Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2020	Oct. 30, 2020
Entity Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2020
Document Transition Report	false
Entity File Number	1-14756
Entity Registrant Name	Ameren Corporation
Entity Tax Identification Number	43-1723446
Entity Incorporation, State or Country Code	MO
Entity Address, Address Line One	1901 Chouteau Avenue
Entity Address, City or Town	St. Louis
Entity Address, State or Province	MO
Entity Address, Postal Zip Code	63103
City Area Code	(314)
Local Phone Number	621-3222
Title of 12(b) Security	Common Stock, $0.01 par value per share
Trading Symbol(s)	AEE
Security Exchange Name	NYSE
Entity 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		247,206,978
Entity Central Index Key	0001002910
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Amendment Flag	false
Union Electric Company
Entity Information [Line Items]
Entity File Number	1-2967
Entity Registrant Name	Union Electric Company
Entity Tax Identification Number	43-0559760
Entity Incorporation, State or Country Code	MO
Entity Address, Address Line One	1901 Chouteau Avenue
Entity Address, City or Town	St. Louis
Entity Address, State or Province	MO
Entity Address, Postal Zip Code	63103
City Area Code	(314)
Local Phone Number	621-3222
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		102,123,834
Entity Central Index Key	0000100826
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Amendment Flag	false
Ameren Illinois Company
Entity Information [Line Items]
Entity File Number	1-3672
Entity Registrant Name	Ameren Illinois Company
Entity Tax Identification Number	37-0211380
Entity Incorporation, State or Country Code	IL
Entity Address, Address Line One	10 Executive Drive
Entity Address, City or Town	Collinsville
Entity Address, State or Province	IL
Entity Address, Postal Zip Code	62234
City Area Code	(618)
Local Phone Number	343-8150
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		25,452,373
Entity Central Index Key	0000018654
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Amendment Flag	false

Consolidated Statement of Income (Loss) and Comprehensive Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,628	$ 1,659	$ 4,466	$ 4,594
Operating Expenses:
Fuel	141	147	400	409
Purchased Power	140	148	383	440
Natural gas purchased for resale	34	31	183	236
Other operations and maintenance	418	434	1,240	1,301
Depreciation and amortization	273	248	799	745
Taxes other than income taxes	128	131	372	375
Total operating expenses	1,134	1,139	3,377	3,506
Operating Income	494	520	1,089	1,088
Other Income (Expense), Net	48	34	117	99
Interest Charges	110	96	311	290
Income Before Income Taxes	432	458	895	897
Income Taxes	63	92	134	158
Net income	369	366	761	739
Less: Net Income Attributable to Noncontrolling Interests	2	2	5	5
Net income attributable to Ameren common shareholders	367	364	756	734
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	1	0	2	1
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	370	366	763	740
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	2	2	5	5
Comprehensive Income	$ 368	$ 364	$ 758	$ 735
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic	$ 1.48	$ 1.48	$ 3.06	$ 2.99
Earnings Per Share, Diluted	$ 1.47	$ 1.47	$ 3.04	$ 2.97
Average Common Shares Outstanding - Basic (in shares)	247.1	245.9	246.8	245.5
Weighted Average Number of Shares Outstanding, Diluted (in shares)	249.2	247.5	248.4	247.0
Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,489	$ 1,528	$ 3,846	$ 3,928
Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	139	131	620	666
Union Electric Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	1,001	1,059	2,473	2,615
Operating Expenses:
Fuel	141	147	400	409
Purchased Power	48	49	124	160
Natural gas purchased for resale	5	6	29	41
Other operations and maintenance	221	242	662	720
Depreciation and amortization	154	138	448	417
Taxes other than income taxes	92	96	254	256
Total operating expenses	661	678	1,917	2,003
Operating Income	340	381	556	612
Other Income (Expense), Net	26	15	55	43
Interest Charges	50	44	140	136
Income Before Income Taxes	316	352	471	519
Income Taxes	18	51	29	70
Net income	298	301	442	449
Preferred Stock Dividends	1	1	3	3
Net Income (Loss) Attributable to Parent	297	300	439	446
Union Electric Company \| Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	984	1,040	2,386	2,517
Union Electric Company \| Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	17	19	87	98
Ameren Illinois Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	589	564	1,879	1,873
Operating Expenses:
Purchased Power	97	101	271	284
Natural gas purchased for resale	29	25	154	195
Other operations and maintenance	197	193	578	580
Depreciation and amortization	109	102	323	304
Taxes other than income taxes	33	33	107	110
Total operating expenses	465	454	1,433	1,473
Operating Income	124	110	446	400
Other Income (Expense), Net	17	13	45	39
Interest Charges	39	38	116	111
Income Before Income Taxes	102	85	375	328
Income Taxes	25	20	93	79
Net income	77	65	282	249
Preferred Stock Dividends	0	0	2	2
Net Income (Loss) Attributable to Parent	77	65	280	247
Ameren Illinois Company \| Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	467	452	1,346	1,305
Ameren Illinois Company \| Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 122	$ 112	$ 533	$ 568

Consolidated Statement of Income (Loss) and Comprehensive Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Pension and other postretirement benefit plan activity, tax	$ 0	$ 0	$ 0	$ 0

Consolidated Balance Sheet - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Current Assets:
Cash and cash equivalents	$ 6	$ 16
Accounts receivable - trade (less allowance for doubtful accounts)	525	393
Unbilled revenue	221	278
Miscellaneous accounts receivable	71	63
Inventories	557	494
Other current assets	210	187
Total current assets	1,590	1,431
Property, Plant, and Equipment, Net	25,541	24,376
Investments and Other Assets:
Nuclear decommissioning trust fund	904	847
Goodwill	411	411
Regulatory assets	1,078	992
Other assets	993	876
Total investments and other assets	3,386	3,126
TOTAL ASSETS	30,517	28,933
Current Liabilities:
Current maturities of long-term debt	357	442
Short-term debt	272	440
Accounts and wages payable	640	874
Taxes accrued	199	37
Customer deposits	109	111
Current regulatory liabilities	96	164
Other current liabilities	432	437
Total current liabilities	2,105	2,505
Long-term Debt, Net	10,172	8,915
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes and investment tax credits, net	3,151	2,919
Regulatory liabilities	4,972	4,887
Asset retirement obligations	675	638
Pension and other postretirement benefits	373	401
Other deferred credits and liabilities	438	467
Total deferred credits and other liabilities	9,609	9,312
Commitments and Contingencies
Shareholders’ Equity:
Common Stock	2	2
Other paid-in capital, principally premium on common stock	5,733	5,694
Retained earnings	2,769	2,380
Accumulated other comprehensive loss	(15)	(17)
Total shareholders’ equity	8,489	8,059
Noncontrolling Interests	142	142
Total equity	8,631	8,201
TOTAL LIABILITIES AND EQUITY	30,517	28,933
Union Electric Company
Current Assets:
Cash and cash equivalents	0	9
Advances to money pool	5	0
Accounts receivable - trade (less allowance for doubtful accounts)	263	164
Accounts receivable – affiliates	12	30
Unbilled revenue	117	139
Miscellaneous accounts receivable	44	33
Inventories	408	373
Other current assets	119	66
Total current assets	968	814
Property, Plant, and Equipment, Net	12,991	12,635
Investments and Other Assets:
Nuclear decommissioning trust fund	904	847
Regulatory assets	310	285
Other assets	350	356
Total investments and other assets	1,564	1,488
TOTAL ASSETS	15,523	14,937
Current Liabilities:
Current maturities of long-term debt	7	92
Short-term debt	0	234
Accounts and wages payable	256	465
Accounts payable – affiliates	70	52
Taxes accrued	164	24
Interest accrued	59	48
Current asset retirement obligations	53	53
Current regulatory liabilities	40	62
Other current liabilities	102	96
Total current liabilities	751	1,126
Long-term Debt, Net	4,561	4,098
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes and investment tax credits, net	1,689	1,612
Regulatory liabilities	2,898	2,937
Asset retirement obligations	670	634
Pension and other postretirement benefits	129	141
Other deferred credits and liabilities	37	40
Total deferred credits and other liabilities	5,423	5,364
Commitments and Contingencies
Shareholders’ Equity:
Common Stock	511	511
Other paid-in capital, principally premium on common stock	2,027	2,027
Preferred stock	80	80
Retained earnings	2,170	1,731
Total shareholders’ equity	4,788	4,349
TOTAL LIABILITIES AND EQUITY	15,523	14,937
Ameren Illinois Company
Current Assets:
Cash and cash equivalents	0	0
Accounts receivable - trade (less allowance for doubtful accounts)	247	215
Accounts receivable – affiliates	15	28
Unbilled revenue	104	139
Miscellaneous accounts receivable	24	25
Inventories	149	121
Current regulatory assets	29	57
Other current assets	31	29
Total current assets	599	614
Property, Plant, and Equipment, Net	10,861	10,083
Investments and Other Assets:
Goodwill	411	411
Regulatory assets	748	694
Other assets	480	383
Total investments and other assets	1,639	1,488
TOTAL ASSETS	13,099	12,185
Current Liabilities:
Short-term debt	242	53
Accounts and wages payable	318	299
Accounts payable – affiliates	43	82
Customer deposits	80	77
Current regulatory liabilities	47	84
Other current liabilities	234	249
Total current liabilities	964	844
Long-term Debt, Net	3,576	3,575
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes and investment tax credits, net	1,314	1,224
Regulatory liabilities	1,968	1,849
Pension and other postretirement benefits	204	214
Environmental remediation	66	87
Other deferred credits and liabilities	245	260
Total deferred credits and other liabilities	3,797	3,634
Commitments and Contingencies
Shareholders’ Equity:
Common Stock	0	0
Other paid-in capital, principally premium on common stock	2,538	2,188
Preferred stock	62	62
Retained earnings	2,162	1,882
Total shareholders’ equity	4,762	4,132
TOTAL LIABILITIES AND EQUITY	$ 13,099	$ 12,185

Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Accounts Receivable, Allowance for Credit Loss, Current	$ 44	$ 17
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	400,000,000.0	400,000,000.0
Common Stock, Shares, Outstanding	247,200,000	246,200,000
Union Electric Company
Accounts Receivable, Allowance for Credit Loss, Current	$ 14	$ 7
Common stock, par value (in dollars per share)	$ 5,000,000	$ 5,000,000
Common stock, shares authorized (in shares)	150,000,000.0	150,000,000.0
Common Stock, Shares, Outstanding	102,100,000	102,100,000
Ameren Illinois Company
Accounts Receivable, Allowance for Credit Loss, Current	$ 30	$ 10
Common stock, no par value (in dollars per share)	$ 0	$ 0
Common stock, shares authorized (in shares)	45,000,000.0	45,000,000.0
Common Stock, Shares, Outstanding	25,500,000	25,500,000

Consolidated Statement of Cash Flows - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Cash Flows From Operating Activities:
Net income	$ 761	$ 739
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	802	745
Amortization of nuclear fuel	68	56
Amortization of debt issuance costs and premium/discounts	16	14
Deferred income taxes and investment tax credits, net	125	144
Allowance for equity funds used during construction	(25)	(20)
Stock-based compensation costs	16	15
Other	14	(11)
Changes in assets and liabilities:
Receivables	(113)	10
Inventories	(61)	(4)
Accounts and wages payable	(190)	(205)
Taxes accrued	154	118
Regulatory assets and liabilities	(55)	147
Assets, other	(66)	(56)
Liabilities, other	(76)	11
Pension and other postretirement benefits	(41)	(35)
Net cash provided by operating activities	1,329	1,668
Cash Flows From Investing Activities:
Capital expenditures	(1,884)	(1,761)
Nuclear fuel expenditures	(61)	(26)
Purchases of securities – nuclear decommissioning trust fund	(169)	(192)
Sales and maturities of securities – nuclear decommissioning trust fund	135	184
Purchase of bonds	0	(207)
Proceeds from sale of remarketed bonds	0	207
Other	(2)	(3)
Net cash used in investing activities	(1,981)	(1,798)
Cash Flows From Financing Activities:
Dividends on common stock	(367)	(350)
Dividends paid to noncontrolling interest holders	(5)	(5)
Short-term debt, net	(168)	(53)
Maturities of long-term debt	(85)	(329)
Issuances of long-term debt	1,263	900
Issuances of common stock	37	54
Employee payroll taxes related to stock-based compensation	(20)	(29)
Debt issuance costs	(11)	(10)
Net cash provided by financing activities	644	178
Net change in cash, cash equivalents, and restricted cash	(8)	48
Cash, cash equivalents, and restricted cash at beginning of year	176	107
Cash, cash equivalents, and restricted cash at end of period	168	155
Union Electric Company
Cash Flows From Operating Activities:
Net income	442	449
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	452	419
Amortization of nuclear fuel	68	56
Amortization of debt issuance costs and premium/discounts	4	4
Deferred income taxes and investment tax credits, net	(1)	(9)
Allowance for equity funds used during construction	(15)	(14)
Other	13	10
Changes in assets and liabilities:
Receivables	(93)	(32)
Inventories	(33)	3
Accounts and wages payable	(168)	(153)
Taxes accrued	158	148
Regulatory assets and liabilities	(56)	5
Assets, other	(10)	(37)
Liabilities, other	(46)	(4)
Pension and other postretirement benefits	(6)	(5)
Net cash provided by operating activities	709	840
Cash Flows From Investing Activities:
Capital expenditures	(778)	(751)
Nuclear fuel expenditures	(61)	(26)
Purchases of securities – nuclear decommissioning trust fund	(169)	(192)
Sales and maturities of securities – nuclear decommissioning trust fund	135	184
Purchase of bonds	0	(207)
Proceeds from sale of remarketed bonds	0	207
Money pool advances, net	5	0
Other	1	0
Net cash used in investing activities	(877)	(785)
Cash Flows From Financing Activities:
Dividends on common stock	0	(250)
Dividends on preferred stock	(3)	(3)
Short-term debt, net	(234)	89
Maturities of long-term debt	(85)	(329)
Issuances of long-term debt	465	450
Debt issuance costs	(4)	(6)
Net cash provided by financing activities	139	(49)
Net change in cash, cash equivalents, and restricted cash	(29)	6
Cash, cash equivalents, and restricted cash at beginning of year	39	8
Cash, cash equivalents, and restricted cash at end of period	10	14
Ameren Illinois Company
Cash Flows From Operating Activities:
Net income	282	249
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	322	303
Amortization of debt issuance costs and premium/discounts	9	9
Deferred income taxes and investment tax credits, net	72	42
Allowance for equity funds used during construction	(10)	(6)
Other	0	8
Changes in assets and liabilities:
Receivables	(9)	18
Inventories	(28)	(7)
Accounts and wages payable	(12)	(48)
Taxes accrued	(20)	14
Regulatory assets and liabilities	11	147
Assets, other	(52)	(15)
Liabilities, other	(30)	13
Pension and other postretirement benefits	(30)	(27)
Net cash provided by operating activities	515	706
Cash Flows From Investing Activities:
Capital expenditures	(1,031)	(900)
Other	1	(3)
Net cash used in investing activities	(1,030)	(903)
Cash Flows From Financing Activities:
Dividends on preferred stock	(2)	(2)
Short-term debt, net	189	237
Capital contributions from parent	350	0
Other	0	(1)
Net cash provided by financing activities	537	234
Net change in cash, cash equivalents, and restricted cash	22	37
Cash, cash equivalents, and restricted cash at beginning of year	125	80
Cash, cash equivalents, and restricted cash at end of period	$ 147	$ 117

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 Company Common Stock	Union Electric Company Other Paid-in Capital	Union Electric Company Preferred Stock	Union Electric Company Retained Earnings	Ameren Illinois Company	Ameren Illinois Company Common Stock	Ameren Illinois Company Other Paid-in Capital	Ameren Illinois Company Preferred Stock	Ameren Illinois Company Retained Earnings
Beginning of period at Dec. 31, 2018			$ 5,627	$ 2,024	$ (22)		$ 142						$ 1,735			$ 2,173		$ 1,539
Shares issued under the DRPlus and 401(k) plan			54
Stock-based compensation activity			(8)
Capital contribution from parent														$ 0		0
Net income	$ 739								$ 449				449	249				249
Net income attributable to Ameren common shareholders	734			734
Common stock dividends				(350)									(250)
Preferred stock dividends													(3)					(2)
Change in deferred retirement benefit costs	1				1
Net income attributable to noncontrolling interest holders	$ (5)						5
Dividends paid to noncontrolling interest holders							(5)
Common stock shares outstanding at beginning of year at Dec. 31, 2018	244,500,000
Shares issued under the DRPlus and 401(k) plan	700,000
Shares issued for stock-based compensation	800,000
Common stock shares outstanding at end of period at Sep. 30, 2019	246,000,000.0
End of period at Sep. 30, 2019	$ 8,204	$ 2	5,673	2,408	(21)	$ (21)	142			$ 511	$ 1,903	$ 80	1,931		$ 0	2,173	$ 62	1,786
Dividends per common share (in dollars per share)	$ 1.4250
Shareholders' equity, end of year at Sep. 30, 2019								$ 8,062	4,425					4,021
Beginning of period at Jun. 30, 2019			5,649	2,161	(21)		142						1,781			2,173		1,721
Shares issued under the DRPlus and 401(k) plan			17
Stock-based compensation activity			7
Capital contribution from parent																0
Net income	$ 366								301				301	65				65
Net income attributable to Ameren common shareholders	364			364
Common stock dividends				(117)									(150)
Preferred stock dividends													(1)					0
Change in deferred retirement benefit costs	0				0
Net income attributable to noncontrolling interest holders	$ (2)						2
Dividends paid to noncontrolling interest holders							(2)
Common stock shares outstanding at beginning of year at Jun. 30, 2019	245,800,000
Shares issued under the DRPlus and 401(k) plan	200,000
Shares issued for stock-based compensation	0
Common stock shares outstanding at end of period at Sep. 30, 2019	246,000,000.0
End of period at Sep. 30, 2019	$ 8,204	2	5,673	2,408	(21)	(21)	142			511	1,903	80	1,931		0	2,173	62	1,786
Dividends per common share (in dollars per share)	$ 0.4750
Shareholders' equity, end of year at Sep. 30, 2019								8,062	4,425					4,021
Beginning of period at Dec. 31, 2019	$ 8,201		5,694	2,380	(17)		142						1,731			2,188		1,882
Shares issued under the DRPlus and 401(k) plan			37
Stock-based compensation activity			2
Capital contribution from parent														(350)		350
Net income	761								$ 442				442	$ 282				282
Net income attributable to Ameren common shareholders	756			756
Common stock dividends				(367)									0
Preferred stock dividends													(3)					(2)
Change in deferred retirement benefit costs	2				2
Net income attributable to noncontrolling interest holders	$ (5)						5
Dividends paid to noncontrolling interest holders							(5)
Common stock shares outstanding at beginning of year at Dec. 31, 2019	246,200,000								102,100,000					25,500,000
Shares issued under the DRPlus and 401(k) plan	500,000
Shares issued for stock-based compensation	500,000
Common stock shares outstanding at end of period at Sep. 30, 2020	247,200,000								102,100,000					25,500,000
End of period at Sep. 30, 2020	$ 8,631	2	5,733	2,769	(15)	(15)	142			511	2,027	80	2,170		0	2,538	62	2,162
Dividends per common share (in dollars per share)	$ 1.4850
Shareholders' equity, end of year at Sep. 30, 2020	$ 8,489							8,489	$ 4,788					$ 4,762
Beginning of period at Jun. 30, 2020			5,716	2,525	(16)		142						1,873			2,538		2,085
Shares issued under the DRPlus and 401(k) plan			10
Stock-based compensation activity			7
Capital contribution from parent																0
Net income	369								$ 298				298	$ 77				77
Net income attributable to Ameren common shareholders	367			367
Common stock dividends				(123)									0
Preferred stock dividends													(1)					0
Change in deferred retirement benefit costs	1				1
Net income attributable to noncontrolling interest holders	$ (2)						2
Dividends paid to noncontrolling interest holders							(2)
Common stock shares outstanding at beginning of year at Jun. 30, 2020	247,100,000
Shares issued under the DRPlus and 401(k) plan	100,000
Shares issued for stock-based compensation	0
Common stock shares outstanding at end of period at Sep. 30, 2020	247,200,000								102,100,000					25,500,000
End of period at Sep. 30, 2020	$ 8,631	$ 2	$ 5,733	$ 2,769	$ (15)	$ (15)	$ 142			$ 511	$ 2,027	$ 80	$ 2,170		$ 0	$ 2,538	$ 62	$ 2,162
Dividends per common share (in dollars per share)	$ 0.4950
Shareholders' equity, end of year at Sep. 30, 2020	$ 8,489							$ 8,489	$ 4,788					$ 4,762

