, transmission, and distribution business and a rate-regulated natural gas distribution business in Missouri. • Ameren Illinois Company, doing business as Ameren Illinois, operates rate-regulated electric transmission, electric distribution, and natural gas distribution businesses in Illinois. • ATXI operates a FERC rate-regulated electric transmission business in the MISO. The COVID-19 pandemic continues to affect our results of operations, financial position, and liquidity, but we expect a gradual improvement in sales volumes in 2021, compared to 2020. In the first three months of 2021, our sales volumes were comparable to the same period in 2020, excluding the estimated effects of weather and customer energy-efficiency programs. However, we experienced 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. In general, restrictions on social activities and nonessential businesses implemented in our service territories in 2020 have been relaxed. However, certain restrictions remain in place that limit individual activities and the operation of nonessential businesses and additional restrictions may be imposed in the future. We continue to assess the impacts the COVID-19 pandemic is having on our businesses, including but not limited to impacts on our liquidity; demand for residential, commercial, and industrial electric and natural gas services; changes in deferred payment arrangements for customers; the timing and extent to which recovery of incremental costs incurred, net of savings, and forgone customer late fee revenues at Ameren Missouri is allowed by the MoPSC; changes in our ability to disconnect customers for nonpayment; bad debt expense; supply chain operations; the availability of our employees and contractors; counterparty credit; capital construction; infrastructure operations and maintenance; energy-efficiency programs; and pension valuations. In March 2021, the MoPSC approved accounting authority orders that allowed Ameren Missouri to accumulate $9 million of certain costs incurred related to the COVID-19 pandemic, net of savings, as well as forgone customer late fees and reconnection fee revenues from March 2020 to March 2021, for potential recovery in the current electric and natural gas service regulatory rate reviews. While the revenues from Ameren Illinois’ electric distribution business, residential and small nonresidential customers of Ameren Illinois’ natural gas distribution business, and Ameren Illinois’ and ATXI’s electric transmission businesses are decoupled from changes in sales volumes, earnings at Ameren Missouri and those associated with Ameren Illinois’ large nonresidential natural gas customers are exposed to such changes. Regarding uncollectible accounts receivable, Ameren Illinois’ electric distribution and natural gas distribution businesses have bad debt riders, which provide for recovery of bad debt write-offs, net of any subsequent recoveries. Ameren Missouri does not have a bad debt rider or tracker, and thus its earnings are exposed to increases in bad debt expense, absent regulatory relief. However, Ameren Missouri does not expect a material impact to earnings from increases in bad debt expense. As of March 31, 2021, accounts receivable balances that were 30 days or greater past due or that were a part of a deferred payment arrangement represented 27%, 19%, and 32%, or $137 million, $33 million, and $104 million, of Ameren’s, Ameren Missouri’s, and Ameren Illinois’ customer trade receivables before allowance for doubtful accounts, respectively. As of March 31, 2020, these percentages were 21%, 16%, and 25%, or $99 million, $29 million, and $70 million, for Ameren, Ameren Missouri, and Ameren Illinois, respectively. For information regarding Ameren Illinois’ suspension and subsequent reinstatement of customer disconnections and late fee charges for nonpayment and Ameren Missouri’s accounting authority orders related to the COVID-19 pandemic, see Note 2 – Rate and Regulatory Matters below. Ameren’s financial statements are prepared on a consolidated basis and therefore include the accounts of its majority-owned subsidiaries. All intercompany transactions have been eliminated. Ameren Missouri and Ameren Illinois have no subsidiaries. All tabular dollar amounts are in millions, unless otherwise indicated. Our accounting policies conform to GAAP. Our financial statements reflect all adjustments (which include normal, recurring adjustments) that are necessary, in our opinion, for a fair statement of our results. The preparation of financial statements in conformity with GAAP requires management to make certain estimates and assumptions. Such estimates and assumptions affect reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the dates of financial statements, and the reported amounts of revenues and expenses during the reported periods. Actual results could differ from those estimates. The results of operations of an interim period may not give a true indication of results that may be expected for a full year. These financial statements should be read in conjunction with the financial statements and accompanying notes included in the Form 10-K. Variable Interest Entities As of March 31, 2021, and December 31, 2020, Ameren had unconsolidated variable interests as a limited partner in various equity method investments, totaling $41 million and $37 million, respectively, included in “Other assets” on Ameren’s consolidated balance sheet. Ameren is not the primary beneficiary of these investments because it does not have the power to direct matters that most significantly affect the activities of these variable interest entities. As of March 31, 2021, the maximum exposure to loss related to these variable interests is limited to the investment in these partnerships of $41 million plus associated outstanding funding commitments of $33 million. Company-owned Life Insurance Ameren and Ameren Illinois have company-owned life insurance, which is recorded at the net cash surrender value. The net cash surrender value is the amount that can be realized under the insurance policies at the balance sheet date. As of March 31, 2021, the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $275 million (December 31, 2020 – $272 million) and $117 million (December 31, 2020 – $115 million), respectively, while total borrowings against the policies were $107 million (December 31, 2020 – $107 million) at both Ameren and Ameren Illinois. Ameren and Ameren Illinois have the right to offset the borrowings against the cash surrender value of the policies and, consequently, present the net asset in “Other assets” on their respective balance sheets.

Rate And Regulatory Matters

Rate And Regulatory Matters	3 Months Ended
	Mar. 31, 2021
Public Utilities, General Disclosures [Abstract]
RATE AND REGULATORY MATTERS	RATE AND REGULATORY MATTERS Below is a summary of updates to significant regulatory proceedings and related legal proceedings. See Note 2 – Rate and Regulatory Matters under Part II, Item 8, of the Form 10-K for additional information and a summary of our regulatory frameworks. We are unable to predict the ultimate outcome of these matters, the timing of final decisions of the various agencies and courts, or the impact on our results of operations, financial position, or liquidity. Missouri 2021 Electric Service Regulatory Rate Review In March 2021, Ameren Missouri filed a request with the MoPSC seeking approval to increase its annual revenues for electric service by $299 million. The electric rate increase request is based on a 9.9% ROE, a capital structure composed of 51.9% common equity, a rate base of $10.0 billion, and a test year ended December 31, 2020, with certain pro-forma adjustments expected through an anticipated true-up date of September 30, 2021. Ameren Missouri also requested the continued use of the FAC and trackers for pension and postretirement benefits, uncertain income tax positions, and certain excess deferred income taxes that the MoPSC previously authorized in earlier electric rate orders. Additionally, Ameren Missouri requested to recover certain estimated costs associated with the Meramec Energy Center, which is expected to be retired in 2022, over a five-year period. Ameren Missouri requested the use of a tracker for any variances between certain costs collected in customer rates associated with the Meramec Energy Center and actual costs incurred after the date new rates become effective, which would be considered for recovery or refund in a future electric regulatory rate review. The electric rate increase request reflects the following: • increased infrastructure investments made under Ameren Missouri’s Smart Energy Plan; • the impact of the transition to a cleaner generation portfolio, including advancing the retirement dates of the Sioux and Rush Island energy centers consistent with Ameren Missouri’s 2020 IRP and 700 MWs of wind generation investment for the High Prairie and Atchison renewable energy centers, which are mitigated by reductions resulting from the request to recover certain Meramec Energy Center costs over a five-year period and the associated tracker; • decreased weather-normalized customer sales volumes; and • increased pension and other post-retirement benefits and tax amortization expenses, partially offset by decreased other operations and maintenance expenses. The MoPSC proceeding relating to the proposed electric service rate changes will take place over a period of up to 11 months, with a decision by the MoPSC expected by February 2022 and new rates effective by March 2022. Ameren Missouri cannot predict the level of any electric service rate change the MoPSC may approve, whether the requested regulatory recovery mechanisms will be approved, or whether any rate change that may eventually be approved will be sufficient for Ameren Missouri to recover its costs and earn a reasonable return on its investments when the rate change goes into effect. Wind Generation Facility In January 2021, Ameren Missouri acquired an up-to 300-MW wind generation project located in northwestern Missouri and partially placed it in service as the Atchison Renewable Energy Center. As of the date of this filing, Ameren Missouri has placed approximately half of the project in service, representing a purchase price of approximately $250 million, including an immaterial amount of transaction costs. Ameren Missouri expects the remaining MWs of the project to be in service by the end of September 2021. The Atchison Renewable Energy Center will support Ameren Missouri’s compliance with the Missouri renewable energy standard. 2021 Natural Gas Delivery Service Regulatory Rate Review In March 2021, Ameren Missouri filed a request with the MoPSC seeking approval to increase its annual revenues for natural gas delivery service by $9 million. The natural gas rate increase request is based on a 9.8% ROE, a capital structure composed of 51.9% common equity, a rate base of $310 million, and a test year ended December 31, 2020, with certain pro-forma adjustments expected through an anticipated true-up date of September 30, 2021. The request includes the continued use of the PGA, ISRS, and DCA and trackers for pension and other postretirement benefits and certain excess deferred taxes that the MoPSC previously authorized in earlier natural gas rate orders. The MoPSC proceeding relating to the proposed natural gas delivery service rate changes will take place over a period of up to 11 months, with a decision by the MoPSC expected by February 2022 and new rates effective by March 2022. Ameren Missouri cannot predict the level of any natural gas delivery service rate change the MoPSC may approve, or whether any rate change that may eventually be approved will be sufficient for Ameren Missouri to recover its costs and earn a reasonable return on its investments when the rate change goes into effect. Accounting Authority Orders Related to COVID-19 Pandemic Costs In March 2021, the MoPSC issued orders approving nonunanimous stipulation and agreements related to Ameren Missouri’s electric and natural gas service accounting authority order requests. The orders allowed Ameren Missouri to accumulate $9 million of certain costs incurred related to the COVID-19 pandemic, net of cost savings, as well as forgone customer late fee and reconnection fee revenues from March 2020 to March 2021, for potential recovery in the electric and natural gas service regulatory rate reviews discussed above. As of March 31, 2021, Ameren Missouri deferred other operations and maintenance expenses of $5 million as a regulatory asset related to the accounting authority orders. If approved for recovery, Ameren Missouri would recognize the remaining $4 million associated with forgone customer late fee and reconnection fee revenue when billed to customers. Illinois Electric Distribution Service Rates In April 2021, Ameren Illinois filed its annual electric distribution service performance-based formula rate update with the ICC, requesting an increase of $64 million in its rates. This update reflects an increase to the annual performance-based formula rate based on 2020 actual costs, an increase to include the 2020 revenue requirement reconciliation adjustment, and an increase for the conclusion of the 2019 revenue requirement reconciliation adjustment, which will be fully refunded to customers in 2021, consistent with the ICC’s December 2020 annual update filing order. It also reflects an increase based on expected net plant additions for 2021. An ICC decision in this proceeding is expected by December 2021, with new rates effective January 2022. Electric Distribution Service Rate Reconciliation Tariff In March 2021, the ICC issued an order approving Ameren Illinois’ requested tariff to reconcile its electric distribution service revenue requirement for a period of up to two years after the final customer rate update under performance-based formula ratemaking. To utilize the reconciliation, the ICC-approved tariff requires Ameren Illinois to file a traditional regulatory rate review for its electric distribution service, which may be based on a future test year, by the end of March in the year following the last year in which an annual performance-based formula rate update was permitted. Pursuant to this order, and without legislative change or Ameren Illinois’ election to no longer use performance-based formula ratemaking, Ameren Illinois’ 2022 and 2023 revenues would reflect each year’s actual costs, year-end rate base, and a return at the applicable WACC, with the ROE based on the annual average of the monthly yields of the 30-year United States Treasury bonds plus 580 basis points. The revenue requirement adjustment will be collected from, or refunded to, customers within two years from the end of the reconciled year. Electric Energy Efficiency Plan In March 2021, Ameren Illinois filed with the ICC an energy-efficiency plan which includes annual investments in electric energy-efficiency programs up to approximately $100 million per year from 2022 through 2025. The ICC has the ability to reduce the amount of electric energy-efficiency savings goals in future plan program years if there are insufficient cost-effective programs available, which could reduce the investments in electric energy-efficiency programs. The electric energy-efficiency program investments and the return on those investments are collected from customers through a rider and are not included in the electric distribution service performance-based formula ratemaking framework. A decision by the ICC in this proceeding is expected by September 2021. QIP Reconciliation Order In March 2021, the ICC issued an order approving Ameren Illinois’ QIP reconciliation for 2018. The ICC also found that Ameren Illinois’ natural gas capital investments recovered under the QIP during 2018 were accurate and prudent. The ICC order effectively dismissed the Illinois Attorney General’s challenge with respect to 2018 capital investments. Service Disconnection Moratorium From March 2020 through March 2021, the ICC limited disconnection activities and late fees for customer nonpayment to varying degrees based on customer class. In March 2021, the ICC issued an order allowing Ameren Illinois to resume disconnection activities for all residential customers through a phased-in approach, which began in April 2021 for customers with the largest past due balances and will resume by June 2021 for all remaining residential customers. The March 2021 order also requires Ameren Illinois to offer deferred payment arrangements, extending to 18 months, to all residential customers through June 2021. In addition, the order requires Ameren Illinois to extend the financial assistance program established by a June 2020 ICC order through 2021. Ameren Illinois is allowed to recover up to $4 million in costs incurred during 2021 related to this financial assistance program. These costs will be deferred as regulatory assets and the portion associated with Ameren Illinois’ electric distribution business will be recovered through its bad debt rider and the portion associated with its natural gas distribution business will be recovered through a special purpose rider. Federal Transmission Formula Rate Revisions In February 2020, the MISO, on behalf of Ameren Missouri, Ameren Illinois, and ATXI, filed requests with the FERC to revise each company’s transmission formula rate calculations with respect to the calculation used for materials and supplies inventories included in rate base. In May 2020, the FERC issued orders approving the revisions prospectively. In addition, the FERC declined to order refunds for earlier periods, as requested by intervenors in Ameren Illinois’ filing, but directed its audit staff to review historical rate recovery in connection with an ongoing FERC audit. In June 2020, Ameren Missouri, Ameren Illinois, and ATXI filed requests for rehearing arguing, among other things, the revisions should be applied retrospectively to include the period January 1, 2019, to June 1, 2020, and that the FERC should not require refunds for periods prior to 2019. In July 2020, the FERC denied the rehearing requests without addressing the issues raised. In July 2020, Ameren Missouri, Ameren Illinois, and ATXI filed an appeal of the July 2020 rehearing denials to the United States Court of Appeals for the District of Columbia Circuit, which is under no deadline to address the appeal. In October 2020, the FERC issued an order reaffirming its May 2020 order and denying the arguments raised in the rehearing requests filed by Ameren Missouri, Ameren Illinois, and ATXI. Regardless of the outcome of the appeal, the impacts of the May 2020 and October 2020 orders are not expected to be material to Ameren’s, Ameren Missouri’s, or Ameren Illinois’ results of operations, financial position, or liquidity. In March 2021, the FERC issued an order related to an intervenor challenge to Ameren Illinois’ 2020 transmission formula rate update. As a result of this order, in March 2021, Ameren Illinois recorded a regulatory liability of $9 million, largely as a reduction of electric operating revenues, to reflect expected refunds, including interest, primarily related to the historical rate recovery of materials and supplies inventories included in rate base. In April 2021, Ameren Illinois filed a request for rehearing with the FERC regarding its March 2021 order. FERC Complaint Cases In November 2013, a customer group filed a complaint case with the FERC seeking a reduction in the allowed base ROE for FERC-regulated transmission rate base under the MISO tariff from 12.38% to 9.15%. In September 2016, the FERC issued an order in the November 2013 complaint case, which lowered the allowed base ROE to 10.32%, or a 10.82% total allowed ROE with the inclusion of a 50 basis point incentive adder for participation in an RTO, that was effective from late September 2016 forward. The September 2016 order also required refunds for the period November 2013 to February 2015, which were paid in 2017. In November 2019, the FERC issued an order addressing the November 2013 complaint case, which set the allowed base ROE at 9.88%, superseding the 10.32% previously ordered, and required refunds, with interest, for the periods November 2013 to February 2015 and from late September 2016 forward. In December 2019, the MISO transmission owners, including Ameren Missouri, Ameren Illinois, and ATXI, filed requests for rehearing with the FERC. In May 2020, the FERC issued an order addressing the requests for rehearing, which set the allowed base ROE at 10.02%, superseding the 9.88% previously ordered, and required refunds, with interest, for the periods November 2013 to February 2015 and from late September 2016 forward. In June 2020, various parties filed requests for rehearing with the FERC, challenging the new ROE methodology established by the May 2020 order. In July 2020, the FERC denied the rehearing requests without addressing the issues raised, and indicated it will address the requests for rehearing in a future order. Also in July 2020, Ameren Missouri, Ameren Illinois, and ATXI filed an appeal of the May 2020 order to the United States Court of Appeals for the District of Columbia Circuit challenging the refunds required for the period from September 2016 to May 2020. The court is under no deadline to address the appeal. As of March 31, 2021, Ameren and Ameren Illinois had recorded current regulatory liabilities of $17 million and $9 million, respectively, to reflect the expected refunds, including interest, associated with the allowed base ROE set by the May 2020 order in the November 2013 complaint case. The increase in the FERC-allowed base ROE resulting from the May 2020 order is not material to Ameren Missouri’s results of operations, financial position, or liquidity.