Summary Of Significant Accounting Policies	9 Months Ended
Summary Of Significant Accounting Policies	Sep. 30, 2020
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 be a rapidly evolving situation. In the first nine months of 2020, we experienced a net decrease in our sales volumes, an increase in our accounts receivable balances that were past due or that were a part of a deferred payment arrangement, and a decline in our cash collections from customers. The continued effect of the COVID-19 pandemic on our results of operations, financial position, and liquidity in subsequent periods will depend on its severity and longevity, future regulatory or legislative actions with respect thereto, and the resulting impact on business, economic, and capital market conditions. Shelter-in-place orders began taking effect in our service territories in mid-March 2020. These orders generally required individuals to remain at home and precluded or limited the operation of businesses that were deemed nonessential. While our business operations were deemed essential and were not directly impacted by the shelter-in-place orders, approximately 65% of our workforce transitioned to remote working arrangements in mid-March 2020. In early June 2020, a small portion of our workforce began the process of returning to our work locations under a phased approach and approximately 50% of our workforce continues to work remotely. In mid-May 2020, shelter-in-place orders effective in our service territories began to be relaxed, with fewer restrictions on social activities and nonessential businesses beginning to reopen. However, certain restrictions remain in place that limit individual activities and the operation of nonessential businesses. Additional restrictions may be imposed in the future. We continue to assess the impacts the pandemic is having on our businesses, including but not limited to impacts on our liquidity; demand for residential, commercial, and industrial electric and natural gas services; more flexible payment plans for customers; the timing and extent to which recovery of incremental costs incurred, net of savings, and forgone customer late fee revenues at Ameren Missouri is allowed by the MoPSC; changes in our ability to disconnect customers for nonpayment; bad debt expense; supply chain operations; the availability of our employees and contractors; counterparty credit; capital construction; infrastructure operations and maintenance; energy-efficiency programs; and pension valuations. While the revenues from Ameren Illinois’ electric distribution business, residential and small nonresidential customers of Ameren Illinois’ natural gas distribution business, and Ameren Illinois’ and ATXI’s electric transmission businesses are decoupled from changes in sales volumes, 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. Pursuant to a June 2020 ICC order, Ameren Illinois’ electric bad debt rider provides for the recovery of bad debt expense in 2020. Ameren Missouri does not have a bad debt rider or regulatory tracking mechanism and its earnings are exposed to increases in bad debt expense. In October 2020, Ameren Missouri filed requests with the MoPSC for accounting authority orders related to costs incurred, net of savings, and forgone customer late fee revenues resulting from the COVID-19 pandemic. Our customers’ payment for our services has been adversely affected by the COVID-19 pandemic, resulting in a decrease to our cash flow from operations. As of September 30, 2020, accounts receivable balances that were 30 days or greater past due or that were a part of a deferred payment arrangement represented 26%, 18%, and 35%, or $151 million, $49 million, and $102 million, of Ameren’s, Ameren Missouri’s, and Ameren Illinois’ customer trade receivables before allowance for doubtful accounts, respectively. As of September 30, 2019, these percentages were 13%, 9%, and 18%, or $65 million, $23 million, and $42 million, for Ameren, Ameren Missouri, and Ameren Illinois, respectively. For information regarding Ameren Missouri’s and Ameren Illinois’ suspension and subsequent reinstatement of customer disconnections and late fee charges for nonpayment and Ameren Missouri’s requests for accounting authority orders related to the COVID-19 pandemic, see Note 2 – Rate and Regulatory Matters below. The Coronavirus Aid, Relief, and Economic Security Act is a federal law enacted in March 2020. Provisions in the act include temporary changes to the utilization of net operating losses, deferral of the payment of the employer portion of Social Security taxes, and additional funding for customer energy assistance, among other things. As of September 30, 2020, the implementation of the act by the Ameren Companies had no material impact to their financial statements. Ameren’s financial statements are prepared on a consolidated basis and therefore include the accounts of its majority-owned subsidiaries. All intercompany transactions have been eliminated, except as disclosed in Note 8 – Related-party Transactions. Ameren Missouri and Ameren Illinois have no subsidiaries. All tabular dollar amounts are in millions, unless otherwise indicated. Our accounting policies conform to GAAP. Our financial statements reflect all adjustments (which include normal, recurring adjustments) that are necessary, in our opinion, for a fair 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 September 30, 2020, Ameren and Ameren Missouri had interests in unconsolidated variable interest entities that were established to construct wind generation facilities and, ultimately, sell those constructed facilities to Ameren Missouri. Neither Ameren nor Ameren Missouri are the primary beneficiary of these variable interest entities because neither has the power to direct matters that most significantly affect the entities' activities, which include designing, financing, and constructing the wind generation facilities. As a result, these variable interest entities have not been consolidated. As of September 30, 2020, the maximum exposure to loss related to these variable interest entities was approximately $18 million, which primarily represents due diligence and legal costs incurred by Ameren Missouri associated with the acquisitions. The risk of a loss was assessed to be remote and, accordingly, Ameren and Ameren Missouri have not recognized a liability associated with any portion of the maximum exposure to loss. See Note 2 – Rate and Regulatory Matters for additional information on the agreements to acquire these wind generation facilities. As of September 30, 2020, and December 31, 2019, Ameren had unconsolidated variable interests as a limited partner in various equity method investments, totaling $34 million and $28 million, respectively, included in “Other assets” on Ameren’s consolidated balance sheet. Ameren is not the primary beneficiary of these investments because it does not have the power to direct matters that most significantly affect the activities of these variable interest entities. As of September 30, 2020, the maximum exposure to loss related to these variable interests is limited to the investment in these partnerships of $34 million plus associated outstanding funding commitments of $37 million. Company-owned Life Insurance Ameren and Ameren Illinois have company-owned life insurance, which is recorded at the net cash surrender value. The net cash surrender value is the amount that can be realized under the insurance policies at the balance sheet date. As of September 30, 2020, the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $260 million (December 31, 2019 – $264 million) and $114 million (December 31, 2019 – $123 million), respectively, while total borrowings against the policies were $107 million (December 31, 2019 – $114 million) at both Ameren and Ameren Illinois. Ameren and Ameren Illinois have the right to offset the borrowings against the cash surrender value of the policies and, consequently, present the net asset in “Other assets” on their respective balance sheets. Accounting and Reporting Developments See Note 1 – Summary of Significant Accounting Policies under Part II, Item 8, of the Form 10-K for additional information about authoritative accounting guidance relating to defined benefit plan disclosures that will be effective for the Ameren Companies in the fourth quarter of 2020. Measurement of Credit Losses on Financial Instruments On January 1, 2020, the Ameren Companies adopted authoritative accounting guidance that requires credit losses on most financial assets carried at amortized cost and off-balance sheet credit exposures, such as financial guarantees or loan commitments, to be measured using a current expected credit loss (CECL) model. The guidance requires an entity to measure expected credit losses using relevant information about past events, current conditions, and reasonable and supportable forecasts that affect the collectibility of the reported amount. In addition, the guidance made certain changes to the impairment model applicable to available-for-sale debt securities, such as requiring credit losses to be presented as an allowance rather than a write-down on impaired debt securities for which there is neither an intent nor a more-likely-than-not requirement to sell. Our adoption of this guidance did not have a material impact on the Ameren Companies’ financial statements and did not result in a cumulative effect adjustment to retained earnings as of the adoption date. See Note 13 – Supplemental Information for additional information regarding credit losses on accounts receivable.

Rate And Regulatory Matters	9 Months Ended
Rate And Regulatory Matters	Sep. 30, 2020
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 the final decisions of the various agencies and courts, or the impact on our results of operations, financial position, or liquidity. Missouri March 2020 MoPSC Electric Rate Order In March 2020, the MoPSC issued an order in Ameren Missouri’s July 2019 electric service regulatory rate review, approving nonunanimous stipulation and agreements. The order resulted in a decrease of $32 million to Ameren Missouri's annual revenue requirement for electric retail service, which reflects infrastructure investments as of December 31, 2019. The order also provided for the continued use of the FAC and trackers for pension and postretirement benefits, uncertain income tax positions, and certain excess deferred income taxes that the MoPSC previously authorized in earlier electric rate orders. The order reduced the annualized base level of net energy costs pursuant to the FAC by approximately $115 million from the base level established in the MoPSC’s March 2017 electric rate order. The order also changed the annualized regulatory asset and liability amortization amounts and the base level of expenses for regulatory tracking mechanisms. On an annualized basis, these changes reflect approximately $20 million of increased revenues and approximate decreases in purchased power expenses of $15 million, other operating and maintenance expenses of $60 million, and income tax expenses of $20 million. Additionally, the annual revenue requirement incorporated increases of approximately $50 million for the reduction in sales volumes resulting from MEEIA programs and approximately $50 million of depreciation and amortization expense for amounts previously deferred under PISA. The increase in the annual revenue requirement related to the MEEIA programs is seasonally weighted to the summer. One of the stipulation and agreements approved by the MoPSC’s March 2020 order states that the net impact of the revenue and expense changes noted above reflects a 9.4% to 9.8% ROE on an unspecified percent of common equity applicable to rate base. In addition, the order required Ameren Missouri to donate $8 million to low-income assistance programs, which was reflected in results of operations in the first quarter of 2020. The new rates, base level of expenses, and amortizations became effective on April 1, 2020. In April 2020, the MoPSC issued another order in Ameren Missouri’s July 2019 electric service regulatory rate review, reaffirming the existing percentage of net energy cost variances allowed to be recovered or refunded under the FAC. Wind Generation Facilities In 2019, Ameren Missouri entered into a build-transfer agreement to acquire, after construction, an up-to 300-megawatt wind generation facility. In 2018, Ameren Missouri entered into a build-transfer agreement to acquire, after construction, an up-to 400-megawatt wind generation facility. These two agreements are subject to customary contract terms and conditions. The two build-transfer acquisitions collectively represent $1.2 billion of capital expenditures and would support Ameren Missouri’s compliance with the Missouri renewable energy standard. Both acquisitions have received all regulatory approvals, and both projects have received all applicable zoning approvals, have entered into RTO interconnection agreements, and are under construction. In 2020, the developers of the wind generation facilities received notices from the wind turbine supplier to each facility, and the developer of the up-to 300-megawatt project received a notice from the construction contractor, of changes in supply and/or construction activities resulting from the COVID-19 pandemic. There have been changes to the schedules for both projects, particularly with regard to wind turbine deliveries. During the third quarter of 2020, all remaining wind turbine deliveries for the up-to 400-megawatt project were completed. Based on the construction schedule, Ameren Missouri expects this project to be placed in-service by the end of 2020. At this time, due to manufacturing, shipping, and other supply chain issues in 2020, and, based on an updated construction schedule from the developer, Ameren Missouri expects the up-to 300-megawatt project to be partially placed in-service by the end of 2020, and the remaining portion of the project, representing approximately $200 million of investment, to be placed in-service in the first quarter of 2021. Ameren Missouri and the developer of the up-to 300-megawatt project continue to monitor the impact to this project's schedule. In May 2020, the IRS issued guidance that extended the in-service date criteria to December 31, 2021, for qualifying for federal production tax credits. As a result of this extension, Ameren does not anticipate that delays in the completion of the wind generation facilities will affect Ameren’s ability to realize anticipated federal production tax credits. MEEIA In August 2020, the MoPSC issued an order approving a unanimous stipulation and agreement with respect to the 2022 program year of Ameren Missouri’s six-year MEEIA 2019 program. The order established performance incentives that would provide Ameren Missouri an opportunity to earn additional revenues, including $11 million if Ameren Missouri achieves certain energy-efficiency goals during the 2022 program year and an additional $1 million if Ameren Missouri exceeds its targeted energy-efficiency goals. Ameren Missouri intends to invest $70 million in energy-efficiency programs during the 2022 program year. The August 2020 order also approved Ameren Missouri’s energy savings results for the first year of the MEEIA 2019 program. As a result of this order and in accordance with revenue recognition guidance, Ameren Missouri recognized revenues of $6 million in the third quarter of 2020. As a result of MoPSC orders issued in September 2017, October 2018, January 2019, and September 2019 related to performance incentives for the MEEIA 2013 and MEEIA 2016 programs, and in accordance with revenue recognition guidance, Ameren Missouri recognized revenues of $38 million during the first nine months of 2019. Requests for Accounting Authority Orders Related to COVID-19 Pandemic Costs Ameren Missouri suspended disconnections for customer nonpayment and charging late fees in mid-March 2020, and resumed those activities for commercial and industrial customers in mid-July 2020 and residential customers in early August 2020. In October 2020, Ameren Missouri filed requests with the MoPSC for accounting authority orders related to its electric and natural gas services. The orders would allow Ameren Missouri to accumulate certain costs incurred related to the COVID-19 pandemic and forgone customer late fee revenues from March 2020 to June 2021, for potential recovery in future electric and natural gas service regulatory rate reviews. These costs would be net of any cost savings Ameren Missouri realizes as a result of the COVID-19 pandemic. The orders would also allow Ameren Missouri to accumulate bad debt write-offs incurred from March 2020 to September 2021 due to the COVID-19 pandemic, for potential recovery in future electric and natural gas service regulatory rate reviews. The requests include an estimated $9 million of costs incurred, net of savings, and forgone customer late fee revenues related to the COVID-19 pandemic from March 2020 through September 2020. The requests did not seek accumulation for potential recovery of forgone revenues associated with decreased sales volumes related to the COVID-19 pandemic. The MoPSC is under no deadline to issue orders, and Ameren Missouri cannot predict the ultimate outcome of these regulatory proceedings. Illinois Electric Distribution Service Rates In April 2020, Ameren Illinois filed its annual electric distribution service formula rate update with the ICC, requesting a reduction of $45 million in its rates. This update reflects a decrease to the annual formula rate based on 2019 actual costs, a decrease to include the 2019 revenue requirement reconciliation adjustment, and a decrease for the conclusion of the 2018 revenue requirement reconciliation adjustment, which will be fully collected from customers in 2020, consistent with the ICC’s December 2019 annual update filing order. It also reflects an increase based on expected net plant additions for 2020. In September 2020, the ICC staff submitted its revised calculation of the revenue requirement included in Ameren Illinois’ update filing, recommending a $49 million decrease in Ameren Illinois’ electric distribution service rates. An ICC decision in this proceeding is expected by December 2020, with new rates effective January 2021. Electric Customer Energy-Efficiency Investments In May 2020, Ameren Illinois filed its annual electric customer energy-efficiency formula rate update to increase its rates by $7 million with the ICC. In August 2020, the ICC staff submitted a calculation of the revenue requirement included in Ameren Illinois’ filing, recommending an amount comparable to that included in Ameren Illinois’ filing. An ICC decision in this proceeding is expected by December 2020, with new rates effective January 2021. 2020 Natural Gas Delivery Service Regulatory Rate Review In February 2020, Ameren Illinois filed a request with the ICC seeking approval to increase its annual revenues for natural gas delivery service. In September 2020, Ameren Illinois filed a revised request seeking to increase its annual revenues for natural gas delivery service by $97 million, which includes an estimated $46 million of annual revenues that would otherwise be recovered under the QIP and other riders. The request is based on a 10.5% ROE, a capital structure composed of 54.1% common equity, and a rate base of $2.1 billion. Ameren Illinois used a 2021 future test year in this proceeding. In October 2020, the ICC staff recommended an increase to annual revenues for natural gas delivery service of $69 million, based on a 9.3% ROE, a capital structure composed of 50.4% common equity, and a rate base of $2.1 billion. A decision by the ICC in this proceeding is required by January 2021, with new rates expected to be effective in February 2021. Ameren Illinois cannot predict the level of any delivery service rate change the ICC may approve, nor whether any rate change that may eventually be approved will be sufficient to enable Ameren Illinois to earn a reasonable return on investments when the rate changes go into effect. QIP Reconciliation Hearing In March 2019, Ameren Illinois filed its annual request with the ICC for a reconciliation hearing to determine the accuracy and prudence of natural gas capital investments recovered under the QIP rider during 2018. In November 2019, the Illinois Attorney General's office challenged the recovery of capital investments, among other things, that were made during 2018, alleging that the amount of investments is excessive based on a comparison to historical investment levels. The Illinois Attorney General's office is not alleging project imprudence or that the investments do not qualify for recovery. In March 2020, the ICC staff filed testimony that supports the prudence and reasonableness of the capital investments made during 2018. Ameren Illinois’ 2018 QIP rate recovery request under review by the ICC is within the rate increase limitations allowed by law. The ICC is under no deadline to issue an order in this proceeding. Service Disconnection Moratorium Proceeding In March 2020, the ICC issued an order requiring all Illinois electric distribution and natural gas utilities to suspend disconnections and late fees for customer nonpayment, on an interim basis, effective March 18, 2020. Pursuant to an ICC order issued in June 2020 and a voluntary extension of the suspension of residential disconnections, Ameren Illinois resumed disconnection activities for commercial and industrial customers for nonpayment in early August 2020 and residential customers in mid-September 2020, with the exception of residential customers classified as low income, expressing a financial hardship, or relying on medical equipment. Disconnections for nonpayment for these residential customers are expected to begin in April 2021, which is after the annual winter moratorium period on disconnections from December 1, 2020 to March 31, 2021. Ameren Illinois also resumed charging late fees to all customers in late July 2020. The June 2020 order requires Ameren Illinois to implement more flexible credit and collection practices, on a temporary basis, including longer deferred payment arrangements, extending to 24 months in certain cases, and programs to provide financial assistance to customers. In addition, the order allows Ameren Illinois to recover up to $8 million in costs incurred related to the financial assistance programs. 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 established by the order. The order also allows Ameren Illinois to recover forgone customer late fees and costs incurred related to the COVID-19 pandemic. The portion of these forgone late fees and costs associated with Ameren Illinois’ electric distribution business will be recovered through formula rates and the portion associated with its natural gas distribution business will be recovered through the special purpose rider. In addition, the order allows Ameren Illinois’ electric distribution business to recover bad debt expense, instead of write-offs net of subsequent recoveries, through its bad debt rider until the end of 2020. Federal Transmission Formula Rate Revisions In February 2020, the MISO, on behalf of Ameren Missouri, Ameren Illinois, and ATXI, filed requests with the FERC to revise each company’s transmission formula rate calculations with respect to calculation inputs for materials and supplies. In May 2020, the FERC issued orders approving the revisions prospectively. In addition, the FERC declined to order refunds for earlier periods, as requested by intervenors in Ameren Illinois’ filing, but directed its audit staff to review historical rate recovery in connection with an ongoing FERC audit. This review could lead the FERC to ultimately require refunds for periods prior to 2019. In June 2020, Ameren Missouri, Ameren Illinois, and ATXI filed requests for rehearing arguing, among other things, the revisions should be applied retrospectively to include the period January 1, 2019, to June 1, 2020, and that the FERC should not require refunds for periods prior to 2019. In July 2020, the FERC denied the rehearing requests without addressing the issues raised. In July 2020, Ameren Missouri, Ameren Illinois, and ATXI filed an appeal of the July 2020 rehearing denials to the United States Court of Appeals for the District of Columbia Circuit, which is under no deadline to address the appeal. In October 2020, the FERC issued an order reaffirming its May 2020 order and denying the arguments raised in the rehearing requests filed by Ameren Missouri, Ameren Illinois, and ATXI. Pursuant to the May 2020 order, in the second quarter of 2020, Ameren and Ameren Illinois recorded a $2 million reduction to revenue for the period of January 2019 through May 2020. Regardless of the outcome of the appeal, the impacts of the May 2020 and 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. 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. With the maximum FERC-allowed refund period for the November 2013 complaint case ending in February 2015, another customer complaint case was filed in February 2015, seeking a further reduction in the allowed base ROE for the period of February 2015 to May 2016. 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. The order also dismissed the February 2015 complaint case. In December 2019, the MISO transmission owners, including Ameren Missouri, Ameren Illinois, and ATXI, filed requests for rehearing with the FERC. Additionally, in December 2019, various parties filed requests for rehearing with the FERC, challenging the dismissal of the February 2015 complaint case. 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. The May 2020 order also denied rehearing of the FERC's dismissal of the February 2015 complaint case. 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 a result of the May 2020 order, which increased the FERC-allowed base ROE from 9.88% to 10.02% for the periods November 2013 to February 2015 and late September 2016 forward, Ameren and Ameren Illinois recognized income of $13 million and $7 million, respectively, during the second quarter of 2020. As of September 30, 2020, Ameren and Ameren Illinois had recorded current regulatory liabilities of $15 million and $7 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. In March 2019, the FERC issued separate Notices of Inquiry regarding its allowed base ROE policy and its transmission incentives policy. The Notice of Inquiry addressing the FERC’s base ROE policy, among other things, broadened the ability to comment on the new methodology beyond electric utilities that are participants in the complaint cases. The transmission incentives Notice of Inquiry was open for comment on the FERC’s transmission incentive policy, including incentive adders to the base ROE. In March 2020, the FERC issued a Notice of Proposed Rulemaking on its transmission incentives policy, which addressed many of the issues in the Notice of Inquiry on transmission incentives. The Notice of Proposed Rulemaking included an increased incentive in the allowed base ROE for participation in an RTO to 100 basis points from the current 50 basis points and revised the parameters for awarding incentives, while limiting the overall incentives to a cap of 250 basis points, among other things. Initial comments were due by July 2020. While the FERC has not formally terminated the Notice of Inquiry regarding its transmission incentives policy, Ameren does not expect further actions relating to it. Ameren is unable to predict the ultimate impact of the Notice of Inquiry regarding its allowed base ROE policy or the Notice of Proposed Rulemaking at this time.

Short-Term Debt And Liquidity

9 Months Ended

Sep. 30, 2020

Debt Disclosure [Abstract]

SHORT-TERM DEBT AND LIQUIDITY

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

Short-Term Borrowings

The Missouri Credit Agreement and the Illinois Credit Agreement are available to support issuances under Ameren (parent)’s, Ameren Missouri’s, and Ameren Illinois’ commercial paper programs, respectively, subject to borrowing sublimits, and the issuance of letters of credit. As of September 30, 2020, based on commercial paper outstanding and letters of credit issued under the Credit Agreements, along with cash and cash equivalents, the net liquidity available to Ameren (parent), Ameren Missouri, and Ameren Illinois, collectively, was $2.0 billion. The Ameren Companies were in compliance with the covenants in their Credit Agreements as of September 30, 2020. As of September 30, 2020, the ratios of consolidated indebtedness to consolidated total capitalization, calculated in accordance with the provisions of the Credit Agreements, were 55%, 48%, and 45% for Ameren, Ameren Missouri, and Ameren Illinois, respectively.

The following table presents commercial paper outstanding, net of issuance discounts, as of September 30, 2020, and December 31, 2019. There were no borrowings outstanding under the Credit Agreements as of September 30, 2020, or December 31, 2019.


	September 30, 2020		December 31, 2019
Ameren (parent)	$	30	$	153
Ameren Missouri	—		234
Ameren Illinois	242		53
Ameren consolidated	$	272	$	440

The following table summarizes the activity and relevant interest rates for Ameren (parent)’s, Ameren Missouri’s, and Ameren Illinois’ commercial paper issuances and borrowings under the Credit Agreements in the aggregate for the nine months ended September 30, 2020 and 2019:


	Ameren (parent)			Ameren Missouri				Ameren Illinois				Ameren Consolidated
2020
Average daily amount outstanding	$	62		$	142			$	53			$	257

Weighted-average interest rate	2.04		%	1.76		%		1.10		%		1.70		%
Peak amount outstanding during period(a)	$	425		$	573			$	243			$	908
Peak interest rate	3.30		%	5.05		%	(b)	3.40		%		5.05		%	(b)
2019
Average daily amount outstanding	$	532		$	141			$	147			$	821

Weighted-average interest rate	2.70		%	2.73		%		2.58		%		2.68		%
Peak amount outstanding during period(a)	$	651		$	549			$	310			$	1,113
Peak interest rate	3.80		%	2.97		%		5.00		%	(c)	5.00		%	(c)

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

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

(c)Ameren’s and Ameren Illinois’ peak interest rate was affected by temporary disruptions in the commercial paper market in the third quarter of 2019.

Money Pools

Ameren has money pool agreements with and among its subsidiaries to coordinate and provide for certain short-term cash and working capital requirements. The average interest rate for borrowings under the utility money pool for the three and nine months ended September 30, 2020, was 0.10% and 0.81%, respectively (2019 – 2.40% and 2.67%, respectively). See Note 8 – Related-party Transactions for the amount of interest income and expense from the utility money pool arrangements recorded by Ameren Missouri and Ameren Illinois for the three and nine months ended September 30, 2020 and 2019.

Long-Term Debt And Equity Financings	9 Months Ended
Long-Term Debt And Equity Financings	Sep. 30, 2020
Long-Term Debt And Equity Financings [Abstract]
LONG-TERM DEBT AND EQUITY FINANCINGS	LONG-TERM DEBT AND EQUITY FINANCINGS Ameren For the three and nine months ended September 30, 2020, Ameren issued a total of 0.1 million and 0.5 million shares of common stock, respectively, under its DRPlus and 401(k) plan, and received proceeds of $10 million and $37 million, respectively. In addition, in the first quarter of 2020, Ameren issued 0.5 million shares of common stock valued at $38 million upon the vesting of stock-based compensation. In August 2019, Ameren entered into a forward sale agreement with a counterparty relating to 7.5 million shares of common stock. The forward sale agreement will be physically settled unless Ameren elects to settle in cash or to net share settle. At September 30, 2020, Ameren could have settled the forward sale agreement with physical delivery of 7.5 million shares of common stock to the counterparty in exchange for $543 million. The forward sale agreement could also have been settled at September 30, 2020, with delivery of approximately $54 million or 0.7 million shares of common stock to the counterparty, if Ameren had elected to net cash or net share settle, respectively. For additional information about the forward sale agreement, see Note 5 – Long-Term Debt and Equity Financings under Part II, Item 8, in the Form 10-K. In April 2020, Ameren (parent) issued $800 million of 3.50% senior unsecured notes due January 2031, with interest payable semiannually on January 15 and July 15 of each year, beginning July 15, 2020. Ameren received net proceeds of $793 million, which were used for general corporate purposes, including to repay outstanding short-term debt, and were used to fund the repayment of Ameren’s $350 million of 2.70% senior unsecured notes, which were redeemed at par plus accrued interest in October 2020. In October 2020, Ameren, Ameren Missouri, and Ameren Illinois filed a Form S-3 shelf registration statement with the SEC, registering the issuance of an unspecified amount of certain types of securities. This registration statement became effective immediately upon filing and expires in October 2023. Ameren Missouri In March 2020, Ameren Missouri issued $465 million of 2.95% first mortgage bonds due March 2030, with interest payable semiannually on March 15 and September 15 of each year, beginning September 15, 2020. Ameren Missouri received net proceeds of $462 million, which were used to repay outstanding short-term debt, including short-term debt that Ameren Missouri incurred in connection with the repayment of $85 million of its 5.00% senior secured notes that matured in February 2020. In October 2020, Ameren Missouri issued $550 million of 2.625% first mortgage bonds due March 2051, with interest payable semiannually on March 15 and September 15 of each year, beginning March 15, 2021. The bonds were issued as green bonds; therefore, the proceeds will be used for eligible green projects. Ameren Missouri received net proceeds of $543 million, which are expected to be used to partially finance the acquisition of two wind generation facilities. See Note 2 – Rate and Regulatory Matters for information about the wind generation facilities. Ameren Illinois Ameren Illinois received cash capital contributions totaling $350 million from Ameren (parent) during the nine months ended September 30, 2020. Indenture Provisions and Other Covenants See Note 5 – Long-Term Debt and Equity Financings under Part II, Item 8, in the Form 10-K for a description of our indenture provisions and other covenants, as well as restrictions on the payment of dividends. At September 30, 2020, the Ameren Companies were in compliance with the provisions and covenants contained in their indentures and articles of incorporation, as applicable, and ATXI was in compliance with the provisions and covenants contained in its note purchase agreement. Off-balance-sheet Arrangements At September 30, 2020, none of the Ameren Companies had any significant off-balance-sheet financing arrangements, other than the forward sale agreement relating to common stock and variable interest entities. See Note 1 – Summary of Significant Accounting Policies for further detail concerning variable interest entities.

Other Income and Expenses

9 Months Ended

Sep. 30, 2020

Other Nonoperating Income (Expense) [Abstract]

OTHER INCOME AND EXPENSES

OTHER INCOME, NET

The following table presents the components of “Other Income, Net” in the Ameren Companies’ statements of income for the three and nine months ended September 30, 2020 and 2019:


	Three Months				Nine Months
	2020		2019		2020			2019
Ameren:

Allowance for equity funds used during construction	$	12	$	7	$	25		$	20
Interest income on industrial development revenue bonds	7		6		19			19
Other interest income	—		2		2			6

Non-service cost components of net periodic benefit income(a)	32		23		85			67
Miscellaneous income	1		2		10			6
Donations	—		(1)		(14)		(b)	(8)
Miscellaneous expense	(4)		(5)		(10)			(11)
Total Other Income, Net	$	48	$	34	$	117		$	99
Ameren Missouri:

Allowance for equity funds used during construction	$	7	$	6	$	15		$	14
Interest income on industrial development revenue bonds	7		6		19			19

Non-service cost components of net periodic benefit income(a)	13		4		32			13
Miscellaneous income	1		2		3			4
Donations	—		(1)		(9)		(b)	(3)
Miscellaneous expense	(2)		(2)		(5)			(4)
Total Other Income, Net	$	26	$	15	$	55		$	43
Ameren Illinois:

Allowance for equity funds used during construction	$	5	$	1	$	10		$	6
Interest income	—		1		2			5

Non-service cost components of net periodic benefit income	12		12		36			36
Miscellaneous income	1		1		6			3
Donations	—		—		(5)			(5)
Miscellaneous expense	(1)		(2)		(4)			(6)
Total Other Income, Net	$	17	$	13	$	45		$	39

(a)For the three and nine months ended September 30, 2020, the non-service cost components of net periodic benefit income were adjusted by amounts deferred of $(3) million and less than $(1) million, respectively, due to a regulatory tracking mechanism for the difference between the level of such costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. The deferral was $7 million and $22 million for three and nine months ended September 30, 2019, respectively.

(b)Includes $8 million pursuant to Ameren Missouri’s March 2020 electric rate order. See Note 2 – Rate and Regulatory Matters for additional information.

Derivative Financial Instruments

9 Months Ended

Sep. 30, 2020

Derivative Instruments and Hedging Activities Disclosure [Abstract]

DERIVATIVE FINANCIAL INSTRUMENTS

We use derivatives to manage the risk of changes in market prices for natural gas, power and uranium, as well as the risk of changes in rail transportation surcharges through fuel oil hedges. Such price fluctuations may cause the following:

•an unrealized appreciation or depreciation of our contracted commitments to purchase or sell when purchase or sale prices under the commitments are compared with current commodity prices;

•market values of natural gas and uranium inventories that differ from the cost of those commodities in inventory;

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

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

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

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

If we determine that a contract meets the definition of a derivative and is not eligible for the NPNS exception, we review the contract to determine whether the resulting gains or losses qualify for regulatory deferral. Derivative contracts that qualify for regulatory deferral are recorded at fair value, with changes in fair value recorded as regulatory assets or liabilities in the period in which the change occurs. We believe derivative losses and gains deferred as regulatory assets and liabilities are probable of recovery, or refund, through future rates charged to customers. Regulatory assets and liabilities are amortized to operating income as related losses and gains are reflected in rates charged to customers. Therefore, gains and losses on these derivatives have no effect on operating income. As of September 30, 2020, and December 31, 2019, all contracts that met the definition of a derivative and were not eligible for the NPNS exception received regulatory deferral. Cash flows for all derivative financial instruments are classified in cash flows from operating activities.

The following table presents open gross commodity contract volumes by commodity type for derivative assets and liabilities as of September 30, 2020, and December 31, 2019. As of September 30, 2020, these contracts extended through October 2023, October 2025, May 2032 and March 2023 for fuel oils, natural gas, power and uranium, respectively.


	Quantity (in millions, except as indicated)
	September 30, 2020			December 31, 2019
Commodity	Ameren Missouri	Ameren Illinois	Ameren	Ameren Missouri	Ameren Illinois	Ameren
Fuel oils (in gallons)	44	—	44	58	—	58
Natural gas (in mmbtu)	32	123	155	20	136	156
Power (in megawatthours)	6	7	13	5	7	12
Uranium (pounds in thousands)	365	—	365	565	—	565

The following table presents the carrying value and balance sheet location of all derivative commodity contracts, none of which were designated as hedging instruments, as of September 30, 2020, and December 31, 2019:


			September 30, 2020					December 31, 2019
	Balance Sheet Location		Ameren Missouri		Ameren Illinois		Ameren			Ameren Missouri		Ameren Illinois		Ameren









Fuel oils	Other current assets	$	2	$	—	$	2		$	4	$	—	$	4
	Other assets		—		—		—			2		—		2


Natural gas	Other current assets		2		10		12			—		3		3
	Other assets		2		4		6			—		1		1


Power	Other current assets		12		—		12			14		—		14
	Other assets		1		—		1			2		—		2


	Total assets	$	19	$	14	$	33		$	22	$	4	$	26





Fuel oils	Other current liabilities	$	14	$	—	$	14		$	4	$	—	$	4
	Other deferred credits and liabilities		6		—		6			3		—		3


Natural gas	Other current liabilities		—		2		2			1		12		13
	Other deferred credits and liabilities		—		1		1			1		6		7


Power	Other current liabilities		6		16		22			2		17		19
	Other deferred credits and liabilities		4		197		201			1		207		208


Uranium	Other deferred credits and liabilities		—		—		—			1		—		1
	Total liabilities	$	30	$	216	$	246		$	13	$	242	$	255

The Ameren Companies elect to present the fair value amounts of derivative assets and derivative liabilities subject to an enforceable master netting arrangement or similar agreement at the gross amounts on the balance sheet. However, if the gross amounts recognized on the balance sheet were netted with derivative instruments and cash collateral received or posted, the net amounts would not be materially different from the gross amounts at September 30, 2020, and December 31, 2019.

Credit Risk

In determining our concentrations of credit risk related to derivative instruments, we review our individual counterparties and categorize each counterparty into groupings according to the primary business in which each engages. As of September 30, 2020, if counterparty groups were to fail completely to perform on contracts, the Ameren Companies’ maximum exposure related to derivative assets would have been immaterial with or without consideration of the application of master netting arrangements or similar agreements and collateral held.

Certain of our derivative instruments contain collateral provisions tied to the Ameren Companies’ credit ratings. If our credit ratings were downgraded below investment grade, or if a counterparty with reasonable grounds for uncertainty regarding our ability to satisfy an obligation requested adequate assurance of performance, additional collateral postings might be required. The additional collateral required is the net liability position allowed under 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 September 30, 2020, the aggregate fair value of derivative instruments with credit risk-related contingent features in a gross liability position, the cash collateral posted, and the aggregate amount of additional collateral that counterparties could require were each immaterial to Ameren, Ameren Missouri, and Ameren Illinois.

Fair Value Measurements

9 Months Ended

Sep. 30, 2020

Fair Value Disclosures [Abstract]

Fair Value Disclosures

FAIR VALUE MEASUREMENTS

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

We consider nonperformance risk in our valuation of derivative instruments by analyzing our own credit standing and the credit standing of our counterparties, and by considering any credit enhancements (e.g., collateral). Included in our valuation, and based on current market conditions, is a valuation adjustment for counterparty default derived from market data such as the price of credit default swaps, bond yields, and credit ratings. No material gains or losses related to valuation adjustments for counterparty default risk were recorded at Ameren, Ameren Missouri, or Ameren Illinois in the three and nine months ended September 30, 2020 or 2019. At September 30, 2020, and December 31, 2019, the counterparty default risk valuation adjustment related to derivative contracts was immaterial for Ameren, Ameren Missouri, and Ameren Illinois.