Short-Term Debt And Liquidity

Short-Term Debt And Liquidity	3 Months Ended
	Mar. 31, 2021
Debt Disclosure [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, in the case of Ameren Missouri and Ameren Illinois, short-term affiliate borrowings. See Note 4 – Short-term Debt and Liquidity under Part II, Item 8, in the Form 10-K for a description of our indebtedness provisions and other covenants as well as a description of money pool arrangements. Short-Term Borrowings The Missouri Credit Agreement and the Illinois Credit Agreement are available to support issuances under Ameren (parent)’s, Ameren Missouri’s, and Ameren Illinois’ commercial paper programs, respectively, subject to borrowing sublimits, and the issuance of letters of credit. As of March 31, 2021, based on commercial paper outstanding and letters of credit issued under the Credit Agreements, along with cash and cash equivalents, the net liquidity available to Ameren (parent), Ameren Missouri, and Ameren Illinois, collectively, was $1.4 billion. The Ameren Companies were in compliance with the covenants in their Credit Agreements as of March 31, 2021. As of March 31, 2021, the ratios of consolidated indebtedness to consolidated total capitalization, calculated in accordance with the provisions of the Credit Agreements, were 57%, 49%, and 46% for Ameren, Ameren Missouri, and Ameren Illinois, respectively. The following table presents commercial paper outstanding, net of issuance discounts, as of March 31, 2021, and December 31, 2020. There were no borrowings outstanding under the Credit Agreements as of March 31, 2021, or December 31, 2020. March 31, 2021 December 31, 2020 Ameren (parent) $ 362 $ 490 Ameren Missouri 204 — Ameren Illinois 323 — Ameren consolidated $ 889 $ 490 The following table summarizes the activity and relevant interest rates for Ameren (parent)’s, Ameren Missouri’s, and Ameren Illinois’ commercial paper issuances and borrowings under the Credit Agreements in the aggregate for the three months ended March 31, 2021 and 2020: Ameren Ameren Ameren Ameren 2021 Average daily amount outstanding $ 454 $ 99 $ 96 $ 649 Weighted-average interest rate 0.25 % 0.22 % 0.21 % 0.24 % Peak amount outstanding during period (a) $ 650 $ 206 $ 353 $ 916 Peak interest rate 0.33 % 0.25 % 0.25 % 0.33 % 2020 Average daily amount outstanding $ 157 $ 395 $ 76 $ 628 Weighted-average interest rate 1.94 % 1.86 % 1.99 % 1.89 % Peak amount outstanding during period (a) $ 425 $ 573 $ 150 $ 908 Peak interest rate 3.30 % 5.05 % (b) 3.40 % 5.05 % (b) (a) The timing of peak outstanding commercial paper issuances and borrowings under the Credit Agreements varies by company. Therefore, the sum of individual company peak amounts may not equal the Ameren consolidated peak for the period. (b) Ameren’s and Ameren Missouri’s peak interest rate was affected by temporary disruptions in the commercial paper market in the first quarter of 2020. Money Pools Ameren has money pool agreements with and among its subsidiaries to coordinate and provide for certain short-term cash and working capital requirements. The average interest rate for borrowings under the utility money pool for the three months ended March 31, 2021, was 0.22% (2020 – 1.93%). See Note 8 – Related-party Transactions for the amount of interest income and expense from the utility money pool arrangements recorded by Ameren Missouri and Ameren Illinois for the three months ended March 31, 2021 and 2020.

Long-Term Debt And Equity Finan

Long-Term Debt And Equity Financings	3 Months Ended
	Mar. 31, 2021
Long-Term Debt And Equity Financings [Abstract]
LONG-TERM DEBT AND EQUITY FINANCINGS	LONG-TERM DEBT AND EQUITY FINANCINGS Ameren For the three months ended March 31, 2021, Ameren issued a total of 0.1 million shares of common stock under its DRPlus and 401(k) plan, and received proceeds of $12 million. In addition, in the first quarter of 2021, Ameren issued 0.5 million shares of common stock valued at $33 million upon the vesting of stock-based compensation. 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 investments. In March 2021, Ameren (parent) issued $450 million of 1.75% senior unsecured notes due March 2028, with interest payable semiannually on March 15 and September 15 of each year, beginning September 15, 2021. Ameren received net proceeds of $447 million, which were used for general corporate purposes, including the repayment of short-term debt. Ameren Missouri Ameren Missouri received capital contributions totaling $113 million from Ameren (parent) during the three months ended March 31, 2021. Ameren Illinois In March 2021, Ameren Illinois redeemed its 6.625% and 7.75% series preferred stock at par for $12 million and $1 million, respectively. The preferred stock of Ameren Illinois is reflected in “Noncontrolling Interests” on Ameren’s consolidated balance sheet. Ameren Illinois received capital contributions totaling $40 million from Ameren (parent) during the three months ended March 31, 2021. Indenture Provisions and Other Covenants See Note 5 – Long-Term Debt and Equity Financings under Part II, Item 8, in the Form 10-K for a description of our indenture provisions and other covenants, as well as restrictions on the payment of dividends. At March 31, 2021, the Ameren Companies were in compliance with the provisions and covenants contained in their indentures and articles of incorporation, as applicable, and ATXI was in compliance with the provisions and covenants contained in its note purchase agreement. Off-balance-sheet Arrangements At March 31, 2021, none of the Ameren Companies had any significant off-balance-sheet financing arrangements, other than variable interest entities. See Note 1 – Summary of Significant Accounting Policies for further detail concerning variable interest entities.

Other Income and Expenses

Other Income and Expenses	3 Months Ended
	Mar. 31, 2021
Other Nonoperating Income (Expense) [Abstract]
OTHER INCOME AND EXPENSES	OTHER INCOME, NET The following table presents the components of “Other Income, Net” in the Ameren Companies’ statements of income for the three months ended March 31, 2021 and 2020: Three Months 2021 2020 Ameren: Allowance for equity funds used during construction $ 7 $ 4 Interest income on industrial development revenue bonds 6 6 Other interest income 1 1 Non-service cost components of net periodic benefit income (a) 34 23 Miscellaneous income 4 2 Donations (3) (13) (b) Miscellaneous expense (3) (2) Total Other Income, Net $ 46 $ 21 Ameren Missouri: Allowance for equity funds used during construction $ 4 $ 2 Interest income on industrial development revenue bonds 6 6 Non-service cost components of net periodic benefit income (a) 14 5 Miscellaneous income 1 1 Donations — (8) (b) Miscellaneous expense (2) (2) Total Other Income, Net $ 23 $ 4 Ameren Illinois: Allowance for equity funds used during construction $ 3 $ 2 Interest income 1 1 Non-service cost components of net periodic benefit income 14 13 Miscellaneous income — 1 Donations (3) (4) Miscellaneous expense (1) (2) Total Other Income, Net $ 14 $ 11 (a) For the three months ended March 31, 2021 and 2020, the non-service cost components of net periodic benefit income were adjusted by amounts deferred of less than $(1) million and $6 million, respectively, due to a regulatory tracking mechanism for the difference between the level of such costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. (b) Includes $8 million pursuant to Ameren Missouri’s March 2020 electric rate order. See Note 2 – Rate and Regulatory Matters under Part II, Item 8, in the Form 10-K for additional information.

Derivative Financial Instrument

Derivative Financial Instruments	3 Months Ended
	Mar. 31, 2021
Derivative Instruments and Hedging Activities Disclosure [Abstract]
DERIVATIVE FINANCIAL INSTRUMENTS	DERIVATIVE FINANCIAL INSTRUMENTS We use derivatives to manage the risk of changes in market prices for natural gas and power, as well as the risk of changes in rail transportation surcharges through fuel oil hedges. Such price fluctuations may cause the following: • an unrealized appreciation or depreciation of our contracted commitments to purchase or sell when purchase or sale prices under the commitments are compared with current commodity prices; • market values of natural gas inventories that differ from the cost of those commodities in inventory; • actual cash outlays for the purchase of these commodities that differ from anticipated cash outlays; and • actual off-system sales revenues that differ from anticipated revenues The derivatives that we use to hedge these risks are governed by our risk management policies for forward contracts, futures, options, and swaps. Our net positions are continually assessed within our structured hedging programs to determine whether new or offsetting transactions are required. The goal of the hedging program is generally to mitigate financial risks while ensuring that sufficient volumes are available to meet our requirements. Contracts we enter into as part of our risk management program may be settled financially, settled by physical delivery, or net settled with the counterparty. All contracts considered to be derivative instruments are required to be recorded on the balance sheet at their fair values, unless the NPNS exception applies. Many of our physical contracts, such as our purchased power contracts, qualify for the NPNS exception to derivative accounting rules. The revenue or expense on NPNS contracts is recognized at the contract price upon physical delivery. The following disclosures exclude NPNS contracts and other non-derivative commodity contracts that are accounted for under the accrual method of accounting. If we determine that a contract meets the definition of a derivative and is not eligible for the NPNS exception, we review the contract to determine whether the resulting gains or losses qualify for regulatory deferral. Derivative contracts that qualify for regulatory deferral are recorded at fair value, with changes in fair value recorded as regulatory assets or liabilities in the period in which the change occurs. We believe derivative losses and gains deferred as regulatory assets and liabilities are probable of recovery, or refund, through future rates charged to customers. Regulatory assets and liabilities are amortized to operating income as related losses and gains are reflected in rates charged to customers. Therefore, gains and losses on these derivatives have no effect on operating income. As of March 31, 2021, and December 31, 2020, all contracts that met the definition of a derivative and were not eligible for the NPNS exception received regulatory deferral. Cash flows for all derivative financial instruments are classified in cash flows from operating activities. The following table presents open gross commodity contract volumes by commodity type for derivative assets and liabilities as of March 31, 2021, and December 31, 2020. As of March 31, 2021, these contracts extended through October 2023, October 2026, and May 2032 for fuel oils, natural gas and power, respectively. Quantity (in millions, except as indicated) March 31, 2021 December 31, 2020 Commodity Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils (in gallons) 36 — 36 43 — 43 Natural gas (in mmbtu) 34 114 148 33 114 147 Power (in MWhs) 7 7 14 6 7 13 The following table presents the carrying value and balance sheet location of all derivative commodity contracts, none of which were designated as hedging instruments, as of March 31, 2021, and December 31, 2020: March 31, 2021 December 31, 2020 Balance Sheet Location Ameren Ameren Ameren Ameren Ameren Ameren Fuel oils Other current assets $ 2 $ — $ 2 $ 2 $ — $ 2 Other assets 3 — 3 — — — Natural gas Other current assets 2 10 12 1 8 9 Other assets 2 4 6 2 2 4 Power Other current assets 4 — 4 7 — 7 Other assets 1 — 1 — — — Total assets $ 14 $ 14 $ 28 $ 12 $ 10 $ 22 Fuel oils Other current liabilities $ 3 $ — $ 3 $ 7 $ — $ 7 Other deferred credits and liabilities — — — 2 — 2 Natural gas Other current liabilities — — — 1 1 2 Other deferred credits and liabilities — — — — 1 1 Power Other current liabilities 9 16 25 3 17 20 Other deferred credits and liabilities 5 169 174 8 181 189 Total liabilities $ 17 $ 185 $ 202 $ 21 $ 200 $ 221 The Ameren Companies elect to present the fair value amounts of derivative assets and derivative liabilities subject to an enforceable master netting arrangement or similar agreement at the gross amounts on the balance sheet. However, if the gross amounts recognized on the balance sheet were netted with derivative instruments and cash collateral received or posted, the net amounts would not be materially different from the gross amounts at March 31, 2021, and December 31, 2020. 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 March 31, 2021, if counterparty groups were to fail completely to perform on contracts, the Ameren Companies’ maximum exposure related to derivative assets would have been immaterial with or without consideration of the application of master netting arrangements or similar agreements and collateral held. Certain of our derivative instruments contain collateral provisions tied to the Ameren Companies’ credit ratings. If our credit ratings were downgraded below investment grade, or if a counterparty with reasonable grounds for uncertainty regarding our ability to satisfy an obligation requested adequate assurance of performance, additional collateral postings might be required. The additional collateral required is the net liability position allowed under master netting arrangements or similar agreements, assuming (1) the credit risk-related contingent features underlying these arrangements were triggered and (2) those counterparties with rights to do so requested collateral. As of March 31, 2021, 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

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2021
Fair Value Disclosures [Abstract]
Fair Value Disclosures	FAIR VALUE MEASUREMENTS Fair value is defined as the price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Fair value measurements are classified in three levels based on the fair value hierarchy as defined by GAAP. See Note 8 – Fair Value Measurements under Part II, Item 8, of the Form 10-K for information related to hierarchy levels and valuation techniques. We consider nonperformance risk in our valuation of derivative instruments by analyzing our own credit standing and the credit standing of our counterparties, and by considering any credit enhancements (e.g., collateral). Included in our valuation, and based on current market conditions, is a valuation adjustment for counterparty default derived from market data such as the price of credit default swaps, bond yields, and credit ratings. No material gains or losses related to valuation adjustments for counterparty default risk were recorded at Ameren, Ameren Missouri, or Ameren Illinois in the three months ended March 31, 2021 or 2020. At March 31, 2021, and December 31, 2020, the counterparty default risk valuation adjustment related to derivative contracts was immaterial for Ameren, Ameren Missouri, and Ameren Illinois. The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of March 31, 2021, and December 31, 2020: March 31, 2021 December 31, 2020 Level 1 Level 2 Level 3 Total Level 1 Level 2 Level 3 Total Assets: Ameren Missouri Derivative assets – commodity contracts: Fuel oils $ 4 $ — $ 1 $ 5 $ — $ — $ 2 $ 2 Natural gas — 4 — 4 — 3 — 3 Power 2 — 3 5 2 — 5 7 Total derivative assets – commodity contracts $ 6 $ 4 $ 4 $ 14 $ 2 $ 3 $ 7 $ 12 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 679 $ — $ — $ 679 $ 680 $ — $ — $ 680 Debt securities: U.S. Treasury and agency securities — 133 — 133 — 115 — 115 Corporate bonds — 128 — 128 — 115 — 115 Other — 63 — 63 — 67 — 67 Total nuclear decommissioning trust fund $ 679 $ 324 $ — $ 1,003 (a) $ 680 $ 297 $ — $ 977 (a) Total Ameren Missouri $ 685 $ 328 $ 4 $ 1,017 $ 682 $ 300 $ 7 $ 989 Ameren Illinois Derivative assets – commodity contracts: Natural gas $ — $ 10 $ 4 $ 14 $ — $ 6 $ 4 $ 10 Ameren Derivative assets – commodity contracts (b) $ 6 $ 14 $ 8 $ 28 $ 2 $ 9 $ 11 $ 22 Nuclear decommissioning trust fund (c) 679 324 — 1,003 (a) 680 297 — 977 (a) Total Ameren $ 685 $ 338 $ 8 $ 1,031 $ 682 $ 306 $ 11 $ 999 Liabilities: Ameren Missouri Derivative liabilities – commodity contracts: Fuel oils $ 1 $ — $ 2 $ 3 $ 6 $ — $ 3 $ 9 Natural gas — — — — — 1 — 1 Power 8 — 6 14 8 — 3 11 Total Ameren Missouri $ 9 $ — $ 8 $ 17 $ 14 $ 1 $ 6 $ 21 Ameren Illinois Derivative liabilities – commodity contracts: Natural gas $ — $ — $ — $ — $ — $ 1 $ 1 $ 2 Power — — 185 185 — — 198 198 Total Ameren Illinois $ — $ — $ 185 $ 185 $ — $ 1 $ 199 $ 200 Ameren Derivative liabilities – commodity contracts (b) $ 9 $ — $ 193 $ 202 $ 14 $ 2 $ 205 $ 221 (a) Balance excludes $7 million and $5 million of cash and cash equivalents, receivables, payables, and accrued income, net, for March 31, 2021, and December 31, 2020, respectively. (b) See the Ameren Missouri and Ameren Illinois sections of the table for a breakout of the fair value of Ameren’s derivative assets and liabilities by type of commodity. (c) See the Ameren Missouri section of the table for a breakout of the fair value of Ameren's nuclear decommissioning trust fund by investment type. Level 3 fuel oils and natural gas derivative contract assets and liabilities measured at fair value on a recurring basis were immaterial for all periods presented. The following table presents the fair value reconciliation of Level 3 power derivative contract assets and liabilities measured at fair value on a recurring basis for the three months ended March 31, 2021 and 2020: 2021 2020 Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren For the three months ended March 31: Beginning balance at January 1 $ 2 $ (198) $ (196) $ 13 $ (224) $ (211) Realized and unrealized gains/(losses) included in regulatory assets/liabilities (5) 9 4 11 (21) (10) Settlements — 4 4 (7) 4 (3) Ending balance at March 31 $ (3) $ (185) $ (188) $ 17 $ (241) $ (224) Change in unrealized gains/(losses) related to assets/liabilities held at March 31 $ (3) $ 9 $ 6 $ 10 $ (21) $ (11) 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 March 31, 2021, and December 31, 2020: Fair Value Weighted Average (b) Commodity Assets Liabilities Valuation Technique(s) Unobservable Input (a) Range 2021 Power (c) $ 3 $ (191) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) 23 – 40 29 Nodal basis ($/MWh) (5) – 0 (1) Trend rate (%) 2 – 5 3 2020 Power (c) $ 5 $ (201) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) 23 – 37 29 Nodal basis ($/MWh) (6) – 0 (2) Trend rate (%) 2 – 6 3 (a) Generally, significant increases (decreases) in these inputs in isolation would result in a significantly higher (lower) fair value measurement. (b) Unobservable inputs were weighted by relative fair value. (c) Valuations through 2029 use visible forward prices adjusted for nodal-to-hub basis differentials. Valuations beyond 2029 use a trend rate factor and are similarly adjusted for nodal-to-hub basis differentials. The following table sets forth the carrying amount and, by level within the fair value hierarchy, the fair value of financial assets and liabilities disclosed, but not recorded, at fair value as of March 31, 2021, and December 31, 2020: Carrying Fair Value Level 1 Level 2 Level 3 Total March 31, 2021 Ameren: Cash, cash equivalents, and restricted cash $ 172 $ 172 $ — $ — $ 172 Investments in industrial development revenue bonds (a) 256 — 256 — 256 Short-term debt 889 — 889 — 889 Long-term debt (including current portion) (a) 11,535 (b) — 12,233 507 (c) 12,740 Ameren Missouri: Cash, cash equivalents, and restricted cash $ 12 $ 12 $ — $ — $ 12 Investments in industrial development revenue bonds (a) 256 — 256 — 256 Short-term debt 204 — 204 — 204 Long-term debt (including current portion) (a) 5,104 (b) — 5,685 — 5,685 Ameren Illinois: Cash, cash equivalents, and restricted cash $ 149 $ 149 $ — $ — $ 149 Short-term debt 323 — 323 — 323 Long-term debt (including current portion) 3,947 (b) — 4,401 — 4,401 December 31, 2020 Ameren: Cash, cash equivalents, and restricted cash $ 301 $ 301 $ — $ — $ 301 Investments in industrial development revenue bonds (a) 256 — 256 — 256 Short-term debt 490 — 490 — 490 Long-term debt (including current portion) (a) 11,086 (b) — 12,778 537 (c) 13,315 Ameren Missouri: Cash, cash equivalents, and restricted cash $ 145 $ 145 $ — $ — $ 145 Advances to money pool 139 — 139 — 139 Investments in industrial development revenue bonds (a) 256 — 256 — 256 Long-term debt (including current portion) (a) 5,104 (b) — 6,160 — 6,160 Ameren Illinois: Cash, cash equivalents, and restricted cash $ 147 $ 147 $ — $ — $ 147 Borrowings from money pool 19 — 19 — 19 Long-term debt (including current portion) 3,946 (b) — 4,822 — 4,822 (a) Ameren and Ameren Missouri have investments in industrial development revenue bonds, classified as held-to-maturity and recorded in “Other Assets,” that are equal to the finance obligations for the Peno Creek and Audrain CT energy centers. As of March 31, 2021, and December 31, 2020, the carrying amount of both the investments in industrial development revenue bonds and the finance obligations approximated fair value. (b) Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $86 million, $36 million, and $36 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of March 31, 2021. Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $84 million, $36 million, and $36 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2020. (c) The Level 3 fair value amount consists of ATXI’s senior unsecured notes.

Related Party Transactions

Related Party Transactions	3 Months Ended
	Mar. 31, 2021
Related Party Transactions [Abstract]
RELATED PARTY TRANSACTIONS	RELATED-PARTY TRANSACTIONS In the ordinary course of business, Ameren Missouri and Ameren Illinois have engaged in, and may in the future engage in, affiliate transactions. These transactions primarily consist of natural gas and power purchases and sales, services received or rendered, and borrowings and lendings. Transactions between Ameren’s subsidiaries are reported as affiliate transactions on their individual financial statements, but those transactions are eliminated in consolidation for Ameren’s consolidated financial statements. For a discussion of material related-party agreements and money pool arrangements, see Note 13 – Related-party Transactions and Note 4 – Short-term Debt and Liquidity under Part II, Item 8, of the Form 10-K. For information Ameren Missouri’s and Ameren Illinois’ capital contributions, see Note 4 – Long-term Debt and Equity Financings. Electric Power Supply Agreement In April 2021, Ameren Illinois conducted a procurement event, administered by the IPA, to purchase energy products. Ameren Missouri was among the winning suppliers in this event. As a result, in April 2021, Ameren Missouri and Ameren Illinois entered into an energy product agreement by which Ameren Missouri agreed to sell and Ameren Illinois agreed to purchase, 33,600 MWhs at an average price of $34 per MWh during the period of July 2022 through November 2022. Tax Allocation Agreement See Note 1 – Summary of Significant Accounting Policies under Part II, Item 8, of the Form 10-K for a discussion of the tax allocation agreement. The following table presents the affiliate balances related to income taxes for Ameren Missouri and Ameren Illinois as of March 31, 2021, and December 31, 2020: March 31, 2021 December 31, 2020 Ameren Missouri Ameren Illinois Ameren Missouri Ameren Illinois Income taxes payable to parent (a) $ — $ 6 $ — $ 6 Income taxes receivable from parent (b) 14 18 9 15 (a) Included in “Accounts payable – affiliates” on the balance sheet. (b) Included in “Accounts receivable – affiliates” on the balance sheet. Effects of Related-party Transactions on the Statement of Income The following table presents the effect on Ameren Missouri and Ameren Illinois of related-party transactions for the three months ended March 31, 2021 and 2020: Three Months Agreement Income Statement Ameren Ameren Ameren Missouri power supply Operating Revenues 2021 $ 2 $ (a) agreements with Ameren Illinois 2020 3 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2021 $ 7 $ (b) rent and facility services 2020 7 1 Ameren Missouri and Ameren Illinois miscellaneous Operating Revenues 2021 $ (b) $ (b) support services and services provided to ATXI 2020 (b) (b) Total Operating Revenues 2021 $ 9 $ (b) 2020 10 1 Ameren Illinois power supply Purchased Power 2021 $ (a) $ 2 agreements with Ameren Missouri 2020 (a) 3 Ameren Missouri and Ameren Illinois Purchased Power 2021 $ 1 $ (b) transmission services from ATXI 2020 (a) (b) Total Purchased Power 2021 $ 1 $ 2 2020 (a) 3 Ameren Missouri and Ameren Illinois Other Operations and Maintenance 2021 $ (b) $ 1 rent and facility services 2020 (b) 1 Ameren Services support services Other Operations and Maintenance 2021 $ 35 $ 33 agreement 2020 35 33 Total Other Operations and 2021 $ 35 $ 34 Maintenance 2020 35 34 Money pool borrowings (advances) (Interest Charges)/Other Income, Net 2021 $ (b) $ (b) 2020 (b) (b) (a) Not applicable. (b) Amount less than $1 million.

Commitments And Contingencies

Commitments And Contingencies	3 Months Ended
	Mar. 31, 2021
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	COMMITMENTS AND CONTINGENCIES We are involved in legal, tax, and regulatory proceedings before various courts, regulatory commissions, authorities, and governmental agencies with respect to matters that arise in the ordinary course of business, some of which involve substantial amounts of money. We believe that the final disposition of these proceedings, except as otherwise disclosed in the notes to our financial statements in this report and in the Form 10-K, will not have a material adverse effect on our results of operations, financial position, or liquidity. Reference is made to Note 1 – Summary of Significant Accounting Policies, Note 2 – Rate and Regulatory Matters, Note 9 – Callaway Energy Center, Note 13 – Related-party Transactions, and Note 14 – Commitments and Contingencies under Part II, Item 8, of the Form 10-K. See also Note 1 – Summary of Significant Accounting Policies, Note 2 – Rate and Regulatory Matters, Note 8 – Related-party Transactions, and Note 10 – Callaway Energy Center of this report. Environmental Matters Our electric and gas generation, transmission, distribution and gas storage operations must comply with a variety of environmental statutory and regulatory requirements, including permitting programs implemented via federal, state, and local authorities. Depending upon the specific business activity of the facility, such laws address air emissions; discharges to water bodies; the storage, handling, and disposal of hazardous substances and waste materials; siting and land use requirements; and potential ecological impacts. Complex and lengthy processes are required to obtain and renew approvals, permits, and licenses for new, existing, or modified facilities. Additionally, the use and handling of various chemicals or hazardous materials require release prevention plans and emergency response procedures. We employ dedicated personnel knowledgeable in environmental matters to oversee our business activities’ compliance with regulatory requirements. Environmental regulations have a significant impact on the electric utility industry and compliance with these regulations could be costly for Ameren Missouri, which operates coal-fired power plants. Clean Air Act regulations that apply to the electric utility industry include the NSPS, the CSAPR, the MATS, and the National Ambient Air Quality Standards, which are subject to periodic review for certain pollutants. Collectively, these regulations cover a variety of pollutants, such as SO 2 , particulate matter, NOx, mercury, toxic metals, and acid gases, and CO 2 emissions from new power plants. Clean Water Act regulations applicable to coal-fired power plants govern both intake and discharges of water 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. Depending upon the scope of modifications ultimately required by state regulators, these capital expenditures could be significant. The management and disposal of coal ash is regulated under the Resource Conservation and Recovery Act and the CCR rule and requires 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. Ameren Missouri complies with current CSAPR requirements by minimizing emissions through the use of low-sulfur coal, operation of two scrubbers at its Sioux Energy Center, and optimization of other existing air pollution control equipment. Ameren Missouri could incur additional costs to lower its emissions at one or more of its energy centers to comply additional CSAPR requirements in future years. These additional costs for compliance are expected to be recovered from customers through the FAC or higher base rates. CO 2 Emissions Standards The EPA’s Affordable Clean Energy Rule repealed the Clean Power Plan and replaced it with a new rule that established emission guidelines for states to follow in developing plans to limit 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. The United States Supreme Court has extended the deadline for seeking review, and a decision on whether the Supreme Court will review the circuit court's ruling could occur in fall 2021. Regardless of the outcome of those legal challenges, the EPA is likely to develop new regulations to address carbon emissions from coal and natural gas electric generating units. At this time, Ameren Missouri cannot predict the outcome of legal challenges to the vacated Affordable Clean Energy Rule or future rulemakings. As such, the impact on the results of operations, financial position, and liquidity of Ameren and Ameren Missouri is uncertain. NSR and Clean Air Litigation In January 2011, the 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 as the Department of Justice previously dismissed claims for penalties. The district court stayed implementation of the majority of the requirements of its order while the case is under appeal to the United States Court of Appeals for the Eighth Circuit. In December 2020, the court of appeals heard oral arguments presented by the parties. The court is under no deadline to issue a ruling in this case and Ameren Missouri is unable to predict the ultimate resolution of this matter. Ameren Missouri expects a ruling by the court of appeals 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 implemented by state regulators 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 in 2020 completed construction of wastewater treatment facilities at three of its four coal-fired energy centers. The Meramec Energy Center is scheduled to close permanently in 2022, and as a result, does not require new wastewater and dry ash handling systems. Estimated capital expenditures to complete these projects are included in the CCR management compliance plan, discussed below. CCR Management The EPA’s CCR rule establishes requirements for the management and disposal of CCR from coal-fired power plants and will result in the closure of surface impoundments at Ameren Missouri’s energy centers. Ameren Missouri is in the process of closing surface impoundments at all of its facilities, and is scheduled to complete the last of such closures in 2023. The EPA has issued a series of revisions to the CCR rule; however, none of those revisions is expected to materially impact our closure schedule. Ameren and Ameren Missouri have AROs of $103 million recorded on their respective balance sheets as of March 31, 2021, associated with CCR storage facilities. Ameren Missouri estimates it will need to make capital expenditures of $75 million to $100 million from 2021 through 2025 to implement its CCR management compliance plan, which includes installation of groundwater monitoring equipment and water treatment facilities. Remediation The Ameren Companies are involved in a number of remediation actions to clean up sites impacted by the use or disposal of materials containing hazardous substances. Federal and state laws can require responsible parties to fund remediation regardless of their degree of fault, the legality of original disposal, or the ownership of a disposal site. As of March 31, 2021, Ameren Illinois has remediated the majority of the 44 former MGP sites in Illinois and could substantially conclude remediation efforts at the remaining sites by 2023. The ICC allows Ameren Illinois to recover such remediation and related litigation costs from its electric and natural gas utility customers through environmental cost riders that are subject to annual prudence review by the ICC. As of March 31, 2021, Ameren Illinois estimated the remaining obligation related to these former MGP sites at $90 million to $150 million. Ameren and Ameren Illinois recorded a liability of $90 million to represent the estimated minimum obligation for these sites, as no other amount within the range was a better estimate. The scope of the remediation activities at these former MGP sites may increase as remediation efforts continue. Considerable uncertainty remains in these estimates because many site-specific factors can influence the ultimate actual costs, including unanticipated underground structures, the degree to which groundwater is encountered, regulatory changes, local ordinances, and site accessibility. The actual costs and timing of completion may vary substantially from these estimates. Our operations or those of our predecessor companies involve the use of, disposal of, and, in appropriate circumstances, the cleanup of substances regulated under environmental laws. We are unable to determine whether such historical practices will result in future environmental commitments or will affect our results of operations, financial position, or liquidity.

Callaway Energy Center

Callaway Energy Center	3 Months Ended
	Mar. 31, 2021
Nuclear Waste Matters [Abstract]
CALLAWAY ENERGY CENTER	CALLAWAY ENERGY CENTER See Note 9 – Callaway Energy Center under Part II, Item 8, of the Form 10-K for information regarding spent nuclear fuel recovery, recovery of decommissioning costs, and the nuclear decommissioning trust fund. The fair value of the trust fund for Ameren Missouri’s Callaway Energy Center is reported as “Nuclear decommissioning trust fund” in Ameren’s and Ameren Missouri’s balance sheets. This amount is legally restricted and may be used only to fund the costs of nuclear decommissioning. Changes in the fair value of the trust fund are recorded as an increase or decrease to the nuclear decommissioning trust fund, with an offsetting adjustment to the related regulatory liability. Ameren and Ameren Missouri have recorded an ARO for the Callaway Energy Center decommissioning costs at fair value, which represents the present value of estimated future cash outflows. Annual decommissioning costs of $7 million are included in the costs used to establish electric rates for Ameren Missouri’s customers. Every three years, the MoPSC requires Ameren Missouri to file an updated cost study and funding analysis for decommissioning its Callaway Energy Center. An updated cost study and funding analysis was filed with the MoPSC in November 2020 and reflected within the ARO. In February 2021, the MoPSC approved no change in electric rates for decommissioning costs based on Ameren Missouri’s updated cost study funding analysis. See Note 13 – Supplemental Information for more information on Ameren Missouri’s AROs. Maintenance Outage During its return to full power after the completion of the last refueling and maintenance outage in late December 2020, the Callaway Energy Center experienced a non-nuclear operating issue related to its generator. A thorough investigation of this matter was conducted. Work continues to replace certain key components of the generator in order to return the energy center to service. Ameren Missouri expects generator repairs of approximately $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 July 2021. In April 2021, Ameren Missouri’s insurance claims were accepted by NEIL, which are expected to cover a significant portion of the capital expenditures and replacement power costs. Replacement power costs of up to $4.5 million weekly are covered by insurance after March 17, 2021. Insurance recoveries related to replacement power costs will be reflected in electric operating revenues and included in net energy costs under the FAC. Insurance recoveries related to the capital expenditures will be reflected as a reduction to property, plant, and equipment. Ameren Missouri continues to review other legal remedies available. Insurance The following table presents insurance coverage at Ameren Missouri’s Callaway Energy Center at April 1, 2021: 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) 13,210 (a) 138 (b) $ 13,660 (c) $ 138 Property damage: NEIL and EMANI April 1, 2021 $ 3,200 (d) $ 25 (e) Replacement power: NEIL April 1, 2021 $ 490 (f) $ 7 (e) (a) Provided through mandatory participation in an industrywide retrospective premium assessment program. The maximum coverage available is dependent on the number of United States commercial reactors participating in the program. (b) Retrospective premium under the Price-Anderson Act. This is subject to retrospective assessment with respect to a covered loss in excess of $450 million in the event of an incident at any licensed United States commercial reactor, payable at $21 million per year. (c) Limit of liability for each incident under the Price-Anderson liability provisions of the Atomic Energy Act of 1954, as amended. This limit is subject to change to account for the effects of inflation and changes in the number of licensed power reactors. (d) NEIL provides $2.7 billion in property damage, stabilization, decontamination, and premature decommissioning insurance for radiation events and $2.3 billion in property damage insurance for nonradiation events. EMANI provides $490 million in property damage insurance for both radiation and nonradiation events. (e) All NEIL-insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL. (f) 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. If losses from a nuclear incident at the Callaway Energy Center exceed the limits of, or are not covered by insurance, or if coverage is unavailable, Ameren Missouri is at risk for any uninsured losses. If a serious nuclear incident were to occur, it could have a material adverse effect on Ameren’s and Ameren Missouri’s results of operations, financial position, or liquidity.