The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of September 30, 2020, and December 31, 2019:


		September 30, 2020									December 31, 2019
		Level 1		Level 2		Level 3		Total			Level 1		Level 2		Level 3		Total
Assets:
Ameren Missouri
	Derivative assets – commodity contracts:
	Fuel oils	$	—	$	—	$	2	$	2		$	—	$	—	$	6	$	6
	Natural gas	—		4		—		4			—		—		—		—
	Power	1		—		12		13			—		2		14		16

	Total derivative assets – commodity contracts	$	1	$	4	$	14	$	19		$	—	$	2	$	20	$	22
	Nuclear decommissioning trust fund:

	Equity securities:
	U.S. large capitalization	$	604	$	—	$	—	$	604		$	569	$	—	$	—	$	569
	Debt securities:
	U.S. Treasury and agency securities	—		112		—		112			—		107		—		107
	Corporate bonds	—		116		—		116			—		93		—		93


	Other	—		66		—		66			—		73		—		73
	Total nuclear decommissioning trust fund	$	604	$	294	$	—	$	898	(a)	$	569	$	273	$	—	$	842	(a)
	Total Ameren Missouri	$	605	$	298	$	14	$	917		$	569	$	275	$	20	$	864
Ameren Illinois
	Derivative assets – commodity contracts:
	Natural gas	$	—	$	9	$	5	$	14		$	—	$	1	$	3	$	4


Ameren
	Derivative assets – commodity contracts(b)	$	1	$	13	$	19	$	33		$	—	$	3	$	23	$	26
	Nuclear decommissioning trust fund(c)	604		294		—		898		(a)	569		273		—		842		(a)
	Total Ameren	$	605	$	307	$	19	$	931		$	569	$	276	$	23	$	868
Liabilities:
Ameren Missouri
	Derivative liabilities – commodity contracts:
	Fuel oils	$	13	$	—	$	7	$	20		$	1	$	—	$	6	$	7
	Natural gas	—		—		—		—			—		2		—		2
	Power	8		—		2		10			—		2		1		3
	Uranium	—		—		—		—			—		—		1		1
	Total Ameren Missouri	$	21	$	—	$	9	$	30		$	1	$	4	$	8	$	13
Ameren Illinois
	Derivative liabilities – commodity contracts:
	Natural gas	$	—	$	2	$	1	$	3		$	3	$	12	$	3	$	18
	Power	—		—		213		213			—		—		224		224
	Total Ameren Illinois	$	—	$	2	$	214	$	216		$	3	$	12	$	227	$	242
Ameren
	Derivative liabilities – commodity contracts(b)	$	21	$	2	$	223	$	246		$	4	$	16	$	235	$	255

(a)Balance excludes $6 million and $5 million of cash and cash equivalents, receivables, payables, and accrued income, net, for September 30, 2020, and December 31, 2019, respectively.

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

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

Level 3 fuel oils, natural gas, and uranium derivative contract assets and liabilities measured at fair value on a recurring basis were immaterial for all periods presented. The following table presents the fair value reconciliation of Level 3 power derivative contract assets and liabilities measured at fair value on a recurring basis for the three and nine months ended September 30, 2020 and 2019:


	2020						2019
	Ameren Missouri		Ameren Illinois		Ameren			Ameren Missouri		Ameren Illinois		Ameren
For the three months ended September 30:
Beginning balance at July 1	$	16	$	(229)	$	(213)		$	15	$	(191)	$	(176)
Realized and unrealized gains/(losses) included in regulatory assets/liabilities	(2)		12		10			(4)		(17)		(21)


Settlements	(4)		4		—			(1)		4		3

Transfers out of Level 3	—		—		—			(2)		—		(2)
Ending balance at September 30	$	10	$	(213)	$	(203)		$	8	$	(204)	$	(196)
Change in unrealized gains/(losses) related to assets/liabilities held at September 30	$	(2)	$	11	$	9		$	(4)	$	(17)	$	(21)
For the nine months ended September 30:
Beginning balance at January 1	$	13	$	(224)	$	(211)		$	—	$	(183)	$	(183)
Realized and unrealized gains/(losses) included in regulatory assets/liabilities	18		(2)		16			12		(32)		(20)


Settlements	(21)		13		(8)			(2)		11		9

Transfers out of Level 3	—		—		—			(2)		—		(2)
Ending balance at September 30	10		(213)		(203)			8		(204)		(196)
Change in unrealized gains/(losses) related to assets/liabilities held at September 30	$	6	$	(1)	$	5		$	8	$	(31)	$	(23)

All gains or losses related to our Level 3 derivative commodity contracts are expected to be recovered or returned through customer rates; therefore, there is no impact to either net income or other comprehensive income resulting from changes in the fair value of these instruments.

The following table describes the valuation techniques and significant unobservable inputs utilized for the fair value of our Level 3 power derivative contract assets and liabilities as of September 30, 2020, and December 31, 2019:


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









2020	Power(c)	$	12	$	(215)	Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps ($/MWh)	20 – 36	28

							Nodal basis ($/MWh)	(5) – 1	(2)


							Trend rate (%)	3 – 4	3








































2019	Power(d)	$	14	$	(225)	Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps ($/MWh)	22 – 34	25

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


							Trend rate (%)	(1) – 0	0

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

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

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

(d)Valuations through 2028 use visible forward prices adjusted for nodal-to-hub basis differentials. Valuations beyond 2028 use a trend rate factor and are similarly adjusted for nodal-to-hub basis differentials.

The following table sets forth the carrying amount and, by level within the fair value hierarchy, the fair value of financial assets and liabilities disclosed, but not recorded, at fair value as of September 30, 2020, and December 31, 2019:


	Carrying Amount			Fair Value
				Level 1		Level 2		Level 3			Total
	September 30, 2020
Ameren:
Cash, cash equivalents, and restricted cash	$	168		$	168	$	—	$	—		$	168
Investments in industrial development revenue bonds(a)	263			—		263		—			263
Short-term debt	272			—		272		—			272
Long-term debt (including current portion)(a)	10,529		(b)	—		11,891		521		(c)	12,412
Ameren Missouri:
Cash, cash equivalents, and restricted cash	$	10		$	10	$	—	$	—		$	10
Advances to money pool	5			—		5		—			5
Investments in industrial development revenue bonds(a)	263			—		263		—			263

Long-term debt (including current portion)(a)	4,568		(b)	—		5,316		—			5,316
Ameren Illinois:
Cash, cash equivalents, and restricted cash	$	147		$	147	$	—	$	—		$	147
Short-term debt	242			—		242		—			242

Long-term debt (including current portion)	3,576		(b)	—		4,431		—			4,431
	December 31, 2019
Ameren:
Cash, cash equivalents, and restricted cash	$	176		$	176	$	—	$	—		$	176
Investments in industrial development revenue bonds(a)	263			—		263		—			263
Short-term debt	440			—		440		—			440
Long-term debt (including current portion)(a)	9,357		(b)	—		9,957		484		(c)	10,441
Ameren Missouri:
Cash, cash equivalents, and restricted cash	$	39		$	39	$	—	$	—		$	39
Investments in industrial development revenue bonds(a)	263			—		263		—			263
Short-term debt	234			—		234		—			234
Long-term debt (including current portion)(a)	4,190		(b)	—		4,772		—			4,772
Ameren Illinois:
Cash, cash equivalents, and restricted cash	$	125		$	125	$	—	$	—		$	125
Short-term debt	53			—		53		—			53
Long-term debt (including current portion)	3,575		(b)	—		4,019		—			4,019

(a)Ameren and Ameren Missouri have investments in industrial development revenue bonds, classified as held-to-maturity and recorded in “Other Assets,” that are equal to the finance obligations for the Peno Creek and Audrain CT energy centers. As of September 30, 2020, and December 31, 2019, the carrying amount of both the investments in industrial development revenue bonds and the finance obligations approximated fair value.

(b)Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $77 million, $32 million, and $33 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of September 30, 2020. Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $72 million, $30 million, and $34 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2019.

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

- Definition

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

+ References + Details

Related Party Transactions

9 Months Ended

Sep. 30, 2020

RELATED PARTY TRANSACTIONS

RELATED-PARTY TRANSACTIONS

In the normal course of business, Ameren Missouri and Ameren Illinois have engaged in, and may in the future engage in, affiliate transactions. These transactions primarily consist of natural gas and power purchases and sales, services received or rendered, and borrowings and lendings. Transactions between Ameren’s subsidiaries are reported as affiliate transactions on their individual financial statements, but those transactions are eliminated in consolidation for Ameren’s consolidated financial statements, except as noted below. For a discussion of material related-party agreements and money pool arrangements, see Note 13 – Related-party Transactions and Note 4 – Short-term Debt and Liquidity under Part II, Item 8, of the Form 10-K.

Electric Power and Capacity Supply Agreements

In April and September 2020, Ameren Illinois conducted procurement events, administered by the IPA, to purchase energy products and acquire capacity. Ameren Missouri was among the winning suppliers in these events. As a result, in April 2020, Ameren Missouri contracted to supply a portion of Ameren Illinois’ capacity requirements for $2 million from June 2021 through May 2023. In September 2020, Ameren Missouri contracted to supply a portion of Ameren Illinois’ capacity requirements for $1 million from June 2021 through May 2023. Additionally, in September 2020, Ameren Missouri and Ameren Illinois entered into an energy product agreement by which Ameren Missouri

agreed to sell, and Ameren Illinois agreed to purchase, 204,800 megawatthours at an average price of $31 per megawatthour during the period of September 2021 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 September 30, 2020, and December 31, 2019:


	September 30, 2020				December 31, 2019
	Ameren Missouri		Ameren Illinois		Ameren Missouri		Ameren Illinois
Income taxes payable to parent(a)	$	38	$	9	$	15	$	43
Income taxes receivable from parent(b)	—		6		15		17

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

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

Effects of Related-party Transactions on the Statement of Income

The following table presents the effect on Ameren Missouri and Ameren Illinois of related-party transactions for the three and nine months ended September 30, 2020 and 2019:


				Three Months				Nine Months
Agreement	Income Statement Line Item			Ameren Missouri		Ameren Illinois		Ameren Missouri		Ameren Illinois
Ameren Missouri power supply	Operating Revenues	2020	$	5	$	(a)	$	11	$	(a)
agreements with Ameren Illinois		2019		1		(a)		3		(a)
Ameren Missouri and Ameren Illinois	Operating Revenues	2020	$	6	$	(b)	$	19	$	1
rent and facility services		2019		7		1		20		2
Ameren Missouri and Ameren Illinois	Operating Revenues	2020	$	(b)	$	1	$	1	$	1
miscellaneous support services		2019		1		(b)		1		1
Ameren Missouri software licensing	Operating Revenues	2020	$	(a)	$	(a)	$	(a)	$	(a)
with Ameren Illinois(c)		2019		(a)		19		(a)		19
Total Operating Revenues		2020	$	11	$	1	$	31	$	2
		2019		9		20		24		22
Ameren Illinois power supply	Purchased Power	2020	$	(a)	$	5	$	(a)	$	11
agreements with Ameren Missouri		2019		(a)		1		(a)		3
Ameren Illinois transmission	Purchased Power	2020	$	(a)	$	(b)	$	(a)	$	1
services with ATXI		2019		(a)		1		(a)		1
Total Purchased Power		2020	$	(a)	$	5	$	(a)	$	12
		2019		(a)		2		(a)		4
Ameren Missouri and Ameren Illinois	Other Operations and Maintenance	2020	$	(b)	$	1	$	(b)	$	3
rent and facility services		2019		(b)		1		1		4
Ameren Services support services	Other Operations and Maintenance	2020	$	36	$	34	$	103	$	98
agreement		2019		34		30		98		91
Total Other Operations and		2020	$	36	$	35	$	103	$	101
Maintenance		2019		34		31		99		95
Money pool borrowings (advances)	(Interest Charges)/Other Income, Net	2020	$	(b)	$	(b)	$	(b)	$	(b)
		2019		(b)		(b)		(b)		(b)

(a)Not applicable.

(b)Amount less than $1 million.

(c)In September 2019, Ameren Missouri purchased a license for advanced metering infrastructure software from Ameren Illinois. The amount of the $24 million cost-based transaction price over the $5 million remaining carrying value of the software was recorded as revenue by Ameren Illinois, with $14 million of revenue recorded at Ameren Illinois Electric Distribution and $5 million recorded at Ameren Illinois Natural Gas. The revenue recorded at Ameren Illinois Electric Distribution was reflected in formula ratemaking, which resulted in no impact to net income. Per authoritative accounting guidance for sales to rate-regulated entities, the revenue recognized by Ameren Illinois was not eliminated upon consolidation by Ameren.

Commitments And Contingencies

9 Months Ended

Sep. 30, 2020

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

COMMITMENTS AND CONTINGENCIESWe 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 10 – 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.

Other Obligations

To supply a portion of the fuel requirements of Ameren Missouri’s energy centers, Ameren Missouri has entered into various long-term commitments for the procurement of coal, natural gas, nuclear fuel, and methane gas. Ameren Missouri and Ameren Illinois also have entered into various long-term commitments for purchased power and natural gas for distribution. The table below presents our estimated minimum fuel, purchased power, and other commitments at September 30, 2020. Ameren’s and Ameren Illinois’ purchased power commitments include the Ameren Illinois agreements entered into as part of the IPA-administered power procurement process. Included in the Other column are minimum purchase commitments under contracts for equipment, design and construction, and meter reading services, among other agreements, at September 30, 2020.


	Coal		Natural Gas(a)		Nuclear Fuel		Purchased Power(b)(c)			Methane Gas		Other		Total
Ameren:
2020	$	94	$	52	$	3	$	57	(d)	$	—	$	39	$	245
2021	242		161		57		156		(d)	3		53		672
2022	193		108		12		63			3		23		402
2023	113		69		45		21			3		23		274
2024	94		28		20		4			3		20		169
Thereafter	55		73		21		—			21		61		231
Total	$	791	$	491	$	158	$	301		$	33	$	219	$	1,993
Ameren Missouri:
2020	$	94	$	12	$	3	$	—		$	—	$	22	$	131
2021	242		43		57		—			3		46		391
2022	193		38		12		—			3		23		269
2023	113		32		45		—			3		23		216
2024	94		13		20		—			3		20		150
Thereafter	55		22		21		—			21		24		143
Total	$	791	$	160	$	158	$	—		$	33	$	158	$	1,300
Ameren Illinois:
2020	$	—	$	40	$	—	$	57	(d)	$	—	$	14	$	111
2021	—		118		—		156		(d)	—		2		276
2022	—		70		—		63			—		—		133
2023	—		37		—		21			—		—		58
2024	—		15		—		4			—		—		19
Thereafter	—		51		—		—			—		—		51
Total	$	—	$	331	$	—	$	301		$	—	$	16	$	648

(a)Includes amounts for generation and for distribution.

(b)The purchased power amounts for Ameren and Ameren Illinois exclude agreements for renewable energy credits through 2035 with various renewable energy suppliers due to the contingent nature of the payment amounts, with the exception of expected payments of $36 million through 2024.

(c)The purchased power amounts for Ameren and Ameren Missouri exclude a 102-megawatt power purchase agreement with a wind farm operator, which expires in 2024, due to the contingent nature of the payment amounts.

(d)In January 2018, as required by the FEJA, Ameren Illinois entered into agreements to acquire zero emission credits through 2026. Annual zero emission credit commitment amounts will be published by the IPA each May prior to the start of the subsequent planning year, which begins each June. The amounts above reflect Ameren Illinois’ commitment to acquire $42 million of zero emission credits through May 2021.

Environmental Matters

We are subject to various environmental laws, including statutes and regulations, enforced by federal, state, and local authorities. The development and operation of electric generation, transmission, and distribution facilities and natural gas storage, transmission, and distribution facilities can trigger compliance obligations with respect to environmental laws. These laws address emissions; discharges to surface and groundwater; water consumption; impacts to air, land, and water; and chemical and waste storage, handling and disposal. 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.

Environmental regulations have a significant impact on the electric utility industry and our operations. Compliance with these regulations could be costly for Ameren Missouri, which operates coal-fired power plants. Regulations that apply to air emissions from the electric utility industry include the NSPS, the CSAPR, the MATS, and the National Ambient Air Quality Standards, which are subject to periodic review for certain pollutants. Collectively, these regulations cover a variety of pollutants, such as SO2, particulate matter, NOx, mercury, toxic metals, and acid gases, and CO2 emissions from new power plants. Water intake and discharges from power plants are regulated under the Clean Water Act. Such regulations require evaluation of the environmental impacts of water intake structures and could require modifications to water intake structures or more stringent limitations on wastewater discharges at Ameren Missouri’s energy centers, either of which could result in significant capital expenditures. The management and disposal of coal ash is regulated as a solid waste under the Resource Conservation Recovery Act and a regulation known as the CCR rule, which will require the closure of our surface impoundments following the installations of dry ash handling systems at several of Ameren Missouri’s 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 $200 million to $250 million from 2020 through 2024 in order to comply with existing environmental regulations. Additional environmental controls beyond 2024 could be required. This estimate of capital expenditures includes ash pond closure and corrective action costs required by the CCR regulations applicable to 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 estimate 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 SO2 and NOx through the reduction of emissions at their source and the use and retirement of emission allowances. CSAPR is implemented through a series of phases, and the second phase became effective in 2017. Additional emission reduction requirements may apply in subsequent years. To achieve compliance with the CSAPR, Ameren Missouri burns low-sulfur coal, operates two scrubbers at its Sioux Energy Center, and optimizes other existing air pollution control equipment. Ameren Missouri expects to incur additional costs to lower its emissions at one or more of its energy centers to comply with the CSAPR in future years. These higher costs are expected to be recovered from customers through the FAC or higher base rates.

CO2 Emissions Standards

The EPA’s Affordable Clean Energy Rule establishes emission guidelines for states to follow in developing plans to limit CO2 emissions from coal-fired electric generating units. The EPA has identified certain efficiency measures as the best system of emission reduction for coal-fired electric generating units. The rule requires the state of Missouri to develop a compliance plan and submit it to the EPA for approval by July 2022. The plan is expected to include a standard of performance for each affected generating unit. We continue to evaluate the impact of the adoption and implementation of the Affordable Clean Energy Rule. Ameren Missouri and other stakeholders are working with the state of Missouri to develop the compliance plan submitted to the EPA. At this time, Ameren Missouri cannot predict the outcome of Missouri’s compliance plan development process. As such, the impact on the results of operations, financial position, and liquidity of Ameren and Ameren Missouri is uncertain. We also cannot predict the outcome of any potential legal challenges to the rule.

NSR and Clean Air Litigation

In January 2011, the Department of Justice, on behalf of the EPA, filed a complaint against Ameren Missouri in the United States District Court for the Eastern District of Missouri alleging that in performing projects at its coal-fired Rush Island Energy Center in 2007 and 2010, Ameren Missouri violated provisions of the Clean Air Act and Missouri law. In January 2017, the district court issued a liability ruling and, in September 2019, entered a final order that required Ameren Missouri to install a flue gas desulfurization system at the Rush Island Energy Center and a dry sorbent injection system at the Labadie Energy Center. There were no fines in the order. In October 2019, Ameren Missouri

appealed the district court’s ruling to the United States Court of Appeals for the Eighth Circuit. The district court has stayed implementation of the majority of the requirements of its order while the case is under appeal. Ameren Missouri believes the district court both misinterpreted and misapplied the law in its ruling. Ameren Missouri is unable to predict the ultimate resolution of this matter. Briefing in this case has been completed, but the appellate court has not yet scheduled oral arguments. The appellate court is under no deadline to issue a ruling in this case.

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, capital expenditures to comply with the district court’s order for installation of a flue gas desulfurization system at the Rush Island Energy Center are estimated at approximately $1 billion. Further, the flue gas desulfurization system would result in additional operation and maintenance expenses of $30 million to $50 million annually for the life of the energy center. Engineering studies required to develop estimated capital expenditures and estimated additional operation and maintenance expenses for the Labadie Energy Center to comply with the district court’s order will not be undertaken while the case is under appeal. As a result of the district court’s stay, Ameren Missouri does not expect to make significant capital expenditures or incur operations and maintenance expenses related to the district court’s order while the case is under appeal.

Clean Water Act

The EPA’s Section 316(b) Rule requires power plant operators to evaluate cooling water intake structures and identify measures for reducing the number of aquatic organisms impinged on a power plant’s cooling water intake screens or entrained through the plant’s cooling water system. All of Ameren Missouri’s coal-fired and nuclear energy centers are subject to the cooling water intake structures rule. Requirements of the rule are being implemented through the permit renewal process of each energy center’s water discharge permit, which is expected to be completed by 2023.

In 2015, the EPA issued a rule to revise the effluent limitation guidelines applicable to steam electric generating units. These guidelines established national standards for water discharges that are based on the effectiveness of available control technology and 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 has completed or will complete construction in 2020 of wastewater treatment facilities and dry ash handling systems at three of its four coal-fired energy centers. The fourth coal-fired energy center is scheduled to close permanently in 2022. Estimated capital expenditures to complete these projects are included in the CCR management compliance plan, discussed below.

CCR Management

In 2015, the EPA issued the CCR rule, which established requirements for the management and disposal of CCR from coal-fired power plants. These regulations affect CCR disposal and handling costs at Ameren Missouri’s energy centers. Ameren Missouri is in the process of closing surface impoundments at three facilities, and is scheduled to complete the last of such closures at all of its energy centers in 2023. While the EPA has issued a series of revisions to the CCR rule, none of those revisions or proposals is expected to materially impact our closure schedule. Ameren and Ameren Missouri have AROs of $125 million recorded on their respective balance sheets as of September 30, 2020, associated with CCR storage facilities. Ameren Missouri estimates it will need to make capital expenditures of $75 million to $125 million from 2020 through 2024 to implement its CCR management compliance plan, which includes installation of dry ash handling systems, wastewater treatment facilities, and groundwater monitoring equipment.

Remediation

The Ameren Companies are involved in a number of remediation actions to clean up sites impacted by the use or disposal of materials containing hazardous substances. Federal and state laws can require responsible parties to fund remediation regardless of their degree of fault, the legality of original disposal, or the ownership of a disposal site.

As of September 30, 2020, Ameren Illinois has remediated the majority of the 44 former MGP sites in Illinois it owned or for which it was otherwise responsible. Ameren Illinois estimates it could substantially conclude remediation efforts at the remaining sites by 2023. The ICC allows Ameren Illinois to recover such remediation and related litigation costs from its electric and natural gas utility customers through environmental cost riders. Costs are subject to annual prudence review by the ICC. As of September 30, 2020, Ameren Illinois estimated the remaining obligation related to these former MGP sites at $106 million to $178 million. Ameren and Ameren Illinois recorded a liability of $106 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

9 Months Ended

Sep. 30, 2020

Nuclear Waste Matters [Abstract]

CALLAWAY ENERGY CENTER

See Note 9 – Callaway Energy Center under Part II, Item 8, of the Form 10-K for information regarding spent nuclear fuel recovery, recovery of decommissioning costs, and the nuclear decommissioning trust fund. The fair value of the trust fund for Ameren Missouri’s Callaway Energy Center is reported as “Nuclear decommissioning trust fund” in Ameren’s and Ameren Missouri’s balance sheets. This amount is legally restricted and may be used only to fund the costs of nuclear decommissioning. Changes in the fair value of the trust fund are recorded as an increase or decrease to the nuclear decommissioning trust fund, with an offsetting adjustment to the related regulatory liability. Ameren and Ameren Missouri have recorded an ARO for the Callaway Energy Center decommissioning costs at fair value, which represents the present value of estimated future cash outflows. Annual decommissioning costs of $7 million are included in the costs used to establish electric rates for Ameren Missouri’s customers. Every three years, the MoPSC requires Ameren Missouri to file an updated cost study and funding analysis for decommissioning its Callaway Energy Center. An updated cost study and funding analysis was filed with the MoPSC in November 2020 and reflected within the ARO. See Note 13 - Supplemental Information for more information on Ameren Missouri’s AROs.

Insurance

The following table presents insurance coverage at Ameren Missouri’s Callaway Energy Center at September 30, 2020:


Type and Source of Coverage	Most Recent Renewal Date	Maximum Coverages			Maximum Assessments for Single Incidents
Public liability and nuclear worker liability:
American Nuclear Insurers	January 1, 2020	$	450		$	—
Pool participation	(a)	13,348		(a)	138		(b)
		$	13,798	(c)	$	138
Property damage:
NEIL and EMANI	April 1, 2020	$	3,200	(d)	$	25	(e)
Replacement power:
NEIL	April 1, 2020	$	490	(f)	$	7	(e)

(a)Provided through mandatory participation in an industrywide retrospective premium assessment program. The maximum coverage available is dependent on the number of United States commercial reactors participating in the program.

(b)Retrospective premium under the Price-Anderson Act. This is subject to retrospective assessment with respect to a covered loss in excess of $450 million in the event of an incident at any licensed United States commercial reactor, payable at $21 million per year.

(c)Limit of liability for each incident under the Price-Anderson liability provisions of the Atomic Energy Act of 1954, as amended. This limit is subject to change to account for the effects of inflation and changes in the number of licensed power reactors.

(d)NEIL provides $2.7 billion in property damage, stabilization, decontamination, and premature decommissioning insurance for radiation events and $2.3 billion in property damage insurance for nonradiation events. EMANI provides $490 million in property damage insurance for both radiation and nonradiation events.

(e)All NEIL-insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL.

(f)Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity up to $4.5 million for 52 weeks, which commences after the first 12 weeks of an outage, plus up to $3.6 million per week for a minimum of 71 weeks thereafter for a total not exceeding the policy limit of $490 million. Nonradiation events are limited to $328 million.

The Price-Anderson Act is a federal law that limits the liability for claims from an incident involving any licensed United States commercial nuclear energy center. The limit is based on the number of licensed reactors. The limit of liability and the maximum potential annual payments are adjusted at least every five years for inflation to reflect changes in the Consumer Price Index. The most recent five-year inflationary adjustment became effective in November 2018. Owners of a nuclear reactor cover this exposure through a combination of private insurance and mandatory participation in a financial protection pool, as established by the Price-Anderson Act.

Losses resulting from terrorist attacks on nuclear facilities insured by NEIL are subject to industrywide aggregates, such that terrorist acts against one or more commercial nuclear power plants within a stated time period would be treated as a single event, and the owners of the nuclear power plants would share the limit of liability. NEIL policies have an aggregate limit of $3.2 billion within a 12-month period for radiation events, or $1.8 billion for events not involving radiation contamination, resulting from terrorist attacks. The EMANI policies are not subject to industrywide aggregates in the event of terrorist attacks on nuclear facilities.

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

Retirement Benefits

9 Months Ended

Sep. 30, 2020

Retirement Benefits [Abstract]

RETIREMENT BENEFITS

The following table presents the components of the net periodic benefit cost (income) incurred for Ameren’s pension and postretirement benefit plans for the three and nine months ended September 30, 2020 and 2019:


	Pension Benefits								Postretirement Benefits
	Three Months				Nine Months				Three Months				Nine Months
	2020		2019		2020		2019		2020		2019		2020		2019
Service cost(a)	$	28	$	22	$	83	$	66	$	4	$	4	$	14	$	13
Non-service cost components:
Interest cost	43		46		130		139		10		11		29		32
Expected return on plan assets	(73)		(69)		(218)		(207)		(20)		(19)		(60)		(57)
Amortization of:

Prior service benefit	—		—		(1)		—		(1)		(1)		(3)		(4)
Actuarial loss (gain)	15		6		45		19		(3)		(4)		(7)		(11)
Total non-service cost components(b)	$	(15)	$	(17)	$	(44)	$	(49)	$	(14)	$	(13)	$	(41)	$	(40)

Net periodic benefit cost (income)	$	13	$	5	$	39	$	17	$	(10)	$	(9)	$	(27)	$	(27)

(a)Service cost, net of capitalization, is reflected in “Operating Expenses – Other operations and maintenance” on Ameren’s statement of income.

(b)Non-service cost components are reflected in “Other Income, Net” on Ameren’s statement of income. See Note 5 – Other Income, Net for additional information.

Ameren Missouri and Ameren Illinois are responsible for their respective share of Ameren’s pension and other postretirement costs. The following table presents the respective share of net periodic pension and other postretirement benefit costs (income) incurred for the three and nine months ended September 30, 2020 and 2019:


	Pension Benefits								Postretirement Benefits
	Three Months				Nine Months				Three Months				Nine Months
	2020		2019		2020		2019		2020		2019		2020		2019
Ameren Missouri(a)	$	5	$	1	$	16	$	3	$	(2)	$	(1)	$	(4)	$	(4)
Ameren Illinois	8		5		24		15		(8)		(8)		(24)		(23)
Other	—		(1)		(1)		(1)		—		—		1		—
Ameren(a)	$	13	$	5	$	39	$	17	$	(10)	$	(9)	$	(27)	$	(27)

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

Funding

Based on its assumptions at September 30, 2020, its investment performance in 2020, and its pension funding policy, the estimated aggregate contributions through 2024 has not materially changed from the $70 million expected aggregate contributions at December 31, 2019.

- Definition

The entire disclosure for retirement benefits.

+ References

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+ Details

Income Taxes

9 Months Ended

Sep. 30, 2020

Income Tax Disclosure [Abstract]

Income Taxes

INCOME TAXES

The following table presents a reconciliation of the federal statutory corporate income tax rate to the effective income tax rate for the three and nine months ended September 30, 2020 and 2019:


	Ameren			Ameren Missouri			Ameren Illinois
	2020		2019	2020		2019	2020	2019
Three Months
Federal statutory corporate income tax rate:	21%		21%	21%		21%	21%	21%
Increases (decreases) from:
Amortization of deferred investment tax credit	(1)		(1)	(1)		(1)	—	—
Amortization of excess deferred taxes	(9)	(a)	(7)	(16)	(a)	(11)	(3)	(3)
Depreciation differences	—		—	—		—	(1)	—

Renewable and other tax credits	(1)		—	(2)		—	—	—
State tax	4		6	4		5	7	6
Stock-based compensation	1		1	—		—	—	—

Effective income tax rate	15%		20%	6%		14%	24%	24%


	Ameren			Ameren Missouri			Ameren Illinois
	2020		2019	2020		2019	2020	2019
Nine 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)	(a)	(7)	(16)	(a)	(12)	(3)	(4)


Renewable and other tax credits	(1)		—	(1)		—	—	—
State tax	5		6	3		5	7	7
Stock-based compensation	(1)		(1)	—		—	—	—

Effective income tax rate	15%		18%	6%		13%	25%	24%

(a)Increase in the amortization of excess deferred taxes pursuant to the MoPSC’s March 2020 electric rate order. See Note 2 – Rate and Regulatory Matters for additional information.

- Definition

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

+ References + Details

Supplemental Information

9 Months Ended

Sep. 30, 2020

Supplemental Information [Abstract]

Supplemental Information

SUPPLEMENTAL INFORMATION

Cash, Cash Equivalents, and Restricted Cash

The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets and the statements of cash flows as of September 30, 2020, and December 31, 2019:


	September 30, 2020						December 31, 2019
	Ameren		Ameren Missouri		Ameren Illinois		Ameren		Ameren Missouri		Ameren Illinois
Cash and cash equivalents	$	6	$	—	$	—	$	16	$	9	$	—
Restricted cash included in “Other current assets”	14		4		5		14		4		5
Restricted cash included in “Other assets”	142		—		142		120		—		120
Restricted cash included in “Nuclear decommissioning trust fund”	6		6		—		26		26		—
Total cash, cash equivalents, and restricted cash	$	168	$	10	$	147	$	176	$	39	$	125

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

Accounts Receivable

“Accounts receivable – trade” on Ameren’s and Ameren Illinois’ balance sheets include certain receivables purchased at a discount from alternative retail electric suppliers that elect to participate in the utility consolidated billing program. At September 30, 2020, and December 31, 2019, “Other current liabilities” on Ameren’s and Ameren Illinois’ balance sheets included payables for purchased receivables of $36 million and $32 million, respectively.

The following table provides a reconciliation of the beginning and ending amount of the allowance for doubtful accounts for the three and nine months ended September 30, 2020 and 2019:


	Three Months				Nine Months
	2020		2019		2020		2019
Ameren:
Beginning of period	$	25	$	19	$	17	$	18
Bad debt expense	21		14		31		22

Net write-offs	(2)		(14)		(4)		(21)

End of period	$	44	$	19	$	44	$	19
Ameren Missouri:
Beginning of period	$	9	$	7	$	7	$	7
Bad debt expense	5		3		9		6

Net write-offs	—		(2)		(2)		(5)
End of period	$	14	$	8	$	14	$	8
Ameren Illinois:(a)
Beginning of Period	$	16	$	12	$	10	$	11
Bad debt expense	16		11		22		16

Net write-offs	(2)		(12)		(2)		(16)

End of Period	$	30	$	11	$	30	$	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. In 2020, the rate-adjustment mechanism for electric distribution allows for recovery of bad debt expense recognized under GAAP. See Note 2 – Rate and Regulatory Matters for additional information.

Net write-offs decreased for the three and nine months ended September 30, 2020, compared with the year-ago periods, due to the temporary suspension of disconnecting customers 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 nine months ended September 30, 2020 and 2019:


	September 30, 2020						September 30, 2019
	Ameren		Ameren Missouri				Ameren Illinois	Ameren		Ameren Missouri		Ameren Illinois
Investing

Exchange of bond investments for the extinguishment of senior unsecured notes	$	—	$	—	$	—		$	17	$	—	$	17
Accrued capital expenditures	311		115		191			273		138		128

Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	43		43		—			100		100		—
Financing
Exchange of bond investments for the extinguishment of senior unsecured notes	$	—	$	—	$	—		$	(17)	$	—	$	(17)
Issuance of common stock for stock-based compensation	38		—		—			54		—		—

Asset Retirement Obligations

The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the nine months ended September 30, 2020:


	Ameren Missouri			Ameren Illinois			Ameren
Balance at December 31, 2019	$	687		$	4	(a)	$	691	(b)

Liabilities settled	(42)			—			(42)
Accretion	21		(c)	1		(c)	22		(c)
Change in estimates	57		(d)	—			57		(d)
Balance at September 30, 2020	$	723		$	5	(a)	$	728	(b)

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

(b)Balance included $53 million in “Other current liabilities” on the balance sheet as of both December 31, 2019, and September 30, 2020.

(c)Accretion expense attributable to Ameren Missouri and Ameren Illinois was recorded as a decrease to regulatory liabilities and an increase to regulatory assets, respectively.

(d)Ameren Missouri changed its fair value estimate primarily due to an update to the decommissioning of the Callaway Energy Center to reflect the cost study and funding analysis filed with the MoPSC in November 2020 and an increase in the cost estimate for closure of certain CCR storage facilities.

Stock-based Compensation

On January 1, 2020, Ameren granted 294,320 performance share units with a grant date fair value of $24 million and 132,307 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 (252,370 performance share units) or based on the achievement of renewable generation and energy storage installation targets (41,950 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.

On September 18, 2020, Ameren granted certain executive officers an additional 37,104 restricted share units with a grant date fair value of $3 million, which will vest after three years on September 17, 2023. The awards do not provide for pro rata vesting in connection with the executive officer's retirement.

For the nine months ended September 30, 2020 and 2019, excess tax benefits associated with the settlement of stock-based compensation awards reduced income tax expense by $8 million and $14 million, respectively.

Deferred Compensation

As of September 30, 2020, and December 31, 2019, the present value of benefits to be paid for deferred compensation obligations was $88 million and $86 million, respectively, which was primarily reflected in “Other deferred credits and liabilities” on Ameren's consolidated balance sheet.

Operating Revenues

As of September 30, 2020 and 2019, our remaining performance obligations for contracts with a term greater than one year were immaterial. The Ameren Companies elected not to disclose the aggregate amount of the transaction price allocated to the performance obligations that are unsatisfied as of the end of the reporting period for contracts with an initial expected term of one year or less.

See Note 14 – Segment Information for disaggregated revenue information.

Excise Taxes

Ameren Missouri and Ameren Illinois collect from their customers excise taxes, including municipal and state excise taxes and gross receipts taxes that are levied on the sale or distribution of natural gas and electricity. The following table presents the excise taxes recorded on a gross basis in “Operating Revenues – Electric,” “Operating Revenues – Natural gas” and “Operating Expenses – Taxes other than income taxes” on the statements of income for the three and nine months ended September 30, 2020 and 2019:


	Three Months				Nine Months
	2020		2019		2020		2019
Ameren Missouri	$	45	$	49	$	111	$	118
Ameren Illinois	26		27		87		91
Ameren	$	71	$	76	$	198	$	209

Earnings per Share

The following table reconciles the basic weighted-average number of common shares outstanding to the diluted weighted-average number of common shares outstanding for the three and nine months ended September 30, 2020 and 2019:


	Three Months		Nine Months
	2020	2019	2020	2019


Weighted-average Common Shares Outstanding – Basic	247.1	245.9	246.8	245.5
Assumed settlement of performance share units and restricted stock units	1.5	1.4	1.2	1.4
Dilutive effect of forward sale agreement	0.6	0.2	0.4	0.1
Weighted-average Common Shares Outstanding – Diluted(a)	249.2	247.5	248.4	247.0

(a)There were no potentially dilutive securities excluded from the earnings per diluted share calculations for the three and nine months ended September 30, 2020 and 2019.

Segment Information

9 Months Ended

Sep. 30, 2020

Segment Reporting [Abstract]

SEGMENT INFORMATION

The following tables present revenues, net income (loss) attributable to common shareholders, and capital expenditures by segment at Ameren and Ameren Illinois for the three and nine months ended September 30, 2020 and 2019. 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 2020:
External revenues	$	990	$	390	$	122	$	126		$	—	$	—	$	1,628
Intersegment revenues	11		1		—		15		(a)	—		(27)		—
Net income (loss) attributable to Ameren common shareholders	297		34		2		62		(b)	(28)		—		367
Capital expenditures	262		129		81		185			2		(3)		656
Three Months 2019:
External revenues	$	1,050	$	374	$	107	$	109		$	—	$	—	$	1,640
Intersegment revenues	9		15		5		19		(a)	—		(29)		19		(c)
Net income (loss) attributable to Ameren common shareholders	300		32		(1)		53		(b)	(20)		—		364
Capital expenditures	256		139		113		129			(7)		6		636
Nine Months 2020:
External revenues	$	2,442	$	1,131	$	533	$	360		$	—	$	—	$	4,466
Intersegment revenues	31		2		—		40		(a)	—		(73)		—
Net income (loss) attributable to Ameren common shareholders	439		107		66		168		(b)	(24)		—		756
Capital expenditures	778		391		221		490			4		—		1,884
Nine Months 2019:
External revenues	$	2,591	$	1,118	$	563	$	303		$	—	$	—	$	4,575
Intersegment revenues	24		17		5		48		(a)	—		(75)		19		(c)
Net income (loss) attributable to Ameren common shareholders	446		105		57		139		(b)	(13)		—		734
Capital expenditures	751		390		241		377			3		(1)		1,761

(a)Ameren Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution.