Retirement Benefits

Retirement Benefits	3 Months Ended
	Mar. 31, 2021
Retirement Benefits [Abstract]
RETIREMENT BENEFITS	RETIREMENT BENEFITS The following table presents the components of the net periodic benefit cost (income) incurred for Ameren’s pension and postretirement benefit plans for the three months ended March 31, 2021 and 2020: Pension Benefits Postretirement Benefits Three Months Three Months 2021 2020 2021 2020 Service cost (a) $ 33 $ 27 $ 6 $ 4 Non-service cost components: Interest cost 38 43 8 10 Expected return on plan assets (75) (73) (20) (20) Amortization of: Prior service benefit — — (1) (1) Actuarial loss (gain) 17 14 (1) (2) Total non-service cost components (b) $ (20) $ (16) $ (14) $ (13) Net periodic benefit cost (income) $ 13 $ 11 $ (8) $ (9) (a) Service cost, net of capitalization, is reflected in “Operating Expenses – Other operations and maintenance” on Ameren’s statement of income. (b) Non-service cost components are reflected in “Other Income, Net” on Ameren’s statement of income. See Note 5 – Other Income, Net, for additional information. Ameren Missouri and Ameren Illinois are responsible for their respective share of Ameren’s pension and other postretirement costs. The following table presents the respective share of net periodic pension and other postretirement benefit costs (income) incurred for the three months ended March 31, 2021 and 2020: Pension Benefits Postretirement Benefits Three Months Three Months 2021 2020 2021 2020 Ameren Missouri (a) $ 6 $ 4 $ (1) $ (1) Ameren Illinois 8 7 (7) (8) Other (1) — — — Ameren (a) $ 13 $ 11 $ (8) $ (9) (a) Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. Funding Based on its assumptions at March 31, 2021, its investment performance in 2021, and its pension funding policy, the estimated aggregate contributions through 2025 has not changed from the $60 million expected aggregate contributions at December 31, 2020.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2021
Income Tax Disclosure [Abstract]
Income Taxes	INCOME TAXES The following table presents a reconciliation of the federal statutory corporate income tax rate to the effective income tax rate for the three months ended March 31, 2021 and 2020: Ameren Ameren Missouri Ameren Illinois 2021 2020 2021 2020 2021 2020 Three Months Federal statutory corporate income tax rate: 21% 21% 21% 21% 21% 21% Increases (decreases) from: Amortization of deferred investment tax credit (1) — (1) (1) — — Amortization of excess deferred taxes (9) (9) (17) (15) (2) (3) Depreciation differences — (1) 1 — (1) (1) Renewable and other tax credits (6) — (11) — — — State tax 6 6 3 3 7 7 Stock-based compensation (1) (5) — — — — Effective income tax rate 10% 12% (4)% 8% 25% 24%

Supplemental Information

Supplemental Information	3 Months Ended
	Mar. 31, 2021
Supplemental Information [Abstract]
Supplemental Information	SUPPLEMENTAL INFORMATION Cash, Cash Equivalents, and Restricted Cash The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets and the statements of cash flows as of March 31, 2021, and December 31, 2020: March 31, 2021 December 31, 2020 Ameren Ameren Ameren Ameren Ameren Ameren Cash and cash equivalents $ 6 $ 1 $ — $ 139 $ 136 $ — Restricted cash 134 — 125 17 — 6 Restricted cash included in “Other current assets” — 3 — — 5 — Restricted cash included in “Other assets” 24 — 24 141 — 141 Restricted cash included in “Nuclear decommissioning trust fund” 8 8 — 4 4 — Total cash, cash equivalents, and restricted cash $ 172 $ 12 $ 149 $ 301 $ 145 $ 147 Restricted cash included in “Other current assets” primarily represents funds held by an irrevocable Voluntary Employee Beneficiary Association (VEBA) trust, which provides health care benefits for active employees. As of December 31, 2020, restricted cash included in “Other assets” on Ameren’s and Ameren Illinois’ balance sheets primarily represents amounts collected under a cost recovery rider restricted for use in the procurement of renewable energy credits and amounts in a trust fund restricted for the use of funding certain asbestos-related claims. As of March 31, 2021, the amounts collected under a cost recovery rider restricted for use in Ameren Illinois’ procurement of renewable energy credits was reclassified to current as the amount is expected to be refunded to customers within a year. Accounts Receivable “Accounts receivable – trade” on Ameren’s and Ameren Illinois’ balance sheets include certain receivables purchased at a discount from alternative retail electric suppliers that elect to participate in the utility consolidated billing program. At March 31, 2021, and December 31, 2020, “Other current liabilities” on Ameren’s and Ameren Illinois’ balance sheets included payables for purchased receivables of $31 million and $28 million, respectively. The following table provides a reconciliation of the beginning and ending amount of the allowance for doubtful accounts for the three months ended March 31, 2021 and 2020: Three Months 2021 2020 Ameren: Beginning of period $ 50 $ 17 Bad debt expense 4 3 Net write-offs (7) (1) End of period $ 47 $ 19 Ameren Missouri: Beginning of period $ 16 $ 7 Bad debt expense 1 2 Net write-offs (2) (1) End of period $ 15 $ 8 Ameren Illinois: (a) Beginning of Period $ 34 $ 10 Bad debt expense 3 1 Net write-offs (5) — End of Period $ 32 $ 11 (a) Ameren Illinois has rate-adjustment mechanisms that allow it to recover the difference between its actual net bad debt write-offs under GAAP, including those associated with receivables purchased from alternative retail electric suppliers, and the amount of net bad debt write-offs included in its base rates. Net write-offs increased for the three months ended March 31, 2021, compared with the year-ago period, due to the resumption of disconnection activities for nonpayment. See Note 2 – Rate and Regulatory Matters for additional information. Supplemental Cash Flow Information The following table provides noncash financing and investing activity excluded from the statements of cash flows for the three months ended March 31, 2021 and 2020: March 31, 2021 March 31, 2020 Ameren Ameren Ameren Ameren Ameren Ameren Investing Accrued capital expenditures, including wind generation expenditures $ 271 $ 141 $ 139 $ 235 $ 97 $ 127 Accrued nuclear fuel expenditures — — — 7 7 — Net realized and unrealized gain (loss) – nuclear decommissioning trust fund 22 22 — (111) (111) — Financing Issuance of common stock for stock-based compensation 33 — — 38 — — Asset Retirement Obligations The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the three months ended March 31, 2021: Ameren Ameren Ameren Balance at December 31, 2020 $ 751 $ 5 (a) $ 756 (b) Liabilities incurred (c) 10 — 10 Liabilities settled (3) — (3) Accretion 8 (d) — 8 (d) Change in estimates (7) (e) — (7) (e) Balance at March 31, 2021 $ 759 $ 5 (a) $ 764 (b) (a) Included in “Other deferred credits and liabilities” on the balance sheet. (b) Balance included $59 million and $60 million in “Other current liabilities” on the balance sheet as of March 31, 2021, and December 31, 2020, respectively. (c) In the first quarter of 2021, Ameren Missouri recorded an ARO related to the decommissioning for the Atchison Renewable Energy Center. (d) Accretion expense attributable to Ameren Missouri was recorded as a decrease to regulatory liabilities. (e) Ameren Missouri changed its fair value estimate primarily due to a decrease in the cost estimate for closure of certain CCR storage facilities. Stock-based Compensation On January 1, 2021, Ameren granted 293,058 performance share units with a grant date fair value of $25 million and 125,562 restricted share units with a grant date fair value of $10 million. Awards vest approximately 38 months after the grant date or on a pro-rata basis upon death or eligible retirement. The performance share units vest based on the achievement of certain specified market performance measures (251,177 performance share units) or based on the achievement of renewable generation and energy storage installation targets (41,881 performance share units). The exact number of shares issued pursuant to a performance share unit varies from 0% to 200% of the target award, depending on actual company performance relative to the performance goals. For the three months ended March 31, 2021 and 2020, excess tax benefits associated with the settlement of stock-based compensation awards reduced income tax expense by $5 million and $8 million, respectively. Deferred Compensation As of both March 31, 2021, and December 31, 2020, the present value of benefits to be paid for deferred compensation obligations was $90 million, which was primarily reflected in “Other deferred credits and liabilities” on Ameren's consolidated balance sheet. Operating Revenues As of March 31, 2021 and 2020, our remaining performance obligations for contracts with a term greater than one year were immaterial. The Ameren Companies elected not to disclose the aggregate amount of the transaction price allocated to the performance obligations that are unsatisfied as of the end of the reporting period for contracts with an initial expected term of one year or less. See Note 14 – Segment Information for disaggregated revenue information. Excise Taxes Ameren Missouri and Ameren Illinois collect from their customers excise taxes, including municipal and state excise taxes and gross receipts taxes that are levied on the sale or distribution of natural gas and electricity. The following table presents the excise taxes recorded on a gross basis in “Operating Revenues – Electric,” “Operating Revenues – Natural gas” and “Operating Expenses – Taxes other than income taxes” on the statements of income for the three months ended March 31, 2021 and 2020: Three Months 2021 2020 Ameren Missouri $ 31 $ 30 Ameren Illinois 39 35 Ameren $ 70 $ 65 Earnings per Share The following table reconciles the basic weighted-average number of common shares outstanding to the diluted weighted-average number of common shares outstanding for the three months ended March 31, 2021 and 2020: Three Months 2021 2020 Weighted-average Common Shares Outstanding – Basic 254.4 246.4 Assumed settlement of performance share units and restricted stock units 1.5 1.1 Dilutive effect of forward sale agreement — 0.6 Weighted-average Common Shares Outstanding – Diluted (a) 255.9 248.1 (a) There were no potentially dilutive securities excluded from the earnings per diluted share calculations for the three months ended March 31, 2021 and 2020.

Segment Information

Segment Information	3 Months Ended
	Mar. 31, 2021
Segment Reporting [Abstract]
SEGMENT INFORMATION	SEGMENT INFORMATION The following tables present revenues, net income (loss) attributable to common shareholders, and capital expenditures by segment at Ameren and Ameren Illinois for the three months ended March 31, 2021 and 2020. Ameren, Ameren Missouri, and Ameren Illinois management review segment capital expenditure information rather than any individual or total asset amount. For additional information about our segments, see Note 16 – Segment Information under Part II, Item 8, of the Form 10-K. Ameren Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Other Intersegment Eliminations Ameren Three Months 2021: External revenues $ 695 $ 411 $ 347 $ 113 $ — $ — $ 1,566 Intersegment revenues 9 — — 17 — (26) — Net income attributable to Ameren common shareholders 47 46 75 47 (a) 18 — 233 Capital expenditures 534 (b) 157 48 141 1 6 887 (b) Three Months 2020: External revenues $ 670 $ 389 $ 271 $ 110 $ — $ — $ 1,440 Intersegment revenues 10 1 — 13 — (24) — Net income (loss) attributable to Ameren common shareholders (10) 37 55 47 (a) 17 — 146 Capital expenditures 278 123 61 170 3 1 636 (a) Ameren Transmission earnings reflect an allocation of financing costs from Ameren (parent). (b) Includes $193 million at Ameren and Ameren Missouri for wind generation expenditures for the three months ended March 31, 2021. Ameren Illinois Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Ameren Illinois Three Months 2021: External revenues $ 411 $ 347 $ 65 $ — $ 823 Intersegment revenues — — 16 (16) — Net income available to common shareholder 46 75 28 — 149 Capital expenditures 157 48 132 — 337 Three Months 2020: External revenues $ 390 $ 271 $ 62 $ — $ 723 Intersegment revenues — — 12 (12) — Net income available to common shareholder 37 55 28 — 120 Capital expenditures 123 61 140 — 324 The following tables present disaggregated revenues by segment at Ameren and Ameren Illinois for the three months ended March 31, 2021 and 2020. Economic factors affect the nature, timing, amount, and uncertainty of revenues and cash flows in a similar manner across customer classes. Revenues from alternative revenue programs have a similar distribution among customer classes as revenues from contracts with customers. Other revenues not associated with contracts with customers are presented in the Other customer classification, along with electric transmission and off-system revenues. Ameren Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Intersegment Eliminations Ameren Three Months 2021: Residential $ 312 $ 229 $ — $ — $ — $ 541 Commercial 216 132 — — — 348 Industrial 52 34 — — — 86 Other 61 16 — 130 (26) 181 Total electric revenues $ 641 $ 411 $ — $ 130 $ (26) $ 1,156 Residential $ 34 $ — $ 251 $ — $ — $ 285 Commercial 15 — 64 — — 79 Industrial 1 — 14 — — 15 Other 13 — 18 — — 31 Total natural gas revenues $ 63 $ — $ 347 $ — $ — $ 410 Total revenues (a) $ 704 $ 411 $ 347 $ 130 $ (26) $ 1,566 Three Months 2020: Residential $ 297 $ 220 $ — $ — $ — $ 517 Commercial 221 126 — — — 347 Industrial 53 35 — — — 88 Other 60 9 — 123 (24) 168 Total electric revenues $ 631 $ 390 $ — $ 123 $ (24) $ 1,120 Residential $ 33 $ — $ 213 $ — $ — $ 246 Commercial 13 — 54 — — 67 Industrial 1 — 3 — — 4 Other 2 — 1 — — 3 Total natural gas revenues $ 49 $ — $ 271 $ — $ — $ 320 Total revenues (a) $ 680 $ 390 $ 271 $ 123 $ (24) $ 1,440 (a) The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the three months ended March 31, 2021 and 2020: Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Ameren Three Months 2021: Revenues from alternative revenue programs $ (10) $ 61 $ 3 $ (1) $ 53 Other revenues not from contracts with customers (2) 3 1 — 2 Three Months 2020: Revenues from alternative revenue programs $ (3) $ 46 $ 11 $ 12 $ 66 Other revenues not from contracts with customers 8 1 1 — 10 Ameren Illinois Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Ameren Illinois Three Months 2021: Residential $ 229 $ 251 $ — $ — $ 480 Commercial 132 64 — — 196 Industrial 34 14 — — 48 Other 16 18 81 (16) 99 Total revenues (a) $ 411 $ 347 $ 81 $ (16) $ 823 Three Months 2020: Residential $ 220 $ 213 $ — $ — $ 433 Commercial 126 54 — — 180 Industrial 35 3 — — 38 Other 9 1 74 (12) 72 Total revenues (a) $ 390 $ 271 $ 74 $ (12) $ 723 (a) The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the Ameren Illinois segments for the three months ended March 31, 2021 and 2020: Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Ameren Illinois Three Months 2021: Revenues from alternative revenue programs $ 61 $ 3 $ (1) $ 63 Other revenues not from contracts with customers 3 1 — 4 Three Months 2020: Revenues from alternative revenue programs $ 46 $ 11 $ 10 $ 67 Other revenues not from contracts with customers 1 1 — 2

Summary Of Significant Accoun_2

Summary Of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Nature of Operations	Ameren, headquartered in St. Louis, Missouri, is a public utility holding company whose primary assets are its equity interests in its subsidiaries. Ameren’s subsidiaries are separate, independent legal entities with separate businesses, assets, and liabilities. Dividends on Ameren’s common stock and the payment of expenses by Ameren depend on distributions made to it by its subsidiaries. Ameren’s principal subsidiaries are listed below. Ameren has other subsidiaries that conduct other activities, such as providing shared services. • Union Electric Company, doing business as Ameren Missouri, operates a rate-regulated electric generation, transmission, and distribution business and a rate-regulated natural gas distribution business in Missouri. • Ameren Illinois Company, doing business as Ameren Illinois, operates rate-regulated electric transmission, electric distribution, and natural gas distribution businesses in Illinois. • ATXI operates a FERC rate-regulated electric transmission business in the MISO.
Consolidation	Ameren’s financial statements are prepared on a consolidated basis and therefore include the accounts of its majority-owned subsidiaries. All intercompany transactions have been eliminated. Ameren Missouri and Ameren Illinois have no subsidiaries. All tabular dollar amounts are in millions, unless otherwise indicated.Our accounting policies conform to GAAP. Our financial statements reflect all adjustments (which include normal, recurring adjustments) that are necessary, in our opinion, for a fair statement of our results. The preparation of financial statements in conformity with GAAP requires management to make certain estimates and assumptions. Such estimates and assumptions affect reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the dates of financial statements, and the reported amounts of revenues and expenses during the reported periods. Actual results could differ from those estimates. The results of operations of an interim period may not give a true indication of results that may be expected for a full year. These financial statements should be read in conjunction with the financial statements and accompanying notes included in the Form 10-K.
Consolidation, Variable Interest Entity, Policy	Variable Interest Entities As of March 31, 2021, and December 31, 2020, Ameren had unconsolidated variable interests as a limited partner in various equity method investments, totaling $41 million and $37 million, respectively, included in “Other assets” on Ameren’s consolidated balance sheet. Ameren is not the primary beneficiary of these investments because it does not have the power to direct matters that most significantly affect the activities of these variable interest entities. As of March 31, 2021, the maximum exposure to loss related to these variable interests is limited to the investment in these partnerships of $41 million plus associated outstanding funding commitments of $33 million.
Derivatives, Policy	We use derivatives to manage the risk of changes in market prices for natural gas and power, as well as the risk of changes in rail transportation surcharges through fuel oil hedges. Such price fluctuations may cause the following: • an unrealized appreciation or depreciation of our contracted commitments to purchase or sell when purchase or sale prices under the commitments are compared with current commodity prices; • market values of natural gas inventories that differ from the cost of those commodities in inventory; • 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.
Deferred Compensation	Deferred Compensation As of both March 31, 2021, and December 31, 2020, the present value of benefits to be paid for deferred compensation obligations was $90 million, which was primarily reflected in “Other deferred credits and liabilities” on Ameren's consolidated balance sheet.
Revenue from Contract with Customer	Operating Revenues As of March 31, 2021 and 2020, our remaining performance obligations for contracts with a term greater than one year were immaterial. The Ameren Companies elected not to disclose the aggregate amount of the transaction price allocated to the performance obligations that are unsatisfied as of the end of the reporting period for contracts with an initial expected term of one year or less. See Note 14 – Segment Information for disaggregated revenue information.
Excise Taxes	Excise TaxesAmeren Missouri and Ameren Illinois collect from their customers excise taxes, including municipal and state excise taxes and gross receipts taxes that are levied on the sale or distribution of natural gas and electricity.