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

(c)Intersegment revenues at Ameren include $14 million and $5 million of revenue from Ameren Illinois Electric Distribution and Ameren Illinois Natural Gas, respectively, for the three and nine months ended September 30, 2019, for a software licensing agreement with Ameren Missouri. Under authoritative accounting guidance for rate-regulated entities, the revenue recognized by Ameren Illinois was not eliminated upon consolidation. See Note 8 – Related-party Transactions under Part I, Item 1, of this report for additional information.

Ameren Illinois


	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Illinois Transmission			Intersegment Eliminations		Ameren Illinois
Three Months 2020:
External revenues	$	391	$	122	$	76		$	—	$	589
Intersegment revenues	—		—		15		(a)	(15)		—
Net income available to common shareholder	34		2		41			—		77
Capital expenditures	129		81		160			—		370
Three Months 2019:
External revenues	$	389	$	112	$	63		$	—	$	564
Intersegment revenues	—		—		18		(a)	(18)		—
Net income (loss) available to common shareholder	32		(1)		34			—		65
Capital expenditures	139		113		92			—		344


	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Illinois Transmission			Intersegment Eliminations		Ameren Illinois
Nine Months 2020:
External revenues	$	1,133	$	533	$	213		$	—	$	1,879
Intersegment revenues	—		—		39		(a)	(39)		—
Net income available to common shareholder	107		66		107			—		280
Capital expenditures	391		221		419			—		1,031
Nine Months 2019:
External revenues	$	1,135	$	568	$	170		$	—	$	1,873
Intersegment revenues	—		—		47		(a)	(47)		—
Net income available to common shareholder	105		57		85			—		247
Capital expenditures	390		241		269			—		900

(a)Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution.

The following tables present disaggregated revenues by segment at Ameren and Ameren Illinois for the three and nine months ended September 30, 2020 and 2019. 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 2020:
Residential	$	455	$	234		$	—		$	—	$	—	$	689
Commercial	343		127			—			—		—		470
Industrial	87		26			—			—		—		113
Other	99		4			—			141		(27)		217
Total electric revenues	$	984	$	391		$	—		$	141	$	(27)	$	1,489
Residential	$	8	$	—		$	68		$	—	$	—	$	76
Commercial	3		—			18			—		—		21
Industrial	1		—			5			—		—		6
Other	5		—			31			—		—		36
Total gas revenues	$	17	$	—		$	122		$	—	$	—	$	139
Total revenues(a)	$	1,001	$	391		$	122		$	141	$	(27)	$	1,628
Three Months 2019:
Residential	$	489	$	224		$	—		$	—	$	—	$	713
Commercial	394		123			—			—		—		517
Industrial	94		27			—			—		—		121
Other	63		15		(b)	—			128		(29)		177
Total electric revenues	$	1,040	$	389		$	—		$	128	$	(29)	$	1,528
Residential	$	8	$	—		$	65		$	—	$	—	$	73
Commercial	4		—			17			—		—		21
Industrial	1		—			2			—		—		3
Other	6		—			28		(b)	—		—		34
Total gas revenues	$	19	$	—		$	112		$	—	$	—	$	131
Total revenues(a)	$	1,059	$	389		$	112		$	128	$	(29)	$	1,659


	Ameren Missouri		Ameren Illinois Electric Distribution			Ameren Illinois Natural Gas			Ameren Transmission		Intersegment Eliminations		Ameren
Nine Months 2020:
Residential	$	1,111	$	664		$	—		$	—	$	—	$	1,775
Commercial	828		365			—			—		—		1,193
Industrial	207		91			—			—		—		298
Other	240		13			—			400		(73)		580
Total electric revenues	$	2,386	$	1,133		$	—		$	400	$	(73)	$	3,846
Residential	$	52	$	—		$	375		$	—	$	—	$	427
Commercial	20		—			94			—		—		114
Industrial	3		—			11			—		—		14
Other	12		—			53			—		—		65
Total natural gas revenues	$	87	$	—		$	533		$	—	$	—	$	620
Total revenues(a)	$	2,473	$	1,133		$	533		$	400	$	(73)	$	4,466
Nine Months 2019:
Residential	$	1,134	$	640		$	—		$	—	$	—	$	1,774
Commercial	943		370			—			—		—		1,313
Industrial	226		94			—			—		—		320
Other	214		31		(b)	—			351		(75)		521
Total electric revenues	$	2,517	$	1,135		$	—		$	351	$	(75)	$	3,928
Residential	$	56	$	—		$	399		$	—	$	—	$	455
Commercial	24		—			105			—		—		129
Industrial	3		—			9			—		—		12
Other	15		—			55		(b)	—		—		70
Total natural gas revenues	$	98	$	—		$	568		$	—	$	—	$	666
Total revenues(a)	$	2,615	$	1,135		$	568		$	351	$	(75)	$	4,594

(a)The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the three and nine months ended September 30, 2020 and 2019:


	Ameren Missouri		Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Transmission		Ameren
Three Months 2020:
Revenues from alternative revenue programs	$	1	$	(110)	$	3	$	4	$	(102)
Other revenues not from contracts with customers	6		5		—		—		11

Three Months 2019:
Revenues from alternative revenue programs	$	26	$	(145)	$	1	$	(12)	$	(130)
Other revenues not from contracts with customers	5		1		1		—		7

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

Nine Months 2019:
Revenues from alternative revenue programs	$	41	$	(111)	$	2	$	(25)	$	(93)
Other revenues not from contracts with customers	14		5		2		—		21

(b)Includes $14 million and $5 million for Ameren Illinois Electric Distribution and Ameren Illinois Natural Gas, respectively, for the three and nine months ended September 30, 2019, for a software licensing agreement with Ameren Missouri. See Note 8 – Related-party Transactions for additional information.

Ameren Illinois


	Ameren Illinois Electric Distribution			Ameren Illinois Natural Gas			Ameren Illinois Transmission		Intersegment Eliminations		Ameren Illinois
Three Months 2020:
Residential	$	234		$	68		$	—	$	—	$	302
Commercial	127			18			—		—		145
Industrial	26			5			—		—		31
Other	4			31			91		(15)		111
Total revenues(a)	$	391		$	122		$	91	$	(15)	$	589
Three Months 2019:
Residential	$	224		$	65		$	—	$	—	$	289
Commercial	123			17			—		—		140
Industrial	27			2			—		—		29
Other	15		(b)	28		(b)	81		(18)		106
Total revenues(a)	$	389		$	112		$	81	$	(18)	$	564
Nine Months 2020:
Residential	$	664		$	375		$	—	$	—	$	1,039
Commercial	365			94			—		—		459
Industrial	91			11			—		—		102
Other	13			53			252		(39)		279
Total revenues(a)	$	1,133		$	533		$	252	$	(39)	$	1,879
Nine Months 2019:
Residential	$	640		$	399		$	—	$	—	$	1,039
Commercial	370			105			—		—		475
Industrial	94			9			—		—		103
Other	31		(b)	55		(b)	217		(47)		256
Total revenues(a)	$	1,135		$	568		$	217	$	(47)	$	1,873

(a)The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the Ameren Illinois segments for the three and nine months ended September 30, 2020 and 2019:


	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Illinois Transmission		Ameren Illinois
Three Months 2020:
Revenues from alternative revenue programs	$	(110)	$	3	$	5	$	(102)
Other revenues not from contracts with customers	5		—		—		5

Three Months 2019:
Revenues from alternative revenue programs	$	(145)	$	1	$	(12)	$	(156)
Other revenues not from contracts with customers	1		1		—		2

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

Nine Months 2019:
Revenues from alternative revenue programs	$	(111)	$	2	$	(26)	$	(135)
Other revenues not from contracts with customers	5		2		—		7

(b)Includes $14 million and $5 million for Ameren Illinois Electric Distribution and Ameren Illinois Natural Gas, respectively, for the three and nine months ended September 30, 2019, for a software licensing agreement with Ameren Missouri. See Note 8 – Related-party Transactions for additional information.

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 31
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8924-108599

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8981-108599

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8721-108599

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 40
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9031-108599

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
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+ Details

Summary Of Significant Accounting Policies (Policies)	9 Months Ended
Summary Of Significant Accounting Policies (Policies)	Sep. 30, 2020
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, except as disclosed in Note 8 – Related-party Transactions. Ameren Missouri and Ameren Illinois have no subsidiaries. All tabular dollar amounts are in millions, unless otherwise indicated. Our accounting policies conform to GAAP. Our financial statements reflect all adjustments (which include normal, recurring adjustments) that are necessary, in our opinion, for a fair 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 September 30, 2020, Ameren and Ameren Missouri had interests in unconsolidated variable interest entities that were established to construct wind generation facilities and, ultimately, sell those constructed facilities to Ameren Missouri. Neither Ameren nor Ameren Missouri are the primary beneficiary of these variable interest entities because neither has the power to direct matters that most significantly affect the entities' activities, which include designing, financing, and constructing the wind generation facilities. As a result, these variable interest entities have not been consolidated. As of September 30, 2020, the maximum exposure to loss related to these variable interest entities was approximately $18 million, which primarily represents due diligence and legal costs incurred by Ameren Missouri associated with the acquisitions. The risk of a loss was assessed to be remote and, accordingly, Ameren and Ameren Missouri have not recognized a liability associated with any portion of the maximum exposure to loss. See Note 2 – Rate and Regulatory Matters for additional information on the agreements to acquire these wind generation facilities. As of September 30, 2020, and December 31, 2019, Ameren had unconsolidated variable interests as a limited partner in various equity method investments, totaling $34 million and $28 million, respectively, included in “Other assets” on Ameren’s consolidated balance sheet. Ameren is not the primary beneficiary of these investments because it does not have the power to direct matters that most significantly affect the activities of these variable interest entities. As of September 30, 2020, the maximum exposure to loss related to these variable interests is limited to the investment in these partnerships of $34 million plus associated outstanding funding commitments of $37 million.
Company-owned Life Insurance	Company-owned Life Insurance Ameren and Ameren Illinois have company-owned life insurance, which is recorded at the net cash surrender value. The net cash surrender value is the amount that can be realized under the insurance policies at the balance sheet date. As of September 30, 2020, the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $260 million (December 31, 2019 – $264 million) and $114 million (December 31, 2019 – $123 million), respectively, while total borrowings against the policies were $107 million (December 31, 2019 – $114 million) at both Ameren and Ameren Illinois. Ameren and Ameren Illinois have the right to offset the borrowings against the cash surrender value of the policies and, consequently, present the net asset in “Other assets” on their respective balance sheets.
Accounting and Reporting Developments	Accounting and Reporting Developments See Note 1 – Summary of Significant Accounting Policies under Part II, Item 8, of the Form 10-K for additional information about authoritative accounting guidance relating to defined benefit plan disclosures that will be effective for the Ameren Companies in the fourth quarter of 2020. Measurement of Credit Losses on Financial Instruments On January 1, 2020, the Ameren Companies adopted authoritative accounting guidance that requires credit losses on most financial assets carried at amortized cost and off-balance sheet credit exposures, such as financial guarantees or loan commitments, to be measured using a current expected credit loss (CECL) model. The guidance requires an entity to measure expected credit losses using relevant information about past events, current conditions, and reasonable and supportable forecasts that affect the collectibility of the reported amount. In addition, the guidance made certain changes to the impairment model applicable to available-for-sale debt securities, such as requiring credit losses to be presented as an allowance rather than a write-down on impaired debt securities for which there is neither an intent nor a more-likely-than-not requirement to sell. Our adoption of this guidance did not have a material impact on the Ameren Companies’ financial statements and did not result in a cumulative effect adjustment to retained earnings as of the adoption date. See Note 13 – Supplemental Information for additional information regarding credit losses on accounts receivable.
Derivatives, Policy	We use derivatives to manage the risk of changes in market prices for natural gas, power and uranium, as well as the risk of changes in rail transportation surcharges through fuel oil hedges. Such price fluctuations may cause the following: •an unrealized appreciation or depreciation of our contracted commitments to purchase or sell when purchase or sale prices under the commitments are compared with current commodity prices; •market values of natural gas and uranium inventories that differ from the cost of those commodities in inventory; •actual cash outlays for the purchase of these commodities that differ from anticipated cash outlays; and •actual off-system sales revenues that differ from anticipated revenues The derivatives that we use to hedge these risks are governed by our risk management policies for forward contracts, futures, options, and swaps. Our net positions are continually assessed within our structured hedging programs to determine whether new or offsetting transactions are required. The goal of the hedging program is generally to mitigate financial risks while ensuring that sufficient volumes are available to meet our requirements. Contracts we enter into as part of our risk management program may be settled financially, settled by physical delivery, or net settled with the counterparty. 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.
Deferred Compensation	Deferred Compensation As of September 30, 2020, and December 31, 2019, the present value of benefits to be paid for deferred compensation obligations was $88 million and $86 million, respectively, which was primarily reflected in “Other deferred credits and liabilities” on Ameren's consolidated balance sheet.
Revenue from Contract with Customer	Operating Revenues As of September 30, 2020 and 2019, 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 (Tables)

9 Months Ended

Sep. 30, 2020

Debt Disclosure [Abstract]

Schedule of Short-term Debt

The following table presents commercial paper outstanding, net of issuance discounts, as of September 30, 2020, and December 31, 2019. There were no borrowings outstanding under the Credit Agreements as of September 30, 2020, or December 31, 2019.


	September 30, 2020		December 31, 2019
Ameren (parent)	$	30	$	153
Ameren Missouri	—		234
Ameren Illinois	242		53
Ameren consolidated	$	272	$	440

The following table summarizes the activity and relevant interest rates for Ameren (parent)’s, Ameren Missouri’s, and Ameren Illinois’ commercial paper issuances and borrowings under the Credit Agreements in the aggregate for the nine months ended September 30, 2020 and 2019:


	Ameren (parent)			Ameren Missouri				Ameren Illinois				Ameren Consolidated
2020
Average daily amount outstanding	$	62		$	142			$	53			$	257

Weighted-average interest rate	2.04		%	1.76		%		1.10		%		1.70		%
Peak amount outstanding during period(a)	$	425		$	573			$	243			$	908
Peak interest rate	3.30		%	5.05		%	(b)	3.40		%		5.05		%	(b)
2019
Average daily amount outstanding	$	532		$	141			$	147			$	821

Weighted-average interest rate	2.70		%	2.73		%		2.58		%		2.68		%
Peak amount outstanding during period(a)	$	651		$	549			$	310			$	1,113
Peak interest rate	3.80		%	2.97		%		5.00		%	(c)	5.00		%	(c)

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

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

(c)Ameren’s and Ameren Illinois’ peak interest rate was affected by temporary disruptions in the commercial paper market in the third quarter of 2019.

Other Income and Expenses (Tables)

9 Months Ended

Sep. 30, 2020

Other Nonoperating Income (Expense) [Abstract]

Other Income And Expenses

The following table presents the components of “Other Income, Net” in the Ameren Companies’ statements of income for the three and nine months ended September 30, 2020 and 2019:


	Three Months				Nine Months
	2020		2019		2020			2019
Ameren:

Allowance for equity funds used during construction	$	12	$	7	$	25		$	20
Interest income on industrial development revenue bonds	7		6		19			19
Other interest income	—		2		2			6

Non-service cost components of net periodic benefit income(a)	32		23		85			67
Miscellaneous income	1		2		10			6
Donations	—		(1)		(14)		(b)	(8)
Miscellaneous expense	(4)		(5)		(10)			(11)
Total Other Income, Net	$	48	$	34	$	117		$	99
Ameren Missouri:

Allowance for equity funds used during construction	$	7	$	6	$	15		$	14
Interest income on industrial development revenue bonds	7		6		19			19

Non-service cost components of net periodic benefit income(a)	13		4		32			13
Miscellaneous income	1		2		3			4
Donations	—		(1)		(9)		(b)	(3)
Miscellaneous expense	(2)		(2)		(5)			(4)
Total Other Income, Net	$	26	$	15	$	55		$	43
Ameren Illinois:

Allowance for equity funds used during construction	$	5	$	1	$	10		$	6
Interest income	—		1		2			5

Non-service cost components of net periodic benefit income	12		12		36			36
Miscellaneous income	1		1		6			3
Donations	—		—		(5)			(5)
Miscellaneous expense	(1)		(2)		(4)			(6)
Total Other Income, Net	$	17	$	13	$	45		$	39

(a)For the three and nine months ended September 30, 2020, the non-service cost components of net periodic benefit income were adjusted by amounts deferred of $(3) million and less than $(1) million, respectively, due to a regulatory tracking mechanism for the difference between the level of such costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. The deferral was $7 million and $22 million for three and nine months ended September 30, 2019, respectively.

(b)Includes $8 million pursuant to Ameren Missouri’s March 2020 electric rate order. See Note 2 – Rate and Regulatory Matters for additional information.

Derivative Financial Instruments (Tables)

9 Months Ended

Sep. 30, 2020

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Open Gross Derivative Volumes By Commodity Type

The following table presents open gross commodity contract volumes by commodity type for derivative assets and liabilities as of September 30, 2020, and December 31, 2019. As of September 30, 2020, these contracts extended through October 2023, October 2025, May 2032 and March 2023 for fuel oils, natural gas, power and uranium, respectively.