Short-Term Debt and Liquidity (

Short-Term Debt and Liquidity (Tables)	3 Months Ended
	Mar. 31, 2021
Debt Disclosure [Abstract]
Schedule of Short-term Debt	The following table presents commercial paper outstanding, net of issuance discounts, as of March 31, 2021, and December 31, 2020. There were no borrowings outstanding under the Credit Agreements as of March 31, 2021, or December 31, 2020. March 31, 2021 December 31, 2020 Ameren (parent) $ 362 $ 490 Ameren Missouri 204 — Ameren Illinois 323 — Ameren consolidated $ 889 $ 490 The following table summarizes the activity and relevant interest rates for Ameren (parent)’s, Ameren Missouri’s, and Ameren Illinois’ commercial paper issuances and borrowings under the Credit Agreements in the aggregate for the three months ended March 31, 2021 and 2020: Ameren Ameren Ameren Ameren 2021 Average daily amount outstanding $ 454 $ 99 $ 96 $ 649 Weighted-average interest rate 0.25 % 0.22 % 0.21 % 0.24 % Peak amount outstanding during period (a) $ 650 $ 206 $ 353 $ 916 Peak interest rate 0.33 % 0.25 % 0.25 % 0.33 % 2020 Average daily amount outstanding $ 157 $ 395 $ 76 $ 628 Weighted-average interest rate 1.94 % 1.86 % 1.99 % 1.89 % Peak amount outstanding during period (a) $ 425 $ 573 $ 150 $ 908 Peak interest rate 3.30 % 5.05 % (b) 3.40 % 5.05 % (b) (a) The timing of peak outstanding commercial paper issuances and borrowings under the Credit Agreements varies by company. Therefore, the sum of individual company peak amounts may not equal the Ameren consolidated peak for the period. (b) Ameren’s and Ameren Missouri’s peak interest rate was affected by temporary disruptions in the commercial paper market in the first quarter of 2020.

Other Income and Expenses (Tabl

Other Income and Expenses (Tables)	3 Months Ended
	Mar. 31, 2021
Other Nonoperating Income (Expense) [Abstract]
Other Income And Expenses	The following table presents the components of “Other Income, Net” in the Ameren Companies’ statements of income for the three months ended March 31, 2021 and 2020: Three Months 2021 2020 Ameren: Allowance for equity funds used during construction $ 7 $ 4 Interest income on industrial development revenue bonds 6 6 Other interest income 1 1 Non-service cost components of net periodic benefit income (a) 34 23 Miscellaneous income 4 2 Donations (3) (13) (b) Miscellaneous expense (3) (2) Total Other Income, Net $ 46 $ 21 Ameren Missouri: Allowance for equity funds used during construction $ 4 $ 2 Interest income on industrial development revenue bonds 6 6 Non-service cost components of net periodic benefit income (a) 14 5 Miscellaneous income 1 1 Donations — (8) (b) Miscellaneous expense (2) (2) Total Other Income, Net $ 23 $ 4 Ameren Illinois: Allowance for equity funds used during construction $ 3 $ 2 Interest income 1 1 Non-service cost components of net periodic benefit income 14 13 Miscellaneous income — 1 Donations (3) (4) Miscellaneous expense (1) (2) Total Other Income, Net $ 14 $ 11 (a) For the three months ended March 31, 2021 and 2020, the non-service cost components of net periodic benefit income were adjusted by amounts deferred of less than $(1) million and $6 million, respectively, due to a regulatory tracking mechanism for the difference between the level of such costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. (b) Includes $8 million pursuant to Ameren Missouri’s March 2020 electric rate order. See Note 2 – Rate and Regulatory Matters under Part II, Item 8, in the Form 10-K for additional information.

Derivative Financial Instrume_2

Derivative Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2021
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Open Gross Derivative Volumes By Commodity Type	The following table presents open gross commodity contract volumes by commodity type for derivative assets and liabilities as of March 31, 2021, and December 31, 2020. As of March 31, 2021, these contracts extended through October 2023, October 2026, and May 2032 for fuel oils, natural gas and power, respectively. Quantity (in millions, except as indicated) March 31, 2021 December 31, 2020 Commodity Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils (in gallons) 36 — 36 43 — 43 Natural gas (in mmbtu) 34 114 148 33 114 147 Power (in MWhs) 7 7 14 6 7 13
Derivative Instruments Carrying Value	The following table presents the carrying value and balance sheet location of all derivative commodity contracts, none of which were designated as hedging instruments, as of March 31, 2021, and December 31, 2020: March 31, 2021 December 31, 2020 Balance Sheet Location Ameren Ameren Ameren Ameren Ameren Ameren Fuel oils Other current assets $ 2 $ — $ 2 $ 2 $ — $ 2 Other assets 3 — 3 — — — Natural gas Other current assets 2 10 12 1 8 9 Other assets 2 4 6 2 2 4 Power Other current assets 4 — 4 7 — 7 Other assets 1 — 1 — — — Total assets $ 14 $ 14 $ 28 $ 12 $ 10 $ 22 Fuel oils Other current liabilities $ 3 $ — $ 3 $ 7 $ — $ 7 Other deferred credits and liabilities — — — 2 — 2 Natural gas Other current liabilities — — — 1 1 2 Other deferred credits and liabilities — — — — 1 1 Power Other current liabilities 9 16 25 3 17 20 Other deferred credits and liabilities 5 169 174 8 181 189 Total liabilities $ 17 $ 185 $ 202 $ 21 $ 200 $ 221

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2021
Fair Value Disclosures [Abstract]
Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis	The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of March 31, 2021, and December 31, 2020: March 31, 2021 December 31, 2020 Level 1 Level 2 Level 3 Total Level 1 Level 2 Level 3 Total Assets: Ameren Missouri Derivative assets – commodity contracts: Fuel oils $ 4 $ — $ 1 $ 5 $ — $ — $ 2 $ 2 Natural gas — 4 — 4 — 3 — 3 Power 2 — 3 5 2 — 5 7 Total derivative assets – commodity contracts $ 6 $ 4 $ 4 $ 14 $ 2 $ 3 $ 7 $ 12 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 679 $ — $ — $ 679 $ 680 $ — $ — $ 680 Debt securities: U.S. Treasury and agency securities — 133 — 133 — 115 — 115 Corporate bonds — 128 — 128 — 115 — 115 Other — 63 — 63 — 67 — 67 Total nuclear decommissioning trust fund $ 679 $ 324 $ — $ 1,003 (a) $ 680 $ 297 $ — $ 977 (a) Total Ameren Missouri $ 685 $ 328 $ 4 $ 1,017 $ 682 $ 300 $ 7 $ 989 Ameren Illinois Derivative assets – commodity contracts: Natural gas $ — $ 10 $ 4 $ 14 $ — $ 6 $ 4 $ 10 Ameren Derivative assets – commodity contracts (b) $ 6 $ 14 $ 8 $ 28 $ 2 $ 9 $ 11 $ 22 Nuclear decommissioning trust fund (c) 679 324 — 1,003 (a) 680 297 — 977 (a) Total Ameren $ 685 $ 338 $ 8 $ 1,031 $ 682 $ 306 $ 11 $ 999 Liabilities: Ameren Missouri Derivative liabilities – commodity contracts: Fuel oils $ 1 $ — $ 2 $ 3 $ 6 $ — $ 3 $ 9 Natural gas — — — — — 1 — 1 Power 8 — 6 14 8 — 3 11 Total Ameren Missouri $ 9 $ — $ 8 $ 17 $ 14 $ 1 $ 6 $ 21 Ameren Illinois Derivative liabilities – commodity contracts: Natural gas $ — $ — $ — $ — $ — $ 1 $ 1 $ 2 Power — — 185 185 — — 198 198 Total Ameren Illinois $ — $ — $ 185 $ 185 $ — $ 1 $ 199 $ 200 Ameren Derivative liabilities – commodity contracts (b) $ 9 $ — $ 193 $ 202 $ 14 $ 2 $ 205 $ 221 (a) Balance excludes $7 million and $5 million of cash and cash equivalents, receivables, payables, and accrued income, net, for March 31, 2021, and December 31, 2020, respectively. (b) See the Ameren Missouri and Ameren Illinois sections of the table for a breakout of the fair value of Ameren’s derivative assets and liabilities by type of commodity. (c) See the Ameren Missouri section of the table for a breakout of the fair value of Ameren's nuclear decommissioning trust fund by investment type.
Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level Three In The Fair Value Hierarchy	The following table presents the fair value reconciliation of Level 3 power derivative contract assets and liabilities measured at fair value on a recurring basis for the three months ended March 31, 2021 and 2020: 2021 2020 Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren For the three months ended March 31: Beginning balance at January 1 $ 2 $ (198) $ (196) $ 13 $ (224) $ (211) Realized and unrealized gains/(losses) included in regulatory assets/liabilities (5) 9 4 11 (21) (10) Settlements — 4 4 (7) 4 (3) Ending balance at March 31 $ (3) $ (185) $ (188) $ 17 $ (241) $ (224) Change in unrealized gains/(losses) related to assets/liabilities held at March 31 $ (3) $ 9 $ 6 $ 10 $ (21) $ (11)
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis, Valuation Techniques	The following table describes the valuation techniques and significant unobservable inputs utilized for the fair value of our Level 3 power derivative contract assets and liabilities as of March 31, 2021, and December 31, 2020: Fair Value Weighted Average (b) Commodity Assets Liabilities Valuation Technique(s) Unobservable Input (a) Range 2021 Power (c) $ 3 $ (191) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) 23 – 40 29 Nodal basis ($/MWh) (5) – 0 (1) Trend rate (%) 2 – 5 3 2020 Power (c) $ 5 $ (201) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) 23 – 37 29 Nodal basis ($/MWh) (6) – 0 (2) Trend rate (%) 2 – 6 3 (a) Generally, significant increases (decreases) in these inputs in isolation would result in a significantly higher (lower) fair value measurement. (b) Unobservable inputs were weighted by relative fair value. (c) Valuations through 2029 use visible forward prices adjusted for nodal-to-hub basis differentials. Valuations beyond 2029 use a trend rate factor and are similarly adjusted for nodal-to-hub basis differentials.
Schedule of Financial Assets and Liabilities	The following table sets forth the carrying amount and, by level within the fair value hierarchy, the fair value of financial assets and liabilities disclosed, but not recorded, at fair value as of March 31, 2021, and December 31, 2020: Carrying Fair Value Level 1 Level 2 Level 3 Total March 31, 2021 Ameren: Cash, cash equivalents, and restricted cash $ 172 $ 172 $ — $ — $ 172 Investments in industrial development revenue bonds (a) 256 — 256 — 256 Short-term debt 889 — 889 — 889 Long-term debt (including current portion) (a) 11,535 (b) — 12,233 507 (c) 12,740 Ameren Missouri: Cash, cash equivalents, and restricted cash $ 12 $ 12 $ — $ — $ 12 Investments in industrial development revenue bonds (a) 256 — 256 — 256 Short-term debt 204 — 204 — 204 Long-term debt (including current portion) (a) 5,104 (b) — 5,685 — 5,685 Ameren Illinois: Cash, cash equivalents, and restricted cash $ 149 $ 149 $ — $ — $ 149 Short-term debt 323 — 323 — 323 Long-term debt (including current portion) 3,947 (b) — 4,401 — 4,401 December 31, 2020 Ameren: Cash, cash equivalents, and restricted cash $ 301 $ 301 $ — $ — $ 301 Investments in industrial development revenue bonds (a) 256 — 256 — 256 Short-term debt 490 — 490 — 490 Long-term debt (including current portion) (a) 11,086 (b) — 12,778 537 (c) 13,315 Ameren Missouri: Cash, cash equivalents, and restricted cash $ 145 $ 145 $ — $ — $ 145 Advances to money pool 139 — 139 — 139 Investments in industrial development revenue bonds (a) 256 — 256 — 256 Long-term debt (including current portion) (a) 5,104 (b) — 6,160 — 6,160 Ameren Illinois: Cash, cash equivalents, and restricted cash $ 147 $ 147 $ — $ — $ 147 Borrowings from money pool 19 — 19 — 19 Long-term debt (including current portion) 3,946 (b) — 4,822 — 4,822 (a) Ameren and Ameren Missouri have investments in industrial development revenue bonds, classified as held-to-maturity and recorded in “Other Assets,” that are equal to the finance obligations for the Peno Creek and Audrain CT energy centers. As of March 31, 2021, and December 31, 2020, the carrying amount of both the investments in industrial development revenue bonds and the finance obligations approximated fair value. (b) Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $86 million, $36 million, and $36 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of March 31, 2021. Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $84 million, $36 million, and $36 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2020. (c) The Level 3 fair value amount consists of ATXI’s senior unsecured notes.

Related Party Transactions (Tab

Related Party Transactions (Tables)	3 Months Ended
	Mar. 31, 2021
Related Party Transactions [Abstract]
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 March 31, 2021, and December 31, 2020: March 31, 2021 December 31, 2020 Ameren Missouri Ameren Illinois Ameren Missouri Ameren Illinois Income taxes payable to parent (a) $ — $ 6 $ — $ 6 Income taxes receivable from parent (b) 14 18 9 15 (a) Included in “Accounts payable – affiliates” on the balance sheet. (b) Included in “Accounts receivable – affiliates” on the balance sheet.
Schedule of Related Party Transactions	The following table presents the effect on Ameren Missouri and Ameren Illinois of related-party transactions for the three months ended March 31, 2021 and 2020: Three Months Agreement Income Statement Ameren Ameren Ameren Missouri power supply Operating Revenues 2021 $ 2 $ (a) agreements with Ameren Illinois 2020 3 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2021 $ 7 $ (b) rent and facility services 2020 7 1 Ameren Missouri and Ameren Illinois miscellaneous Operating Revenues 2021 $ (b) $ (b) support services and services provided to ATXI 2020 (b) (b) Total Operating Revenues 2021 $ 9 $ (b) 2020 10 1 Ameren Illinois power supply Purchased Power 2021 $ (a) $ 2 agreements with Ameren Missouri 2020 (a) 3 Ameren Missouri and Ameren Illinois Purchased Power 2021 $ 1 $ (b) transmission services from ATXI 2020 (a) (b) Total Purchased Power 2021 $ 1 $ 2 2020 (a) 3 Ameren Missouri and Ameren Illinois Other Operations and Maintenance 2021 $ (b) $ 1 rent and facility services 2020 (b) 1 Ameren Services support services Other Operations and Maintenance 2021 $ 35 $ 33 agreement 2020 35 33 Total Other Operations and 2021 $ 35 $ 34 Maintenance 2020 35 34 Money pool borrowings (advances) (Interest Charges)/Other Income, Net 2021 $ (b) $ (b) 2020 (b) (b) (a) Not applicable. (b) Amount less than $1 million.

Callaway Energy Center (Tables)

Callaway Energy Center (Tables)	3 Months Ended
	Mar. 31, 2021
Nuclear Waste Matters [Abstract]
Schedule of Insurance Coverage at Callaway Energy Center	The following table presents insurance coverage at Ameren Missouri’s Callaway Energy Center at April 1, 2021: 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) 13,210 (a) 138 (b) $ 13,660 (c) $ 138 Property damage: NEIL and EMANI April 1, 2021 $ 3,200 (d) $ 25 (e) Replacement power: NEIL April 1, 2021 $ 490 (f) $ 7 (e) (a) Provided through mandatory participation in an industrywide retrospective premium assessment program. The maximum coverage available is dependent on the number of United States commercial reactors participating in the program. (b) Retrospective premium under the Price-Anderson Act. This is subject to retrospective assessment with respect to a covered loss in excess of $450 million in the event of an incident at any licensed United States commercial reactor, payable at $21 million per year. (c) Limit of liability for each incident under the Price-Anderson liability provisions of the Atomic Energy Act of 1954, as amended. This limit is subject to change to account for the effects of inflation and changes in the number of licensed power reactors. (d) NEIL provides $2.7 billion in property damage, stabilization, decontamination, and premature decommissioning insurance for radiation events and $2.3 billion in property damage insurance for nonradiation events. EMANI provides $490 million in property damage insurance for both radiation and nonradiation events. (e) All NEIL-insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL. (f) 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)	3 Months Ended
	Mar. 31, 2021
Retirement Benefits [Abstract]
Components Of Net Periodic Benefit Cost	The following table presents the components of the net periodic benefit cost (income) incurred for Ameren’s pension and postretirement benefit plans for the three months ended March 31, 2021 and 2020: Pension Benefits Postretirement Benefits Three Months Three Months 2021 2020 2021 2020 Service cost (a) $ 33 $ 27 $ 6 $ 4 Non-service cost components: Interest cost 38 43 8 10 Expected return on plan assets (75) (73) (20) (20) Amortization of: Prior service benefit — — (1) (1) Actuarial loss (gain) 17 14 (1) (2) Total non-service cost components (b) $ (20) $ (16) $ (14) $ (13) Net periodic benefit cost (income) $ 13 $ 11 $ (8) $ (9) (a) Service cost, net of capitalization, is reflected in “Operating Expenses – Other operations and maintenance” on Ameren’s statement of income. (b) Non-service cost components are reflected in “Other Income, Net” on Ameren’s statement of income. See Note 5 – Other Income, Net, for additional information.
Summary Of Benefit Plan Costs Incurred	Ameren Missouri and Ameren Illinois are responsible for their respective share of Ameren’s pension and other postretirement costs. The following table presents the respective share of net periodic pension and other postretirement benefit costs (income) incurred for the three months ended March 31, 2021 and 2020: Pension Benefits Postretirement Benefits Three Months Three Months 2021 2020 2021 2020 Ameren Missouri (a) $ 6 $ 4 $ (1) $ (1) Ameren Illinois 8 7 (7) (8) Other (1) — — — Ameren (a) $ 13 $ 11 $ (8) $ (9) (a) Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates.

Income Taxes Income Taxes (Tabl

Income Taxes Income Taxes (Tables)	3 Months Ended
	Mar. 31, 2021
Income Tax Disclosure [Abstract]
Schedule of Effective Income Tax Rate Reconciliation	The following table presents a reconciliation of the federal statutory corporate income tax rate to the effective income tax rate for the three months ended March 31, 2021 and 2020: Ameren Ameren Missouri Ameren Illinois 2021 2020 2021 2020 2021 2020 Three Months Federal statutory corporate income tax rate: 21% 21% 21% 21% 21% 21% Increases (decreases) from: Amortization of deferred investment tax credit (1) — (1) (1) — — Amortization of excess deferred taxes (9) (9) (17) (15) (2) (3) Depreciation differences — (1) 1 — (1) (1) Renewable and other tax credits (6) — (11) — — — State tax 6 6 3 3 7 7 Stock-based compensation (1) (5) — — — — Effective income tax rate 10% 12% (4)% 8% 25% 24%

Supplemental Information (Table

Supplemental Information (Tables)	3 Months Ended
	Mar. 31, 2021
Supplemental Information [Abstract]
Schedule of Cash and Cash Equivalents Including Restricted Cash	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets and the statements of cash flows as of March 31, 2021, and December 31, 2020: March 31, 2021 December 31, 2020 Ameren Ameren Ameren Ameren Ameren Ameren Cash and cash equivalents $ 6 $ 1 $ — $ 139 $ 136 $ — Restricted cash 134 — 125 17 — 6 Restricted cash included in “Other current assets” — 3 — — 5 — Restricted cash included in “Other assets” 24 — 24 141 — 141 Restricted cash included in “Nuclear decommissioning trust fund” 8 8 — 4 4 — Total cash, cash equivalents, and restricted cash $ 172 $ 12 $ 149 $ 301 $ 145 $ 147
Schedule of Accounts, Notes, Loans and Financing Receivable	The following table provides a reconciliation of the beginning and ending amount of the allowance for doubtful accounts for the three months ended March 31, 2021 and 2020: Three Months 2021 2020 Ameren: Beginning of period $ 50 $ 17 Bad debt expense 4 3 Net write-offs (7) (1) End of period $ 47 $ 19 Ameren Missouri: Beginning of period $ 16 $ 7 Bad debt expense 1 2 Net write-offs (2) (1) End of period $ 15 $ 8 Ameren Illinois: (a) Beginning of Period $ 34 $ 10 Bad debt expense 3 1 Net write-offs (5) — End of Period $ 32 $ 11 (a) Ameren Illinois has rate-adjustment mechanisms that allow it to recover the difference between its actual net bad debt write-offs under GAAP, including those associated with receivables purchased from alternative retail electric suppliers, and the amount of net bad debt write-offs included in its base rates.
Schedule of Cash Flow, Supplemental Disclosures	Supplemental Cash Flow Information The following table provides noncash financing and investing activity excluded from the statements of cash flows for the three months ended March 31, 2021 and 2020: March 31, 2021 March 31, 2020 Ameren Ameren Ameren Ameren Ameren Ameren Investing Accrued capital expenditures, including wind generation expenditures $ 271 $ 141 $ 139 $ 235 $ 97 $ 127 Accrued nuclear fuel expenditures — — — 7 7 — Net realized and unrealized gain (loss) – nuclear decommissioning trust fund 22 22 — (111) (111) — Financing Issuance of common stock for stock-based compensation 33 — — 38 — —
Asset Retirement Obligation Disclosure	The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the three months ended March 31, 2021: Ameren Ameren Ameren Balance at December 31, 2020 $ 751 $ 5 (a) $ 756 (b) Liabilities incurred (c) 10 — 10 Liabilities settled (3) — (3) Accretion 8 (d) — 8 (d) Change in estimates (7) (e) — (7) (e) Balance at March 31, 2021 $ 759 $ 5 (a) $ 764 (b) (a) Included in “Other deferred credits and liabilities” on the balance sheet. (b) Balance included $59 million and $60 million in “Other current liabilities” on the balance sheet as of March 31, 2021, and December 31, 2020, respectively. (c) In the first quarter of 2021, Ameren Missouri recorded an ARO related to the decommissioning for the Atchison Renewable Energy Center. (d) Accretion expense attributable to Ameren Missouri was recorded as a decrease to regulatory liabilities. (e) Ameren Missouri changed its fair value estimate primarily due to a decrease in the cost estimate for closure of certain CCR storage facilities.
Schedule of excise taxes	The following table presents the excise taxes recorded on a gross basis in “Operating Revenues – Electric,” “Operating Revenues – Natural gas” and “Operating Expenses – Taxes other than income taxes” on the statements of income for the three months ended March 31, 2021 and 2020: Three Months 2021 2020 Ameren Missouri $ 31 $ 30 Ameren Illinois 39 35 Ameren $ 70 $ 65
Schedule of Earnings Per Share, Basic and Diluted	The following table reconciles the basic weighted-average number of common shares outstanding to the diluted weighted-average number of common shares outstanding for the three months ended March 31, 2021 and 2020: Three Months 2021 2020 Weighted-average Common Shares Outstanding – Basic 254.4 246.4 Assumed settlement of performance share units and restricted stock units 1.5 1.1 Dilutive effect of forward sale agreement — 0.6 Weighted-average Common Shares Outstanding – Diluted (a) 255.9 248.1 (a) There were no potentially dilutive securities excluded from the earnings per diluted share calculations for the three months ended March 31, 2021 and 2020.

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2021
Segment Reporting [Abstract]
Schedule Of Segment Reporting Information By Segment	The following tables present revenues, net income (loss) attributable to common shareholders, and capital expenditures by segment at Ameren and Ameren Illinois for the three months ended March 31, 2021 and 2020. Ameren, Ameren Missouri, and Ameren Illinois management review segment capital expenditure information rather than any individual or total asset amount. For additional information about our segments, see Note 16 – Segment Information under Part II, Item 8, of the Form 10-K. Ameren Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Other Intersegment Eliminations Ameren Three Months 2021: External revenues $ 695 $ 411 $ 347 $ 113 $ — $ — $ 1,566 Intersegment revenues 9 — — 17 — (26) — Net income attributable to Ameren common shareholders 47 46 75 47 (a) 18 — 233 Capital expenditures 534 (b) 157 48 141 1 6 887 (b) Three Months 2020: External revenues $ 670 $ 389 $ 271 $ 110 $ — $ — $ 1,440 Intersegment revenues 10 1 — 13 — (24) — Net income (loss) attributable to Ameren common shareholders (10) 37 55 47 (a) 17 — 146 Capital expenditures 278 123 61 170 3 1 636 (a) Ameren Transmission earnings reflect an allocation of financing costs from Ameren (parent). (b) Includes $193 million at Ameren and Ameren Missouri for wind generation expenditures for the three months ended March 31, 2021. Ameren Illinois Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Ameren Illinois Three Months 2021: External revenues $ 411 $ 347 $ 65 $ — $ 823 Intersegment revenues — — 16 (16) — Net income available to common shareholder 46 75 28 — 149 Capital expenditures 157 48 132 — 337 Three Months 2020: External revenues $ 390 $ 271 $ 62 $ — $ 723 Intersegment revenues — — 12 (12) — Net income available to common shareholder 37 55 28 — 120 Capital expenditures 123 61 140 — 324
Disaggregation of Revenue	The following tables present disaggregated revenues by segment at Ameren and Ameren Illinois for the three months ended March 31, 2021 and 2020. Economic factors affect the nature, timing, amount, and uncertainty of revenues and cash flows in a similar manner across customer classes. Revenues from alternative revenue programs have a similar distribution among customer classes as revenues from contracts with customers. Other revenues not associated with contracts with customers are presented in the Other customer classification, along with electric transmission and off-system revenues. Ameren Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Intersegment Eliminations Ameren Three Months 2021: Residential $ 312 $ 229 $ — $ — $ — $ 541 Commercial 216 132 — — — 348 Industrial 52 34 — — — 86 Other 61 16 — 130 (26) 181 Total electric revenues $ 641 $ 411 $ — $ 130 $ (26) $ 1,156 Residential $ 34 $ — $ 251 $ — $ — $ 285 Commercial 15 — 64 — — 79 Industrial 1 — 14 — — 15 Other 13 — 18 — — 31 Total natural gas revenues $ 63 $ — $ 347 $ — $ — $ 410 Total revenues (a) $ 704 $ 411 $ 347 $ 130 $ (26) $ 1,566 Three Months 2020: Residential $ 297 $ 220 $ — $ — $ — $ 517 Commercial 221 126 — — — 347 Industrial 53 35 — — — 88 Other 60 9 — 123 (24) 168 Total electric revenues $ 631 $ 390 $ — $ 123 $ (24) $ 1,120 Residential $ 33 $ — $ 213 $ — $ — $ 246 Commercial 13 — 54 — — 67 Industrial 1 — 3 — — 4 Other 2 — 1 — — 3 Total natural gas revenues $ 49 $ — $ 271 $ — $ — $ 320 Total revenues (a) $ 680 $ 390 $ 271 $ 123 $ (24) $ 1,440 (a) The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the three months ended March 31, 2021 and 2020: Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Ameren Three Months 2021: Revenues from alternative revenue programs $ (10) $ 61 $ 3 $ (1) $ 53 Other revenues not from contracts with customers (2) 3 1 — 2 Three Months 2020: Revenues from alternative revenue programs $ (3) $ 46 $ 11 $ 12 $ 66 Other revenues not from contracts with customers 8 1 1 — 10 Ameren Illinois Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Ameren Illinois Three Months 2021: Residential $ 229 $ 251 $ — $ — $ 480 Commercial 132 64 — — 196 Industrial 34 14 — — 48 Other 16 18 81 (16) 99 Total revenues (a) $ 411 $ 347 $ 81 $ (16) $ 823 Three Months 2020: Residential $ 220 $ 213 $ — $ — $ 433 Commercial 126 54 — — 180 Industrial 35 3 — — 38 Other 9 1 74 (12) 72 Total revenues (a) $ 390 $ 271 $ 74 $ (12) $ 723 (a) The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the Ameren Illinois segments for the three months ended March 31, 2021 and 2020: Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Ameren Illinois Three Months 2021: Revenues from alternative revenue programs $ 61 $ 3 $ (1) $ 63 Other revenues not from contracts with customers 3 1 — 4 Three Months 2020: Revenues from alternative revenue programs $ 46 $ 11 $ 10 $ 67 Other revenues not from contracts with customers 1 1 — 2

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Basis Of Presentation And Significant Accounting Policies [Line Items]
Percentage of accounts receivable balances that are 30 days past due or are part of a deferred payment arrangement	27.00%		21.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 137		$ 99
Noncontrolling Interest in Variable Interest Entity	41	$ 37
Cash Surrender Value of Life Insurance	275	272
Corporate owned life insurance, borrowings	$ 107	107
Union Electric Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Percentage of accounts receivable balances that are 30 days past due or are part of a deferred payment arrangement	19.00%		16.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 33		$ 29
Union Electric Company | Final Rate Order
Basis Of Presentation And Significant Accounting Policies [Line Items]
COVID-19 pandemic costs	$ 9
Ameren Illinois Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Percentage of accounts receivable balances that are 30 days past due or are part of a deferred payment arrangement	32.00%		25.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 104		$ 70
Cash Surrender Value of Life Insurance	117	115
Corporate owned life insurance, borrowings	107	$ 107
Partnership Funding Commitment
Basis Of Presentation And Significant Accounting Policies [Line Items]
Unrecorded Unconditional Purchase Obligation	$ 33

Rate And Regulatory Matters (Na

Rate And Regulatory Matters (Narrative-Missouri) (Detail) $ in Millions	1 Months Ended	3 Months Ended
	Jan. 31, 2021MWh	Mar. 31, 2021USD ($)MWh	Mar. 31, 2020USD ($)	Dec. 31, 2020USD ($)
Rate And Regulatory Matters [Line Items]
Regulatory assets		$ 1,249		$ 1,100
Revenues		1,566	$ 1,440
Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets		413		$ 347
Pending Rate Case | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 299
Public Utilities, Requested Return on Equity, Percentage		9.90%
Public Utilities, Requested Equity Capital Structure, Percentage		51.90%
Rate Base		$ 10,000
Months to complete a rate proceeding		11 months
Pending Rate Case | Natural Gas | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 9
Public Utilities, Requested Return on Equity, Percentage		9.80%
Public Utilities, Requested Equity Capital Structure, Percentage		51.90%
Rate Base		$ 310
Months to complete a rate proceeding		11 months
Wind Generation Facility | Union Electric Company
Rate And Regulatory Matters [Line Items]
Amount of Megawatts | MWh	300	700
Atchison Renewable Energy Center purchase price		$ 250
Final Rate Order | Union Electric Company
Rate And Regulatory Matters [Line Items]
COVID-19 pandemic costs		9
Regulatory assets		5
Revenues		$ 4

Rate And Regulatory Matters (_2

Rate And Regulatory Matters (Narrative-Illinois) (Detail) - Ameren Illinois Company - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2021	Mar. 31, 2021
Rate And Regulatory Matters [Line Items]
Annual investment in energy-efficiency programs		$ 100
Deferred payment arrangement extension		18 months
Financial assistance program costs		$ 4
IEIMA
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage		5.80%
Final Rate Order | IEIMA | Electric Distribution
Rate And Regulatory Matters [Line Items]
Requested electric distribution service rate reconciliation period		2 years
Amortization Period		two years
Subsequent Event | Pending Rate Case | IEIMA | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 64

Rate And Regulatory Matters (_3

Rate And Regulatory Matters (Narrative-Federal) (Detail) - USD ($) $ in Millions	1 Months Ended		3 Months Ended
	May 31, 2020	Nov. 30, 2019	Mar. 31, 2021	Dec. 31, 2020
Rate And Regulatory Matters [Line Items]
Regulatory liabilities			$ 5,230	$ 5,282
Current regulatory liabilities			225	121
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory liabilities			1,988	2,063
Current regulatory liabilities			200	$ 88
Final Rate Order | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory liabilities			$ 9
Midwest Independent Transmission System Operator, Inc
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage			12.38%
Midwest Independent Transmission System Operator, Inc | Final Rate Order
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage	10.02%	9.88%	10.32%
Customer Requested Rate on Equity			9.15%
Incentive adder to FERC allowed base return on common equity			0.50%
Current regulatory liabilities			$ 17
Midwest Independent Transmission System Operator, Inc | Final Rate Order | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities			$ 9
Maximum | Midwest Independent Transmission System Operator, Inc | Final Rate Order
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage			10.82%

Short-Term Debt And Liquidity_2

Short-Term Debt And Liquidity (Narrative) (Details) $ in Billions	3 Months Ended
	Mar. 31, 2021USD ($)	Mar. 31, 2020
Credit Agreements
Short-term Debt [Line Items]
Net Liquidity Available	$ 1.4
Actual debt-to-capital ratio	0.57
Utilities
Short-term Debt [Line Items]
Short-term Debt, Weighted Average Interest Rate, over Time	0.22%	1.93%
Union Electric Company | Missouri Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.49
Ameren Illinois Company | Illinois Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.46

Short-Term Debt and Liquidity_3

Short-Term Debt and Liquidity (Short-Term Debt outstanding) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Short-term Debt [Line Items]
Short-term debt	$ 889	$ 490
Union Electric Company
Short-term Debt [Line Items]
Short-term debt	204	0
Ameren Illinois Company
Short-term Debt [Line Items]
Short-term debt	323	0
Commercial Paper
Short-term Debt [Line Items]
Short-term debt	889	490
Commercial Paper | Ameren (parent)
Short-term Debt [Line Items]
Short-term debt	362	490
Commercial Paper | Union Electric Company
Short-term Debt [Line Items]
Short-term debt	204	0
Commercial Paper | Ameren Illinois Company
Short-term Debt [Line Items]
Short-term debt	$ 323	$ 0