	Quantity (in millions, except as indicated)
	September 30, 2020			December 31, 2019
Commodity	Ameren Missouri	Ameren Illinois	Ameren	Ameren Missouri	Ameren Illinois	Ameren
Fuel oils (in gallons)	44	—	44	58	—	58
Natural gas (in mmbtu)	32	123	155	20	136	156
Power (in megawatthours)	6	7	13	5	7	12
Uranium (pounds in thousands)	365	—	365	565	—	565

Derivative Instruments Carrying Value

The following table presents the carrying value and balance sheet location of all derivative commodity contracts, none of which were designated as hedging instruments, as of September 30, 2020, and December 31, 2019:


			September 30, 2020					December 31, 2019
	Balance Sheet Location		Ameren Missouri		Ameren Illinois		Ameren			Ameren Missouri		Ameren Illinois		Ameren









Fuel oils	Other current assets	$	2	$	—	$	2		$	4	$	—	$	4
	Other assets		—		—		—			2		—		2


Natural gas	Other current assets		2		10		12			—		3		3
	Other assets		2		4		6			—		1		1


Power	Other current assets		12		—		12			14		—		14
	Other assets		1		—		1			2		—		2


	Total assets	$	19	$	14	$	33		$	22	$	4	$	26





Fuel oils	Other current liabilities	$	14	$	—	$	14		$	4	$	—	$	4
	Other deferred credits and liabilities		6		—		6			3		—		3


Natural gas	Other current liabilities		—		2		2			1		12		13
	Other deferred credits and liabilities		—		1		1			1		6		7


Power	Other current liabilities		6		16		22			2		17		19
	Other deferred credits and liabilities		4		197		201			1		207		208


Uranium	Other deferred credits and liabilities		—		—		—			1		—		1
	Total liabilities	$	30	$	216	$	246		$	13	$	242	$	255

Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2020

Fair Value Disclosures [Abstract]

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

The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of September 30, 2020, and December 31, 2019:


		September 30, 2020									December 31, 2019
		Level 1		Level 2		Level 3		Total			Level 1		Level 2		Level 3		Total
Assets:
Ameren Missouri
	Derivative assets – commodity contracts:
	Fuel oils	$	—	$	—	$	2	$	2		$	—	$	—	$	6	$	6
	Natural gas	—		4		—		4			—		—		—		—
	Power	1		—		12		13			—		2		14		16

	Total derivative assets – commodity contracts	$	1	$	4	$	14	$	19		$	—	$	2	$	20	$	22
	Nuclear decommissioning trust fund:

	Equity securities:
	U.S. large capitalization	$	604	$	—	$	—	$	604		$	569	$	—	$	—	$	569
	Debt securities:
	U.S. Treasury and agency securities	—		112		—		112			—		107		—		107
	Corporate bonds	—		116		—		116			—		93		—		93


	Other	—		66		—		66			—		73		—		73
	Total nuclear decommissioning trust fund	$	604	$	294	$	—	$	898	(a)	$	569	$	273	$	—	$	842	(a)
	Total Ameren Missouri	$	605	$	298	$	14	$	917		$	569	$	275	$	20	$	864
Ameren Illinois
	Derivative assets – commodity contracts:
	Natural gas	$	—	$	9	$	5	$	14		$	—	$	1	$	3	$	4


Ameren
	Derivative assets – commodity contracts(b)	$	1	$	13	$	19	$	33		$	—	$	3	$	23	$	26
	Nuclear decommissioning trust fund(c)	604		294		—		898		(a)	569		273		—		842		(a)
	Total Ameren	$	605	$	307	$	19	$	931		$	569	$	276	$	23	$	868
Liabilities:
Ameren Missouri
	Derivative liabilities – commodity contracts:
	Fuel oils	$	13	$	—	$	7	$	20		$	1	$	—	$	6	$	7
	Natural gas	—		—		—		—			—		2		—		2
	Power	8		—		2		10			—		2		1		3
	Uranium	—		—		—		—			—		—		1		1
	Total Ameren Missouri	$	21	$	—	$	9	$	30		$	1	$	4	$	8	$	13
Ameren Illinois
	Derivative liabilities – commodity contracts:
	Natural gas	$	—	$	2	$	1	$	3		$	3	$	12	$	3	$	18
	Power	—		—		213		213			—		—		224		224
	Total Ameren Illinois	$	—	$	2	$	214	$	216		$	3	$	12	$	227	$	242
Ameren
	Derivative liabilities – commodity contracts(b)	$	21	$	2	$	223	$	246		$	4	$	16	$	235	$	255

(a)Balance excludes $6 million and $5 million of cash and cash equivalents, receivables, payables, and accrued income, net, for September 30, 2020, and December 31, 2019, respectively.

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

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

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

The following table presents the fair value reconciliation of Level 3 power derivative contract assets and liabilities measured at fair value on a recurring basis for the three and nine months ended September 30, 2020 and 2019:


	2020						2019
	Ameren Missouri		Ameren Illinois		Ameren			Ameren Missouri		Ameren Illinois		Ameren
For the three months ended September 30:
Beginning balance at July 1	$	16	$	(229)	$	(213)		$	15	$	(191)	$	(176)
Realized and unrealized gains/(losses) included in regulatory assets/liabilities	(2)		12		10			(4)		(17)		(21)


Settlements	(4)		4		—			(1)		4		3

Transfers out of Level 3	—		—		—			(2)		—		(2)
Ending balance at September 30	$	10	$	(213)	$	(203)		$	8	$	(204)	$	(196)
Change in unrealized gains/(losses) related to assets/liabilities held at September 30	$	(2)	$	11	$	9		$	(4)	$	(17)	$	(21)
For the nine months ended September 30:
Beginning balance at January 1	$	13	$	(224)	$	(211)		$	—	$	(183)	$	(183)
Realized and unrealized gains/(losses) included in regulatory assets/liabilities	18		(2)		16			12		(32)		(20)


Settlements	(21)		13		(8)			(2)		11		9

Transfers out of Level 3	—		—		—			(2)		—		(2)
Ending balance at September 30	10		(213)		(203)			8		(204)		(196)
Change in unrealized gains/(losses) related to assets/liabilities held at September 30	$	6	$	(1)	$	5		$	8	$	(31)	$	(23)

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

The following table describes the valuation techniques and significant unobservable inputs utilized for the fair value of our Level 3 power derivative contract assets and liabilities as of September 30, 2020, and December 31, 2019:


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









2020	Power(c)	$	12	$	(215)	Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps ($/MWh)	20 – 36	28

							Nodal basis ($/MWh)	(5) – 1	(2)


							Trend rate (%)	3 – 4	3








































2019	Power(d)	$	14	$	(225)	Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps ($/MWh)	22 – 34	25

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


							Trend rate (%)	(1) – 0	0

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

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

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

(d)Valuations through 2028 use visible forward prices adjusted for nodal-to-hub basis differentials. Valuations beyond 2028 use a trend rate factor and are similarly adjusted for nodal-to-hub basis differentials.

Schedule of Financial Assets and Liabilities

The following table sets forth the carrying amount and, by level within the fair value hierarchy, the fair value of financial assets and liabilities disclosed, but not recorded, at fair value as of September 30, 2020, and December 31, 2019:


	Carrying Amount			Fair Value
				Level 1		Level 2		Level 3			Total
	September 30, 2020
Ameren:
Cash, cash equivalents, and restricted cash	$	168		$	168	$	—	$	—		$	168
Investments in industrial development revenue bonds(a)	263			—		263		—			263
Short-term debt	272			—		272		—			272
Long-term debt (including current portion)(a)	10,529		(b)	—		11,891		521		(c)	12,412
Ameren Missouri:
Cash, cash equivalents, and restricted cash	$	10		$	10	$	—	$	—		$	10
Advances to money pool	5			—		5		—			5
Investments in industrial development revenue bonds(a)	263			—		263		—			263

Long-term debt (including current portion)(a)	4,568		(b)	—		5,316		—			5,316
Ameren Illinois:
Cash, cash equivalents, and restricted cash	$	147		$	147	$	—	$	—		$	147
Short-term debt	242			—		242		—			242

Long-term debt (including current portion)	3,576		(b)	—		4,431		—			4,431
	December 31, 2019
Ameren:
Cash, cash equivalents, and restricted cash	$	176		$	176	$	—	$	—		$	176
Investments in industrial development revenue bonds(a)	263			—		263		—			263
Short-term debt	440			—		440		—			440
Long-term debt (including current portion)(a)	9,357		(b)	—		9,957		484		(c)	10,441
Ameren Missouri:
Cash, cash equivalents, and restricted cash	$	39		$	39	$	—	$	—		$	39
Investments in industrial development revenue bonds(a)	263			—		263		—			263
Short-term debt	234			—		234		—			234
Long-term debt (including current portion)(a)	4,190		(b)	—		4,772		—			4,772
Ameren Illinois:
Cash, cash equivalents, and restricted cash	$	125		$	125	$	—	$	—		$	125
Short-term debt	53			—		53		—			53
Long-term debt (including current portion)	3,575		(b)	—		4,019		—			4,019

(a)Ameren and Ameren Missouri have investments in industrial development revenue bonds, classified as held-to-maturity and recorded in “Other Assets,” that are equal to the finance obligations for the Peno Creek and Audrain CT energy centers. As of September 30, 2020, and December 31, 2019, the carrying amount of both the investments in industrial development revenue bonds and the finance obligations approximated fair value.

(b)Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $77 million, $32 million, and $33 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of September 30, 2020. Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $72 million, $30 million, and $34 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2019.

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

Related Party Transactions (Tables)

9 Months Ended

Sep. 30, 2020

Schedule of Affiliate Receivables and Payables

The following table presents the affiliate balances related to income taxes for Ameren Missouri and Ameren Illinois as of September 30, 2020, and December 31, 2019:


	September 30, 2020				December 31, 2019
	Ameren Missouri		Ameren Illinois		Ameren Missouri		Ameren Illinois
Income taxes payable to parent(a)	$	38	$	9	$	15	$	43
Income taxes receivable from parent(b)	—		6		15		17

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

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

Schedule of Related Party Transactions

The following table presents the effect on Ameren Missouri and Ameren Illinois of related-party transactions for the three and nine months ended September 30, 2020 and 2019:


				Three Months				Nine Months
Agreement	Income Statement Line Item			Ameren Missouri		Ameren Illinois		Ameren Missouri		Ameren Illinois
Ameren Missouri power supply	Operating Revenues	2020	$	5	$	(a)	$	11	$	(a)
agreements with Ameren Illinois		2019		1		(a)		3		(a)
Ameren Missouri and Ameren Illinois	Operating Revenues	2020	$	6	$	(b)	$	19	$	1
rent and facility services		2019		7		1		20		2
Ameren Missouri and Ameren Illinois	Operating Revenues	2020	$	(b)	$	1	$	1	$	1
miscellaneous support services		2019		1		(b)		1		1
Ameren Missouri software licensing	Operating Revenues	2020	$	(a)	$	(a)	$	(a)	$	(a)
with Ameren Illinois(c)		2019		(a)		19		(a)		19
Total Operating Revenues		2020	$	11	$	1	$	31	$	2
		2019		9		20		24		22
Ameren Illinois power supply	Purchased Power	2020	$	(a)	$	5	$	(a)	$	11
agreements with Ameren Missouri		2019		(a)		1		(a)		3
Ameren Illinois transmission	Purchased Power	2020	$	(a)	$	(b)	$	(a)	$	1
services with ATXI		2019		(a)		1		(a)		1
Total Purchased Power		2020	$	(a)	$	5	$	(a)	$	12
		2019		(a)		2		(a)		4
Ameren Missouri and Ameren Illinois	Other Operations and Maintenance	2020	$	(b)	$	1	$	(b)	$	3
rent and facility services		2019		(b)		1		1		4
Ameren Services support services	Other Operations and Maintenance	2020	$	36	$	34	$	103	$	98
agreement		2019		34		30		98		91
Total Other Operations and		2020	$	36	$	35	$	103	$	101
Maintenance		2019		34		31		99		95
Money pool borrowings (advances)	(Interest Charges)/Other Income, Net	2020	$	(b)	$	(b)	$	(b)	$	(b)
		2019		(b)		(b)		(b)		(b)

(a)Not applicable.

(b)Amount less than $1 million.

(c)In September 2019, Ameren Missouri purchased a license for advanced metering infrastructure software from Ameren Illinois. The amount of the $24 million cost-based transaction price over the $5 million remaining carrying value of the software was recorded as revenue by Ameren Illinois, with $14 million of revenue recorded at Ameren Illinois Electric Distribution and $5 million recorded at Ameren Illinois Natural Gas. The revenue recorded at Ameren Illinois Electric Distribution was reflected in formula ratemaking, which resulted in no impact to net income. Per authoritative accounting guidance for sales to rate-regulated entities, the revenue recognized by Ameren Illinois was not eliminated upon consolidation by Ameren.

Commitments and Contingencies (Tables)

9 Months Ended

Sep. 30, 2020

Commitments and Contingencies Disclosure [Abstract]

Long-term Purchase Commitment

The table below presents our estimated minimum fuel, purchased power, and other commitments at September 30, 2020. Ameren’s and Ameren Illinois’ purchased power commitments include the Ameren Illinois agreements entered into as part of the IPA-administered power procurement process. Included in the Other column are minimum purchase commitments under contracts for equipment, design and construction, and meter reading services, among other agreements, at September 30, 2020.


	Coal		Natural Gas(a)		Nuclear Fuel		Purchased Power(b)(c)			Methane Gas		Other		Total
Ameren:
2020	$	94	$	52	$	3	$	57	(d)	$	—	$	39	$	245
2021	242		161		57		156		(d)	3		53		672
2022	193		108		12		63			3		23		402
2023	113		69		45		21			3		23		274
2024	94		28		20		4			3		20		169
Thereafter	55		73		21		—			21		61		231
Total	$	791	$	491	$	158	$	301		$	33	$	219	$	1,993
Ameren Missouri:
2020	$	94	$	12	$	3	$	—		$	—	$	22	$	131
2021	242		43		57		—			3		46		391
2022	193		38		12		—			3		23		269
2023	113		32		45		—			3		23		216
2024	94		13		20		—			3		20		150
Thereafter	55		22		21		—			21		24		143
Total	$	791	$	160	$	158	$	—		$	33	$	158	$	1,300
Ameren Illinois:
2020	$	—	$	40	$	—	$	57	(d)	$	—	$	14	$	111
2021	—		118		—		156		(d)	—		2		276
2022	—		70		—		63			—		—		133
2023	—		37		—		21			—		—		58
2024	—		15		—		4			—		—		19
Thereafter	—		51		—		—			—		—		51
Total	$	—	$	331	$	—	$	301		$	—	$	16	$	648

(a)Includes amounts for generation and for distribution.

(b)The purchased power amounts for Ameren and Ameren Illinois exclude agreements for renewable energy credits through 2035 with various renewable energy suppliers due to the contingent nature of the payment amounts, with the exception of expected payments of $36 million through 2024.

(c)The purchased power amounts for Ameren and Ameren Missouri exclude a 102-megawatt power purchase agreement with a wind farm operator, which expires in 2024, due to the contingent nature of the payment amounts.

(d)In January 2018, as required by the FEJA, Ameren Illinois entered into agreements to acquire zero emission credits through 2026. Annual zero emission credit commitment amounts will be published by the IPA each May prior to the start of the subsequent planning year, which begins each June. The amounts above reflect Ameren Illinois’ commitment to acquire $42 million of zero emission credits through May 2021.

Callaway Energy Center (Tables)

9 Months Ended

Sep. 30, 2020

Nuclear Waste Matters [Abstract]

Schedule of Insurance Coverage at Callaway Energy Center

The following table presents insurance coverage at Ameren Missouri’s Callaway Energy Center at September 30, 2020:


Type and Source of Coverage	Most Recent Renewal Date	Maximum Coverages			Maximum Assessments for Single Incidents
Public liability and nuclear worker liability:
American Nuclear Insurers	January 1, 2020	$	450		$	—
Pool participation	(a)	13,348		(a)	138		(b)
		$	13,798	(c)	$	138
Property damage:
NEIL and EMANI	April 1, 2020	$	3,200	(d)	$	25	(e)
Replacement power:
NEIL	April 1, 2020	$	490	(f)	$	7	(e)

(a)Provided through mandatory participation in an industrywide retrospective premium assessment program. The maximum coverage available is dependent on the number of United States commercial reactors participating in the program.

(b)Retrospective premium under the Price-Anderson Act. This is subject to retrospective assessment with respect to a covered loss in excess of $450 million in the event of an incident at any licensed United States commercial reactor, payable at $21 million per year.

(c)Limit of liability for each incident under the Price-Anderson liability provisions of the Atomic Energy Act of 1954, as amended. This limit is subject to change to account for the effects of inflation and changes in the number of licensed power reactors.

(d)NEIL provides $2.7 billion in property damage, stabilization, decontamination, and premature decommissioning insurance for radiation events and $2.3 billion in property damage insurance for nonradiation events. EMANI provides $490 million in property damage insurance for both radiation and nonradiation events.

(e)All NEIL-insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL.

(f)Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity up to $4.5 million for 52 weeks, which commences after the first 12 weeks of an outage, plus up to $3.6 million per week for a minimum of 71 weeks thereafter for a total not exceeding the policy limit of $490 million. Nonradiation events are limited to $328 million.

Retirement Benefits (Tables)

9 Months Ended

Sep. 30, 2020

Retirement Benefits [Abstract]

Components Of Net Periodic Benefit Cost

The following table presents the components of the net periodic benefit cost (income) incurred for Ameren’s pension and postretirement benefit plans for the three and nine months ended September 30, 2020 and 2019:


	Pension Benefits								Postretirement Benefits
	Three Months				Nine Months				Three Months				Nine Months
	2020		2019		2020		2019		2020		2019		2020		2019
Service cost(a)	$	28	$	22	$	83	$	66	$	4	$	4	$	14	$	13
Non-service cost components:
Interest cost	43		46		130		139		10		11		29		32
Expected return on plan assets	(73)		(69)		(218)		(207)		(20)		(19)		(60)		(57)
Amortization of:

Prior service benefit	—		—		(1)		—		(1)		(1)		(3)		(4)
Actuarial loss (gain)	15		6		45		19		(3)		(4)		(7)		(11)
Total non-service cost components(b)	$	(15)	$	(17)	$	(44)	$	(49)	$	(14)	$	(13)	$	(41)	$	(40)

Net periodic benefit cost (income)	$	13	$	5	$	39	$	17	$	(10)	$	(9)	$	(27)	$	(27)

(a)Service cost, net of capitalization, is reflected in “Operating Expenses – Other operations and maintenance” on Ameren’s statement of income.

(b)Non-service cost components are reflected in “Other Income, Net” on Ameren’s statement of income. See Note 5 – Other Income, Net for additional information.

Summary Of Benefit Plan Costs Incurred

Ameren Missouri and Ameren Illinois are responsible for their respective share of Ameren’s pension and other postretirement costs. The following table presents the respective share of net periodic pension and other postretirement benefit costs (income) incurred for the three and nine months ended September 30, 2020 and 2019:


	Pension Benefits								Postretirement Benefits
	Three Months				Nine Months				Three Months				Nine Months
	2020		2019		2020		2019		2020		2019		2020		2019
Ameren Missouri(a)	$	5	$	1	$	16	$	3	$	(2)	$	(1)	$	(4)	$	(4)
Ameren Illinois	8		5		24		15		(8)		(8)		(24)		(23)
Other	—		(1)		(1)		(1)		—		—		1		—
Ameren(a)	$	13	$	5	$	39	$	17	$	(10)	$	(9)	$	(27)	$	(27)

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

Income Taxes Income Taxes (Tables)

9 Months Ended

Sep. 30, 2020

Income Tax Disclosure [Abstract]

Schedule of Effective Income Tax Rate Reconciliation

The following table presents a reconciliation of the federal statutory corporate income tax rate to the effective income tax rate for the three and nine months ended September 30, 2020 and 2019:


	Ameren			Ameren Missouri			Ameren Illinois
	2020		2019	2020		2019	2020	2019
Three Months
Federal statutory corporate income tax rate:	21%		21%	21%		21%	21%	21%
Increases (decreases) from:
Amortization of deferred investment tax credit	(1)		(1)	(1)		(1)	—	—
Amortization of excess deferred taxes	(9)	(a)	(7)	(16)	(a)	(11)	(3)	(3)
Depreciation differences	—		—	—		—	(1)	—

Renewable and other tax credits	(1)		—	(2)		—	—	—
State tax	4		6	4		5	7	6
Stock-based compensation	1		1	—		—	—	—

Effective income tax rate	15%		20%	6%		14%	24%	24%


	Ameren			Ameren Missouri			Ameren Illinois
	2020		2019	2020		2019	2020	2019
Nine 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)	(a)	(7)	(16)	(a)	(12)	(3)	(4)


Renewable and other tax credits	(1)		—	(1)		—	—	—
State tax	5		6	3		5	7	7
Stock-based compensation	(1)		(1)	—		—	—	—

Effective income tax rate	15%		18%	6%		13%	25%	24%

(a)Increase in the amortization of excess deferred taxes pursuant to the MoPSC’s March 2020 electric rate order. See Note 2 – Rate and Regulatory Matters for additional information.