Short-Term Debt and Liquidity_4

Short-Term Debt and Liquidity (Short-Term Debt Activity) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 649	$ 628
Weighted-average interest rate	0.24%	1.89%
Short-term Debt, Maximum Amount Outstanding During Period	$ 916	$ 908
Peak interest rate	0.33%	5.05%
Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 454	$ 157
Weighted-average interest rate	0.25%	1.94%
Short-term Debt, Maximum Amount Outstanding During Period	$ 650	$ 425
Peak interest rate	0.33%	3.30%
Union Electric Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 99	$ 395
Weighted-average interest rate	0.22%	1.86%
Short-term Debt, Maximum Amount Outstanding During Period	$ 206	$ 573
Peak interest rate	0.25%	5.05%
Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 96	$ 76
Weighted-average interest rate	0.21%	1.99%
Short-term Debt, Maximum Amount Outstanding During Period	$ 353	$ 150
Peak interest rate	0.25%	3.40%

Long-Term debt and Equity Fin_2

Long-Term debt and Equity Financings (Narrative) (Details) - USD ($) shares in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Feb. 11, 2021
Long-Term Debt And Equity Financings [Line Items]
Stock Issued During Period, Shares, New Issues	0.1	0.2
Issuance of common stock	$ 125,000,000	$ 13,000,000
Stock Issued During Period, Shares, Other	0.5	0.5
Issuance of common stock for stock-based compensation	$ 33,000,000	$ 38,000,000
Shares issued under forward sale agreement (in shares)	1.6	0	1.6
Common Stock Value Issued Through Forward Sale Agreement			$ 113,000,000
Dividend reinvestment and 401 (k) plans
Long-Term Debt And Equity Financings [Line Items]
Issuance of common stock	$ 12,000,000
Ameren (parent) | Unsecured Debt | Senior Secured Notes 1.75% Due 2028 [Domain]
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 450,000,000
Debt Instrument, Interest Rate, Stated Percentage	1.75%
Proceeds from Issuance of Debt	$ 447,000,000
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Issuance of common stock for stock-based compensation	0	$ 0
Capital contributions from parent	113,000,000	0
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Issuance of common stock for stock-based compensation	0	0
Capital contributions from parent	40,000,000	100,000,000
Redemption of preferred stock	$ (13,000,000)	$ 0
Ameren Illinois Company | Series6625
Long-Term Debt And Equity Financings [Line Items]
Preferred Stock, Dividend Rate, Percentage	6.625%
Redemption of preferred stock	$ (12,000,000)
Ameren Illinois Company | Series775
Long-Term Debt And Equity Financings [Line Items]
Preferred Stock, Dividend Rate, Percentage	7.75%
Redemption of preferred stock	$ (1,000,000)

Other Income, Net (Detail)

Other Income, Net (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 7	$ 4
Interest income on industrial development revenue bonds	6	6
Other interest income	1	1
Non-service cost components of net periodic benefit income	34	23
Miscellaneous income	4	2
Donations	(3)	(13)
Miscellaneous expense	(3)	(2)
Total Other Income, Net	46	21
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	4	2
Interest income on industrial development revenue bonds	6	6
Non-service cost components of net periodic benefit income	14	5
Miscellaneous income	1	1
Donations	0	(8)
Miscellaneous expense	(2)	(2)
Total Other Income, Net	23	4
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	(1)	6
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	3	2
Other interest income	1	1
Non-service cost components of net periodic benefit income	14	13
Miscellaneous income	0	1
Donations	(3)	(4)
Miscellaneous expense	(1)	(2)
Total Other Income, Net	$ 14	11
Final Rate Order | Electricity | Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Donations		$ (8)

Derivative Financial Instrume_3

Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) gal in Millions, MWh in Millions, MMBTU in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021MMBTUMWhgal	Dec. 31, 2020MWhMMBTUgal
Fuel Oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | gal	36	43
Natural Gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	148	147
Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	14	13
Union Electric Company | Fuel Oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | gal	36	43
Union Electric Company | Natural Gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	34	33
Union Electric Company | Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	7	6
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	114
Ameren Illinois Company | Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	7	7

Derivative Financial Instrume_4

Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Derivative [Line Items]
Derivative assets	$ 28	$ 22
Derivative liabilities	202	221
Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	2	2
Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	3	0
Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	3	7
Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	2
Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	12	9
Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	6	4
Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	2
Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Power | Other Current Assets
Derivative [Line Items]
Derivative assets	4	7
Power | Other Assets
Derivative [Line Items]
Derivative assets	1	0
Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	25	20
Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	174	189
Union Electric Company
Derivative [Line Items]
Derivative assets	14	12
Derivative liabilities	17	21
Union Electric Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	2	2
Union Electric Company | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	3	0
Union Electric Company | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	3	7
Union Electric Company | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	2
Union Electric Company | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	2	1
Union Electric Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	2	2
Union Electric Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Union Electric Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Union Electric Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	4	7
Union Electric Company | Power | Other Assets
Derivative [Line Items]
Derivative assets	1	0
Union Electric Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	9	3
Union Electric Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	5	8
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	14	10
Derivative liabilities	185	200
Ameren Illinois Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	10	8
Ameren Illinois Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	4	2
Ameren Illinois Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Ameren Illinois Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Ameren Illinois Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Power | Other Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	16	17
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	$ 169	$ 181

Fair Value Measurements (Schedu

Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	$ 1,003	$ 977
Assets fair value	1,031	999
Excluded receivables, payables, and accrued income, net	7	5
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	679	680
Assets fair value	685	682
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	324	297
Assets fair value	338	306
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Assets fair value	8	11
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	28	22
Derivative liabilities	202	221
Commodity Contract | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	6	2
Derivative liabilities	9	14
Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	14	9
Derivative liabilities	0	2
Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	8	11
Derivative liabilities	193	205
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	1,003	977
Assets fair value	1,017	989
Union Electric Company | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	679	680
Assets fair value	685	682
Union Electric Company | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	324	297
Assets fair value	328	300
Union Electric Company | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Assets fair value	4	7
Union Electric Company | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	14	12
Derivative liabilities	17	21
Union Electric Company | Commodity Contract | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	6	2
Derivative liabilities	9	14
Union Electric Company | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	3
Derivative liabilities	0	1
Union Electric Company | Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	7
Derivative liabilities	8	6
Union Electric Company | Fuel Oils | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	5	2
Derivative liabilities	3	9
Union Electric Company | Fuel Oils | Commodity Contract | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	0
Derivative liabilities	1	6
Union Electric Company | Fuel Oils | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Union Electric Company | Fuel Oils | Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	2
Derivative liabilities	2	3
Union Electric Company | Natural Gas | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	3
Derivative liabilities	0	1
Union Electric Company | Natural Gas | Commodity Contract | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Union Electric Company | Natural Gas | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	3
Derivative liabilities	0	1
Union Electric Company | Natural Gas | Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Union Electric Company | Power | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	5	7
Derivative liabilities	14	11
Union Electric Company | Power | Commodity Contract | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	2
Derivative liabilities	8	8
Union Electric Company | Power | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Union Electric Company | Power | Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	5
Derivative liabilities	6	3
Union Electric Company | Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	679	680
Union Electric Company | Equity Securities | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	679	680
Union Electric Company | Equity Securities | Significant Other Observable Inputs (Level 2) | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Equity Securities | Level 3 | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt Securities | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	133	115
Union Electric Company | Debt Securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	128	115
Union Electric Company | Debt Securities | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	63	67
Union Electric Company | Debt Securities | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt Securities | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt Securities | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt Securities | Significant Other Observable Inputs (Level 2) | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	133	115
Union Electric Company | Debt Securities | Significant Other Observable Inputs (Level 2) | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	128	115
Union Electric Company | Debt Securities | Significant Other Observable Inputs (Level 2) | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	63	67
Union Electric Company | Debt Securities | Level 3 | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt Securities | Level 3 | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt Securities | Level 3 | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Ameren Illinois Company | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	185	200
Ameren Illinois Company | Commodity Contract | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	1
Ameren Illinois Company | Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	185	199
Ameren Illinois Company | Natural Gas | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	14	10
Derivative liabilities	0	2
Ameren Illinois Company | Natural Gas | Commodity Contract | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Ameren Illinois Company | Natural Gas | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	10	6
Derivative liabilities	0	1
Ameren Illinois Company | Natural Gas | Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	4
Derivative liabilities	0	1
Ameren Illinois Company | Power | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	185	198
Ameren Illinois Company | Power | Commodity Contract | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Power | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Power | Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	$ 185	$ 198

Fair Value Measurements (Sche_2

Fair Value Measurements (Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level Three In The Fair Value Hierarchy) (Detail) - Power - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
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	$ (188)	$ (224)	$ (196)	$ (211)
Included in regulatory assets/liabilities	4	(10)
Settlements, assets	4
Settlement, liabilities		(3)
Change in unrealized gains (losses) related to assets/liabilities held at period end	6	(11)
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	(3)	17	2	13
Included in regulatory assets/liabilities	(5)	11
Settlement, liabilities	0	(7)
Change in unrealized gains (losses) related to assets/liabilities held at period end	(3)	10
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	(185)	(241)	$ (198)	$ (224)
Included in regulatory assets/liabilities	9	(21)
Settlements, assets	4	4
Change in unrealized gains (losses) related to assets/liabilities held at period end	$ 9	$ (21)

Fair Value Measurements (Sche_3

Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) - Power $ in Millions	Mar. 31, 2021USD ($)$ / MWh$ / MMBTU	Dec. 31, 2020USD ($)$ / MWh$ / MMBTU
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	$ 3	$ 5
Derivative liabilities | $	$ (191)	$ (201)
Commodity Forward Price | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	23	23
Commodity Forward Price | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	40	37
Commodity Forward Price | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	29	29
Nodal Basis | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	(5)	(6)
Nodal Basis | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0	0
Nodal Basis | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	(1)	(2)
Commodity Future Price | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU	2	2
Commodity Future Price | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU	5	6
Commodity Future Price | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU	3	3

Fair Value Measurements (Sche_4

Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Financial Assets and Liabilities) (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 172	$ 301	$ 203	$ 176
Short-term debt	889	490
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	12	145	24	39
Advances to money pool	0	139
Short-term debt	204	0
Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	149	147	$ 140	$ 125
Short-term debt	323	0
Borrowings from money pool	0	19
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	172	301
Available-for-sale Securities and Held-to-maturity Securities	256	256
Short-term debt	889	490
Long-term debt (including current portion)	11,535	11,086
Debt Issuance Costs, Net	86	84
Carrying Amount | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	12	145
Advances to money pool		139
Available-for-sale Securities and Held-to-maturity Securities	256	256
Short-term debt	204
Long-term debt (including current portion)	5,104	5,104
Debt Issuance Costs, Net	36	36
Carrying Amount | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	149	147
Short-term debt	323
Long-term debt (including current portion)	3,947	3,946
Debt Issuance Costs, Net	36	36
Borrowings from money pool		19
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	172	301
Investments, Fair Value Disclosure	256	256
Short-term Debt, Fair Value	889	490
Long-term Debt, Fair Value	12,740	13,315
Fair Value | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	172	301
Investments, Fair Value Disclosure	0	0
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	0	0
Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Investments, Fair Value Disclosure	256	256
Short-term Debt, Fair Value	889	490
Long-term Debt, Fair Value	12,233	12,778
Fair Value | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Investments, Fair Value Disclosure	0	0
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	507	537
Fair Value | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	12	145
Advances to money pool		139
Investments, Fair Value Disclosure	256	256
Short-term Debt, Fair Value	204
Long-term Debt, Fair Value	5,685	6,160
Fair Value | Union Electric Company | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	12	145
Advances to money pool		0
Investments, Fair Value Disclosure	0	0
Short-term Debt, Fair Value	0
Long-term Debt, Fair Value	0	0
Fair Value | Union Electric Company | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Advances to money pool		139
Investments, Fair Value Disclosure	256	256
Short-term Debt, Fair Value	204
Long-term Debt, Fair Value	5,685	6,160
Fair Value | Union Electric Company | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Advances to money pool		0
Investments, Fair Value Disclosure	0	0
Short-term Debt, Fair Value	0
Long-term Debt, Fair Value	0	0
Fair Value | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	149	147
Short-term Debt, Fair Value	323
Long-term Debt, Fair Value	4,401	4,822
Borrowings from money pool		19
Fair Value | Ameren Illinois Company | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	149	147
Short-term Debt, Fair Value	0
Long-term Debt, Fair Value	0	0
Borrowings from money pool		0
Fair Value | Ameren Illinois Company | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Short-term Debt, Fair Value	323
Long-term Debt, Fair Value	4,401	4,822
Borrowings from money pool		19
Fair Value | Ameren Illinois Company | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Short-term Debt, Fair Value	0
Long-term Debt, Fair Value	$ 0	0
Borrowings from money pool		$ 0

Related Party Transactions (Nar

Related Party Transactions (Narrative) (Details) - Ameren Illinois Company - April 2021 Procurement - Ameren Illinois Power Supply Agreements with Ameren Missouri	3 Months Ended
	Mar. 31, 2021MWh$ / MWh
Related Party Transaction [Line Items]
Related Party Long Term Contract For Purchase of Electric Power | MWh	33,600
Related Party Long Term Contract For Purchase of Electric Power Rate | $ / MWh	34

Related Party Transactions (Sch

Related Party Transactions (Schedule of Affiliate Receivables and Payables) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Union Electric Company
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	$ 40	$ 46
Accounts Receivable, Related Parties, Current	69	57
Union Electric Company | Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	0	0
Union Electric Company | Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	14	9
Ameren Illinois Company
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	43	51
Accounts Receivable, Related Parties, Current	71	64
Ameren Illinois Company | Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	6	6
Ameren Illinois Company | Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	$ 18	$ 15

Related Party Transactions (Eff

Related Party Transactions (Effects of Related-party Transactions on the Statement of Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Union Electric Company | Ameren Missouri Power Supply Agreements with Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues	$ 2	$ 3
Union Electric Company | Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	7	7
Operating Expenses	1	1
Union Electric Company | Ameren Missouri and Ameren Illinois Miscellaneous Support Services
Related Party Transaction [Line Items]
Operating Revenues	1	1
Union Electric Company | Total Related Party Operating Revenues
Related Party Transaction [Line Items]
Operating Revenues	9	10
Union Electric Company | Ameren Missouri and Ameren Illinois Transmission Services from ATXI
Related Party Transaction [Line Items]
Operating Expenses	1
Union Electric Company | Purchased Power
Related Party Transaction [Line Items]
Operating Expenses	1
Union Electric Company | Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	35	35
Union Electric Company | Total Related Party Other Operations and Maintenance
Related Party Transaction [Line Items]
Operating Expenses	35	35
Union Electric Company | Money pool borrowings (advances)
Related Party Transaction [Line Items]
Interest Charges (Income)	1	1
Ameren Illinois Company | Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	1	1
Operating Expenses	1	1
Ameren Illinois Company | Ameren Missouri and Ameren Illinois Miscellaneous Support Services
Related Party Transaction [Line Items]
Operating Revenues	1	1
Ameren Illinois Company | Total Related Party Operating Revenues
Related Party Transaction [Line Items]
Operating Revenues	1	1
Ameren Illinois Company | Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Operating Expenses	2	3
Ameren Illinois Company | Ameren Missouri and Ameren Illinois Transmission Services from ATXI
Related Party Transaction [Line Items]
Operating Expenses	1	1
Ameren Illinois Company | Purchased Power
Related Party Transaction [Line Items]
Operating Expenses	2	3
Ameren Illinois Company | Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	33	33
Ameren Illinois Company | Total Related Party Other Operations and Maintenance
Related Party Transaction [Line Items]
Operating Expenses	34	34
Ameren Illinois Company | Money pool borrowings (advances)
Related Party Transaction [Line Items]
Interest Charges (Income)	$ 1	$ 1

Commitments And Contingencies (

Commitments And Contingencies (Environmental Matters) (Detail) $ in Millions	Mar. 31, 2021USD ($)centerscrubbersite	Dec. 31, 2020USD ($)
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 764	$ 756
Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	175
Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 225
Union Electric Company
Loss Contingencies [Line Items]
Number of Energy Center Scrubbers | scrubber	2
Number of Energy Centers Constructing Wastewater Treatment Facilities | center	3
Number of energy centers | center	4
Asset Retirement Obligation	$ 759	751
Union Electric Company | Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	175
Union Electric Company | Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	225
Ameren Illinois Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	5	$ 5
Coal Combustion Residuals Estimate | Union Electric Company | Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	75
Coal Combustion Residuals Estimate | Union Electric Company | Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	100
Manufactured Gas Plant
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 90
Manufactured Gas Plant | Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites | site	44
Accrual for environmental loss contingencies	$ 90
Manufactured Gas Plant | Ameren Illinois Company | Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	90
Manufactured Gas Plant | Ameren Illinois Company | Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	150
Rush Island Energy Center | Union Electric Company
Loss Contingencies [Line Items]
Estimated capital costs to comply with preliminary court order	1,000
Rush Island Energy Center | Union Electric Company | Minimum
Loss Contingencies [Line Items]
Estimated operations and maintenance costs to comply with preliminary court order	30
Rush Island Energy Center | Union Electric Company | Maximum
Loss Contingencies [Line Items]
Estimated operations and maintenance costs to comply with preliminary court order	50
New CCR Rules Estimate
Loss Contingencies [Line Items]
Asset Retirement Obligation	103
New CCR Rules Estimate | Union Electric Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 103

Callaway Energy Center (Insuran

Callaway Energy Center (Insurance Disclosure) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021USD ($)
Nuclear Waste Matters [Line Items]
Decommissioning Cost	$ 7
Frequency of Decommissioning Cost Study	3 years
Number Of Years The Limit Of Liability And The Maximum Potential Annual Payments Are Adjusted	5 years
Number Of Weeks Of Coverage After The First Twelve Weeks Of An Outage	1
Number Of Additional Weeks After Initial Indemnity Coverage For Power Outage	1.365
Nuclear Plant | Union Electric Company
Nuclear Waste Matters [Line Items]
Estimated Nuclear Generator Repairs	$ 65
Replacement Power - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Amount Of Weekly Indemnity Coverage Commencing Twelve Weeks After Power Outage	4.5
Insurance Aggregate Maximum Coverage	490
Insurance Maximum Coverage per Incident	7
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 Amount Of Weekly Indemnity Coverage Thereafter Not Exceeding Policy Limit For Non-Nuclear Events	328
Public Liability And Nuclear Worker Liability - American Nuclear Insurers
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	450
Insurance Maximum Coverage per Incident	0
Public Liability And Nuclear Worker Liability - Pool Participation
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	13,210
Insurance Maximum Coverage per Incident	138
Threshold Amount For Retrospective Insurance Assessment For Covered Loss Under Public Liability And Nuclear Worker Liability Insurance Policy	450
Maximum Annual Payment Per Incident At Licensed Commercial Nuclear Reactor	21
Public Liability
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	13,660
Insurance Maximum Coverage per Incident	138
Property Damage - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	3,200
Insurance Maximum Coverage per Incident	25
Radiation Event
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	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]
Insurance Aggregate Maximum Coverage	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]
Insurance Aggregate Maximum Coverage	$ 490

Retirement Benefits (Components

Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	$ (34)	$ (23)
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	33	27
Interest cost	38	43
Expected return on plan assets	(75)	(73)
Prior service cost (benefit)	0	0
Actuarial loss (gain)	17	14
Total non-service cost components	(20)	(16)
Net periodic benefit cost (income)	13	11
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	6	4
Interest cost	8	10
Expected return on plan assets	(20)	(20)
Prior service cost (benefit)	(1)	(1)
Actuarial loss (gain)	(1)	(2)
Total non-service cost components	(14)	(13)
Net periodic benefit cost (income)	$ (8)	$ (9)

Retirement Benefits (Summary of

Retirement Benefits (Summary of Benefit Plan Costs Incurred) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 13	$ 11
Pension Plan | Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	6	4
Pension Plan | Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	8	7
Pension Plan | Other Affiliated Entities And Intercompany Eliminations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(1)	0
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(8)	(9)
Other Postretirement Benefit Plan, Defined Benefit | Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(1)	(1)
Other Postretirement Benefit Plan, Defined Benefit | Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(7)	(8)
Other Postretirement Benefit Plan, Defined Benefit | Other Affiliated Entities And Intercompany Eliminations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 0	$ 0

Retirement Benefits (Narrative)

Retirement Benefits (Narrative) (Details) $ in Millions	Dec. 31, 2020USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan estimated future employer contributions over next five years	$ 60

Income Taxes Income Taxes (Sche

Income Taxes Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Taxes [Line Items]
Federal statutory corporate income tax rate	21.00%	21.00%
Amortization of deferred investment tax credit	(1.00%)	0.00%
Amortization of excess deferred taxes	(9.00%)	(9.00%)
Depreciation differences	0.00%	(1.00%)
Renewable and other tax credits	(6.00%)	0.00%
State tax	6.00%	6.00%
Stock-based compensation	(1.00%)	(5.00%)
Effective income tax rate	10.00%	12.00%
Union Electric Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate	21.00%	21.00%
Amortization of deferred investment tax credit	(1.00%)	(1.00%)
Amortization of excess deferred taxes	(17.00%)	(15.00%)
Depreciation differences	1.00%	0.00%
Renewable and other tax credits	(11.00%)	0.00%
State tax	3.00%	3.00%
Stock-based compensation	0.00%	0.00%
Effective income tax rate	(4.00%)	8.00%
Ameren Illinois Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate	21.00%	21.00%
Amortization of deferred investment tax credit	0.00%	0.00%
Amortization of excess deferred taxes	(2.00%)	(3.00%)
Depreciation differences	(1.00%)	(1.00%)
Renewable and other tax credits	0.00%	0.00%
State tax	7.00%	7.00%
Stock-based compensation	0.00%	0.00%
Effective income tax rate	25.00%	24.00%

Supplemental Information (Narra

Supplemental Information (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Supplemental Information [Abstract]
Payables for purchased receivables	$ 31		$ 28
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred Compensation Liability, Classified, Noncurrent	90		90
Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Amount	5	$ 8
Deferred Compensation Liability, Classified, Noncurrent	$ 90		$ 90
January 1, 2021 Issuance
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	38 months
Performance Shares | January 1, 2021 Issuance
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	293,058
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Granted in Period, Fair Value	$ 25
Stock Issued During Period Percentage Conversion Of Units, Low End	0.00%
Stock Issued During Period Percentage Conversion Of Units, High End	200.00%
Performance Shares | Market performance measures achievement | January 1, 2021 Issuance
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	251,177
Performance Shares | Renewable generation and energy storage installation targets | January 1, 2021 Issuance
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	41,881
Restricted Stock Units (RSUs) | January 1, 2021 Issuance
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	125,562
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Granted in Period, Fair Value	$ 10

Supplemental Information (Cash

Supplemental Information (Cash and Cash Equivalents) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	$ 6	$ 139
Restricted cash included in “Other current assets”	134	17
Restricted cash included in “Other assets”	24	141
Restricted cash included in “Nuclear decommissioning trust fund”	8	4
Total cash, cash equivalents, and restricted cash	172	301	$ 203	$ 176
Union Electric Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	1	136
Restricted cash included in “Other current assets”	3	5
Restricted cash included in “Other assets”	0	0
Restricted cash included in “Nuclear decommissioning trust fund”	8	4
Total cash, cash equivalents, and restricted cash	12	145	24	39
Ameren Illinois Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	0	0
Restricted cash included in “Other current assets”	125	6
Restricted cash included in “Other assets”	24	141
Restricted cash included in “Nuclear decommissioning trust fund”	0	0
Total cash, cash equivalents, and restricted cash	$ 149	$ 147	$ 140	$ 125

Supplemental Information (Allow

Supplemental Information (Allowance for Doubtful Accounts) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning of period	$ 50	$ 17
Bad debt expense	4	3
Net write-offs	(7)	(1)
End of period	47	19
Union Electric Company
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning of period	16	7
Bad debt expense	1	2
Net write-offs	(2)	(1)
End of period	15	8
Ameren Illinois Company
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning of period	34	10
Bad debt expense	3	1
Net write-offs	(5)	0
End of period	$ 32	$ 11

Supplemental Information (Suppl

Supplemental Information (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures, including wind generation expenditures	$ 271	$ 235
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	22	(111)
Issuance of common stock for stock-based compensation	33	38
Nuclear Fuel [Member]
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures, including wind generation expenditures	0	7
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures, including wind generation expenditures	141	97
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	22	(111)
Issuance of common stock for stock-based compensation	0	0
Union Electric Company | Nuclear Fuel [Member]
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures, including wind generation expenditures	0	7
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures, including wind generation expenditures	139	127
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	0	0
Issuance of common stock for stock-based compensation	0	0
Ameren Illinois Company | Nuclear Fuel [Member]
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures, including wind generation expenditures	$ 0	$ 0

Supplemental Information (Sched

Supplemental Information (Schedule of Asset Retirement Obligations) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	$ 756
Asset Retirement Obligation, Liabilities Incurred	10
Liabilities settled	(3)
Accretion	8
Change in estimates	(7)
Asset Retirement Obligation, Ending Balance	764
Other current liabilities	392	$ 407
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	751
Asset Retirement Obligation, Liabilities Incurred	10
Liabilities settled	(3)
Accretion	8
Change in estimates	(7)
Asset Retirement Obligation, Ending Balance	759
Other current liabilities	116	123
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	5
Asset Retirement Obligation, Liabilities Incurred	0
Liabilities settled	0
Accretion	0
Change in estimates	0
Asset Retirement Obligation, Ending Balance	5
Other current liabilities	207	221
Asset Retirement Obligation Balance
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Other current liabilities	$ 59	$ 60

Supplemental Information (Sch_2

Supplemental Information (Schedule Of Excise Taxes) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accounting Policies [Line Items]
Excise tax expense	$ 70	$ 65
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	31	30
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 39	$ 35

Supplemental Information (Earni

Supplemental Information (Earnings Per Share) (Details) - shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Earnings Per Share Reconciliation [Abstract]
Weighted-average Common Shares Outstanding – Basic	254,400,000	246,400,000
Assumed settlement of performance share units and restricted stock units	1,500,000	1,100,000
Dilutive effect of forward sale agreement	0	600,000
Weighted-average Common Shares Outstanding – Diluted	255,900,000	248,100,000
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount (in shares)	0	0

Segment Information (Schedule O

Segment Information (Schedule Of Segment Reporting Information By Segment) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Revenues	$ 1,566	$ 1,440
Net income (loss) attributable to common shareholders	233	146
Capital expenditures	887	636
Wind generation expenditures	(193)	0
Union Electric Company
Segment Reporting Information [Line Items]
Revenues	704	680
Net income (loss) attributable to common shareholders	47	(10)
Wind generation expenditures	(193)	0
Ameren Illinois Company
Segment Reporting Information [Line Items]
Revenues	823	723
Net income (loss) attributable to common shareholders	149	120
Capital expenditures	337	324
Operating Segments | Union Electric Company
Segment Reporting Information [Line Items]
Revenues	695	670
Net income (loss) attributable to common shareholders	47	(10)
Capital expenditures	534	278
Operating Segments | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Revenues	411	389
Net income (loss) attributable to common shareholders	46	37
Capital expenditures	157	123
Operating Segments | Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
Revenues	347	271
Net income (loss) attributable to common shareholders	75	55
Capital expenditures	48	61
Operating Segments | Ameren Transmission
Segment Reporting Information [Line Items]
Revenues	113	110
Net income (loss) attributable to common shareholders	47	47
Capital expenditures	141	170
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Revenues	411	390
Net income (loss) attributable to common shareholders	46	37
Capital expenditures	157	123
Operating Segments | Ameren Illinois Company | Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
Revenues	347	271
Net income (loss) attributable to common shareholders	75	55
Capital expenditures	48	61
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Segment Reporting Information [Line Items]
Revenues	65	62
Net income (loss) attributable to common shareholders	28	28
Capital expenditures	132	140
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenue Not from Contract with Customer	(26)	(24)
Capital expenditures	6	1
Intersegment Eliminations | Union Electric Company
Segment Reporting Information [Line Items]
Revenue Not from Contract with Customer	(9)	(10)
Intersegment Eliminations | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Revenue Not from Contract with Customer	0	(1)
Intersegment Eliminations | Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
Revenue Not from Contract with Customer	0	0
Intersegment Eliminations | Ameren Transmission
Segment Reporting Information [Line Items]
Revenue Not from Contract with Customer	(17)	(13)
Intersegment Eliminations | Ameren Illinois Company
Segment Reporting Information [Line Items]
Revenues	(16)	(12)
Intersegment Eliminations | Ameren Illinois Company | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Revenues	0	0
Intersegment Eliminations | Ameren Illinois Company | Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
Revenues	0	0
Intersegment Eliminations | Ameren Illinois Company | Ameren Illinois Transmission
Segment Reporting Information [Line Items]
Revenues	(16)	(12)
Other
Segment Reporting Information [Line Items]
Net income (loss) attributable to common shareholders	18	17
Capital expenditures	$ 1	$ 3

Segment Information (Disaggrega

Segment Information (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 1,566	$ 1,440
Revenues	1,566	1,440
Revenues from alternative revenue programs	53	66
Other revenues not from contracts with customers	2	10
Union Electric Company
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(10)	(3)
Other revenues not from contracts with customers	(2)	8
Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	61	46
Other revenues not from contracts with customers	3	1
Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	3	11
Other revenues not from contracts with customers	1	1
Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(1)	12
Other revenues not from contracts with customers	0	0
Electricity
Disaggregation of Revenue [Line Items]
Revenues	1,156	1,120
Revenues	1,156	1,120
Electricity | Residential
Disaggregation of Revenue [Line Items]
Revenues	541	517
Electricity | Commercial
Disaggregation of Revenue [Line Items]
Revenues	348	347
Electricity | Industrial
Disaggregation of Revenue [Line Items]
Revenues	86	88
Electricity | Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	181	168
Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	410	320
Revenues	410	320
Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenues	285	246
Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenues	79	67
Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenues	15	4
Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	31	3
Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenues	823	723
Revenues	823	723
Revenues from alternative revenue programs	63	67
Other revenues not from contracts with customers	4	2
Ameren Illinois Company | Residential
Disaggregation of Revenue [Line Items]
Revenues	480	433
Ameren Illinois Company | Commercial
Disaggregation of Revenue [Line Items]
Revenues	196	180
Ameren Illinois Company | Industrial
Disaggregation of Revenue [Line Items]
Revenues	48	38
Ameren Illinois Company | Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	99	72
Ameren Illinois Company | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	61	46
Other revenues not from contracts with customers	3	1
Ameren Illinois Company | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	3	11
Other revenues not from contracts with customers	1	1
Ameren Illinois Company | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(1)	10
Other revenues not from contracts with customers	0	0
Ameren Illinois Company | Electricity
Disaggregation of Revenue [Line Items]
Revenues	476	452
Ameren Illinois Company | Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	347	271
Operating Segments | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenues	695	670
Revenues	704	680
Operating Segments | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	411	389
Revenues	411	390
Operating Segments | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	347	271
Revenues	347	271
Operating Segments | Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenues	113	110
Revenues	130	123
Operating Segments | Electricity | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenues	641	631
Operating Segments | Electricity | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
Revenues	312	297
Operating Segments | Electricity | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
Revenues	216	221
Operating Segments | Electricity | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
Revenues	52	53
Operating Segments | Electricity | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	61	60
Operating Segments | Electricity | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	411	390
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenues	229	220
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenues	132	126
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenues	34	35
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	16	9
Operating Segments | Electricity | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Electricity | Ameren Illinois Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Electricity | Ameren Illinois Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Electricity | Ameren Illinois Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Electricity | Ameren Illinois Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	0	0
Operating Segments | Electricity | Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenues	130	123
Operating Segments | Electricity | Ameren Transmission | Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Electricity | Ameren Transmission | Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Electricity | Ameren Transmission | Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Electricity | Ameren Transmission | Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	130	123
Operating Segments | Natural Gas | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenues	63	49
Operating Segments | Natural Gas | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
Revenues	34	33
Operating Segments | Natural Gas | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
Revenues	15	13
Operating Segments | Natural Gas | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
Revenues	1	1
Operating Segments | Natural Gas | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	13	2
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	0	0
Operating Segments | Natural Gas | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	347	271
Operating Segments | Natural Gas | Ameren Illinois Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenues	251	213
Operating Segments | Natural Gas | Ameren Illinois Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenues	64	54
Operating Segments | Natural Gas | Ameren Illinois Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenues	14	3
Operating Segments | Natural Gas | Ameren Illinois Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	18	1
Operating Segments | Natural Gas | Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Natural Gas | Ameren Transmission | Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Natural Gas | Ameren Transmission | Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Natural Gas | Ameren Transmission | Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Natural Gas | Ameren Transmission | Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	411	390
Operating Segments | Ameren Illinois Company | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	347	271
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues	65	62
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	411	390
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenues	229	220
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenues	132	126
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenues	34	35
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	16	9
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues	81	74
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission | Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission | Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission | Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission | Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	81	74
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	347	271
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenues	251	213
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenues	64	54
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenues	14	3
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	18	1
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	(26)	(24)
Revenues	(26)	(24)
Intersegment Eliminations | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	(9)	(10)
Intersegment Eliminations | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	0	(1)
Intersegment Eliminations | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	0	0
Intersegment Eliminations | Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	(17)	(13)
Intersegment Eliminations | Electricity
Disaggregation of Revenue [Line Items]
Revenues	(26)	(24)
Intersegment Eliminations | Electricity | Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0
Intersegment Eliminations | Electricity | Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0
Intersegment Eliminations | Electricity | Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0
Intersegment Eliminations | Electricity | Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	(26)	(24)
Intersegment Eliminations | Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	0	0
Intersegment Eliminations | Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0
Intersegment Eliminations | Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0
Intersegment Eliminations | Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0
Intersegment Eliminations | Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	0	0
Intersegment Eliminations | Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenues	(16)	(12)
Revenues	(16)	(12)
Intersegment Eliminations | Ameren Illinois Company | Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0
Intersegment Eliminations | Ameren Illinois Company | Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0
Intersegment Eliminations | Ameren Illinois Company | Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0
Intersegment Eliminations | Ameren Illinois Company | Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	(16)	(12)
Intersegment Eliminations | Ameren Illinois Company | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	0	0
Intersegment Eliminations | Ameren Illinois Company | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	0	0
Intersegment Eliminations | Ameren Illinois Company | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues	$ (16)	$ (12)