Supplemental Information (Tables)

9 Months Ended

Sep. 30, 2020

Supplemental Information [Abstract]

Schedule of Cash and Cash Equivalents Including Restricted Cash

The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets and the statements of cash flows as of September 30, 2020, and December 31, 2019:


	September 30, 2020						December 31, 2019
	Ameren		Ameren Missouri		Ameren Illinois		Ameren		Ameren Missouri		Ameren Illinois
Cash and cash equivalents	$	6	$	—	$	—	$	16	$	9	$	—
Restricted cash included in “Other current assets”	14		4		5		14		4		5
Restricted cash included in “Other assets”	142		—		142		120		—		120
Restricted cash included in “Nuclear decommissioning trust fund”	6		6		—		26		26		—
Total cash, cash equivalents, and restricted cash	$	168	$	10	$	147	$	176	$	39	$	125

Schedule of Accounts, Notes, Loans and Financing Receivable

The following table provides a reconciliation of the beginning and ending amount of the allowance for doubtful accounts for the three and nine months ended September 30, 2020 and 2019:


	Three Months				Nine Months
	2020		2019		2020		2019
Ameren:
Beginning of period	$	25	$	19	$	17	$	18
Bad debt expense	21		14		31		22

Net write-offs	(2)		(14)		(4)		(21)

End of period	$	44	$	19	$	44	$	19
Ameren Missouri:
Beginning of period	$	9	$	7	$	7	$	7
Bad debt expense	5		3		9		6

Net write-offs	—		(2)		(2)		(5)
End of period	$	14	$	8	$	14	$	8
Ameren Illinois:(a)
Beginning of Period	$	16	$	12	$	10	$	11
Bad debt expense	16		11		22		16

Net write-offs	(2)		(12)		(2)		(16)

End of Period	$	30	$	11	$	30	$	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. In 2020, the rate-adjustment mechanism for electric distribution allows for recovery of bad debt expense recognized under GAAP. See Note 2 – Rate and Regulatory Matters for additional information.

Schedule of 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 nine months ended September 30, 2020 and 2019:


	September 30, 2020						September 30, 2019
	Ameren		Ameren Missouri				Ameren Illinois	Ameren		Ameren Missouri		Ameren Illinois
Investing

Exchange of bond investments for the extinguishment of senior unsecured notes	$	—	$	—	$	—		$	17	$	—	$	17
Accrued capital expenditures	311		115		191			273		138		128

Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	43		43		—			100		100		—
Financing
Exchange of bond investments for the extinguishment of senior unsecured notes	$	—	$	—	$	—		$	(17)	$	—	$	(17)
Issuance of common stock for stock-based compensation	38		—		—			54		—		—

Asset Retirement Obligation Disclosure

The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the nine months ended September 30, 2020:


	Ameren Missouri			Ameren Illinois			Ameren
Balance at December 31, 2019	$	687		$	4	(a)	$	691	(b)

Liabilities settled	(42)			—			(42)
Accretion	21		(c)	1		(c)	22		(c)
Change in estimates	57		(d)	—			57		(d)
Balance at September 30, 2020	$	723		$	5	(a)	$	728	(b)

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

(b)Balance included $53 million in “Other current liabilities” on the balance sheet as of both December 31, 2019, and September 30, 2020.

(c)Accretion expense attributable to Ameren Missouri and Ameren Illinois was recorded as a decrease to regulatory liabilities and an increase to regulatory assets, respectively.

(d)Ameren Missouri changed its fair value estimate primarily due to an update to the decommissioning of the Callaway Energy Center to reflect the cost study and funding analysis filed with the MoPSC in November 2020 and an increase in the cost estimate for closure of certain CCR storage facilities.

Schedule of excise taxes

The following table presents the excise taxes recorded on a gross basis in “Operating Revenues – Electric,” “Operating Revenues – Natural gas” and “Operating Expenses – Taxes other than income taxes” on the statements of income for the three and nine months ended September 30, 2020 and 2019:


	Three Months				Nine Months
	2020		2019		2020		2019
Ameren Missouri	$	45	$	49	$	111	$	118
Ameren Illinois	26		27		87		91
Ameren	$	71	$	76	$	198	$	209

Schedule of Earnings Per Share, Basic and Diluted

The following table reconciles the basic weighted-average number of common shares outstanding to the diluted weighted-average number of common shares outstanding for the three and nine months ended September 30, 2020 and 2019:


	Three Months		Nine Months
	2020	2019	2020	2019


Weighted-average Common Shares Outstanding – Basic	247.1	245.9	246.8	245.5
Assumed settlement of performance share units and restricted stock units	1.5	1.4	1.2	1.4
Dilutive effect of forward sale agreement	0.6	0.2	0.4	0.1
Weighted-average Common Shares Outstanding – Diluted(a)	249.2	247.5	248.4	247.0

(a)There were no potentially dilutive securities excluded from the earnings per diluted share calculations for the three and nine months ended September 30, 2020 and 2019.

Segment Information (Tables)

9 Months Ended

Sep. 30, 2020

Segment Reporting [Abstract]

Schedule Of Segment Reporting Information By Segment

The following tables present revenues, net income (loss) attributable to common shareholders, and capital expenditures by segment at Ameren and Ameren Illinois for the three and nine months ended September 30, 2020 and 2019. 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 2020:
External revenues	$	990	$	390	$	122	$	126		$	—	$	—	$	1,628
Intersegment revenues	11		1		—		15		(a)	—		(27)		—
Net income (loss) attributable to Ameren common shareholders	297		34		2		62		(b)	(28)		—		367
Capital expenditures	262		129		81		185			2		(3)		656
Three Months 2019:
External revenues	$	1,050	$	374	$	107	$	109		$	—	$	—	$	1,640
Intersegment revenues	9		15		5		19		(a)	—		(29)		19		(c)
Net income (loss) attributable to Ameren common shareholders	300		32		(1)		53		(b)	(20)		—		364
Capital expenditures	256		139		113		129			(7)		6		636
Nine Months 2020:
External revenues	$	2,442	$	1,131	$	533	$	360		$	—	$	—	$	4,466
Intersegment revenues	31		2		—		40		(a)	—		(73)		—
Net income (loss) attributable to Ameren common shareholders	439		107		66		168		(b)	(24)		—		756
Capital expenditures	778		391		221		490			4		—		1,884
Nine Months 2019:
External revenues	$	2,591	$	1,118	$	563	$	303		$	—	$	—	$	4,575
Intersegment revenues	24		17		5		48		(a)	—		(75)		19		(c)
Net income (loss) attributable to Ameren common shareholders	446		105		57		139		(b)	(13)		—		734
Capital expenditures	751		390		241		377			3		(1)		1,761

(a)Ameren Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution.

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

(c)Intersegment revenues at Ameren include $14 million and $5 million of revenue from Ameren Illinois Electric Distribution and Ameren Illinois Natural Gas, respectively, for the three and nine months ended September 30, 2019, for a software licensing agreement with Ameren Missouri. Under authoritative accounting guidance for rate-regulated entities, the revenue recognized by Ameren Illinois was not eliminated upon consolidation. See Note 8 – Related-party Transactions under Part I, Item 1, of this report for additional information.

Ameren Illinois


	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Illinois Transmission			Intersegment Eliminations		Ameren Illinois
Three Months 2020:
External revenues	$	391	$	122	$	76		$	—	$	589
Intersegment revenues	—		—		15		(a)	(15)		—
Net income available to common shareholder	34		2		41			—		77
Capital expenditures	129		81		160			—		370
Three Months 2019:
External revenues	$	389	$	112	$	63		$	—	$	564
Intersegment revenues	—		—		18		(a)	(18)		—
Net income (loss) available to common shareholder	32		(1)		34			—		65
Capital expenditures	139		113		92			—		344


	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Illinois Transmission			Intersegment Eliminations		Ameren Illinois
Nine Months 2020:
External revenues	$	1,133	$	533	$	213		$	—	$	1,879
Intersegment revenues	—		—		39		(a)	(39)		—
Net income available to common shareholder	107		66		107			—		280
Capital expenditures	391		221		419			—		1,031
Nine Months 2019:
External revenues	$	1,135	$	568	$	170		$	—	$	1,873
Intersegment revenues	—		—		47		(a)	(47)		—
Net income available to common shareholder	105		57		85			—		247
Capital expenditures	390		241		269			—		900

(a)Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution.

Disaggregation of Revenue

The following tables present disaggregated revenues by segment at Ameren and Ameren Illinois for the three and nine months ended September 30, 2020 and 2019. 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 2020:
Residential	$	455	$	234		$	—		$	—	$	—	$	689
Commercial	343		127			—			—		—		470
Industrial	87		26			—			—		—		113
Other	99		4			—			141		(27)		217
Total electric revenues	$	984	$	391		$	—		$	141	$	(27)	$	1,489
Residential	$	8	$	—		$	68		$	—	$	—	$	76
Commercial	3		—			18			—		—		21
Industrial	1		—			5			—		—		6
Other	5		—			31			—		—		36
Total gas revenues	$	17	$	—		$	122		$	—	$	—	$	139
Total revenues(a)	$	1,001	$	391		$	122		$	141	$	(27)	$	1,628
Three Months 2019:
Residential	$	489	$	224		$	—		$	—	$	—	$	713
Commercial	394		123			—			—		—		517
Industrial	94		27			—			—		—		121
Other	63		15		(b)	—			128		(29)		177
Total electric revenues	$	1,040	$	389		$	—		$	128	$	(29)	$	1,528
Residential	$	8	$	—		$	65		$	—	$	—	$	73
Commercial	4		—			17			—		—		21
Industrial	1		—			2			—		—		3
Other	6		—			28		(b)	—		—		34
Total gas revenues	$	19	$	—		$	112		$	—	$	—	$	131
Total revenues(a)	$	1,059	$	389		$	112		$	128	$	(29)	$	1,659


	Ameren Missouri		Ameren Illinois Electric Distribution			Ameren Illinois Natural Gas			Ameren Transmission		Intersegment Eliminations		Ameren
Nine Months 2020:
Residential	$	1,111	$	664		$	—		$	—	$	—	$	1,775
Commercial	828		365			—			—		—		1,193
Industrial	207		91			—			—		—		298
Other	240		13			—			400		(73)		580
Total electric revenues	$	2,386	$	1,133		$	—		$	400	$	(73)	$	3,846
Residential	$	52	$	—		$	375		$	—	$	—	$	427
Commercial	20		—			94			—		—		114
Industrial	3		—			11			—		—		14
Other	12		—			53			—		—		65
Total natural gas revenues	$	87	$	—		$	533		$	—	$	—	$	620
Total revenues(a)	$	2,473	$	1,133		$	533		$	400	$	(73)	$	4,466
Nine Months 2019:
Residential	$	1,134	$	640		$	—		$	—	$	—	$	1,774
Commercial	943		370			—			—		—		1,313
Industrial	226		94			—			—		—		320
Other	214		31		(b)	—			351		(75)		521
Total electric revenues	$	2,517	$	1,135		$	—		$	351	$	(75)	$	3,928
Residential	$	56	$	—		$	399		$	—	$	—	$	455
Commercial	24		—			105			—		—		129
Industrial	3		—			9			—		—		12
Other	15		—			55		(b)	—		—		70
Total natural gas revenues	$	98	$	—		$	568		$	—	$	—	$	666
Total revenues(a)	$	2,615	$	1,135		$	568		$	351	$	(75)	$	4,594

(a)The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the three and nine months ended September 30, 2020 and 2019:


	Ameren Missouri		Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Transmission		Ameren
Three Months 2020:
Revenues from alternative revenue programs	$	1	$	(110)	$	3	$	4	$	(102)
Other revenues not from contracts with customers	6		5		—		—		11

Three Months 2019:
Revenues from alternative revenue programs	$	26	$	(145)	$	1	$	(12)	$	(130)
Other revenues not from contracts with customers	5		1		1		—		7

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

Nine Months 2019:
Revenues from alternative revenue programs	$	41	$	(111)	$	2	$	(25)	$	(93)
Other revenues not from contracts with customers	14		5		2		—		21

(b)Includes $14 million and $5 million for Ameren Illinois Electric Distribution and Ameren Illinois Natural Gas, respectively, for the three and nine months ended September 30, 2019, for a software licensing agreement with Ameren Missouri. See Note 8 – Related-party Transactions for additional information.

Ameren Illinois


	Ameren Illinois Electric Distribution			Ameren Illinois Natural Gas			Ameren Illinois Transmission		Intersegment Eliminations		Ameren Illinois
Three Months 2020:
Residential	$	234		$	68		$	—	$	—	$	302
Commercial	127			18			—		—		145
Industrial	26			5			—		—		31
Other	4			31			91		(15)		111
Total revenues(a)	$	391		$	122		$	91	$	(15)	$	589
Three Months 2019:
Residential	$	224		$	65		$	—	$	—	$	289
Commercial	123			17			—		—		140
Industrial	27			2			—		—		29
Other	15		(b)	28		(b)	81		(18)		106
Total revenues(a)	$	389		$	112		$	81	$	(18)	$	564
Nine Months 2020:
Residential	$	664		$	375		$	—	$	—	$	1,039
Commercial	365			94			—		—		459
Industrial	91			11			—		—		102
Other	13			53			252		(39)		279
Total revenues(a)	$	1,133		$	533		$	252	$	(39)	$	1,879
Nine Months 2019:
Residential	$	640		$	399		$	—	$	—	$	1,039
Commercial	370			105			—		—		475
Industrial	94			9			—		—		103
Other	31		(b)	55		(b)	217		(47)		256
Total revenues(a)	$	1,135		$	568		$	217	$	(47)	$	1,873

(a)The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the Ameren Illinois segments for the three and nine months ended September 30, 2020 and 2019:


	Ameren Illinois Electric Distribution		Ameren Illinois Natural Gas		Ameren Illinois Transmission		Ameren Illinois
Three Months 2020:
Revenues from alternative revenue programs	$	(110)	$	3	$	5	$	(102)
Other revenues not from contracts with customers	5		—		—		5

Three Months 2019:
Revenues from alternative revenue programs	$	(145)	$	1	$	(12)	$	(156)
Other revenues not from contracts with customers	1		1		—		2

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

Nine Months 2019:
Revenues from alternative revenue programs	$	(111)	$	2	$	(26)	$	(135)
Other revenues not from contracts with customers	5		2		—		7

(b)Includes $14 million and $5 million for Ameren Illinois Electric Distribution and Ameren Illinois Natural Gas, respectively, for the three and nine months ended September 30, 2019, for a software licensing agreement with Ameren Missouri. See Note 8 – Related-party Transactions for additional information.

Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019
Basis Of Presentation And Significant Accounting Policies [Line Items]
Workforce Utilizing Remote Work Arrangement		50.00%	65.00%
Percentage of accounts receivable balances that are 30 days past due or are part of a deferred payment arrangement	26.00%				13.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 151				$ 65
Unrecorded Unconditional Purchase Obligation	1,993
Cash Surrender Value of Life Insurance	260			$ 264
Corporate owned life insurance, borrowings	$ 107			114
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	18.00%				9.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 49				$ 23
Unrecorded Unconditional Purchase Obligation	$ 1,300
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	35.00%				18.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 102				$ 42
Unrecorded Unconditional Purchase Obligation	648
Cash Surrender Value of Life Insurance	114			123
Corporate owned life insurance, borrowings	107			114
Variable Interest Entity, Not Primary Beneficiary
Basis Of Presentation And Significant Accounting Policies [Line Items]
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	18
Equity Method Investments	34			$ 28
Partnership Funding Commitment
Basis Of Presentation And Significant Accounting Policies [Line Items]
Unrecorded Unconditional Purchase Obligation	$ 37

Rate And Regulatory Matters (Narrative-Missouri) (Detail) $ in Millions	3 Months Ended			7 Months Ended	9 Months Ended		12 Months Ended
	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($) MWh	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2020 USD ($) MWh	Sep. 30, 2019 USD ($)	Dec. 31, 2019 MWh	Dec. 31, 2018 MWh
Rate And Regulatory Matters [Line Items]
Purchased Power		$ 140	$ 148		$ 383	$ 440
Other operations and maintenance		418	434		1,240	1,301
Income Taxes		63	92		134	158
Donations		0	1		14	8
Union Electric Company
Rate And Regulatory Matters [Line Items]
Purchased Power		48	49		124	160
Other operations and maintenance		221	242		662	720
Income Taxes		18	51		29	70
Donations		$ 0	$ 1		9	3
Net costs and forgone late fees related to the COVID-19 Pandemic				$ 9
Final Rate Order \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount					32
Reduction to annualized base level of net energy costs					115
Revenues					20
Purchased Power					15
Other operations and maintenance					60
Income Taxes					20
Revenue requirement change due to MEEIA sales volumes					50
Amounts previously deferred under PISA					50
Donations					$ 8
Wind Generation Facility \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Amount of Megawatts \| MWh		400			300		300	400
Estimated Capital Project Costs					$ 1,200
Wind Generation Facility \| Union Electric Company \| Subsequent Event
Rate And Regulatory Matters [Line Items]
Project Placed In-Service Amount	$ 200
MEEIA 2019 [Member] \| Final Rate Order \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Revenues		$ 6
Incentive Award if Energy Efficiency Goals Are Achieved					11
Incentive Award if Energy Efficiency Goals Are Exceeded					1
MEEIA Energy Efficiency Investments for the 2022 Program Year					$ 70
MEEIA 2013 & 2016 \| Final Rate Order \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Revenues						$ 38
Minimum \| Final Rate Order \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage					9.40%
Maximum \| Final Rate Order \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage					9.80%

Rate And Regulatory Matters (Narrative-Illinois) (Detail) - Ameren Illinois Company - USD ($) $ in Millions	1 Months Ended				3 Months Ended	9 Months Ended
	Oct. 31, 2020	Sep. 30, 2020	May 31, 2020	Apr. 30, 2020	Sep. 30, 2020	Sep. 30, 2020
Rate And Regulatory Matters [Line Items]
Deferred payment arrangement extension						24 months
Financial assistance program costs						$ 8
Pending Rate Case \| Natural Gas
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 97
Revenues					$ 46
Public Utilities, Requested Return on Equity, Percentage					10.50%
Public Utilities, Requested Equity Capital Structure, Percentage					54.10%
Rate Base					$ 2,100
Pending Rate Case \| IEIMA \| Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount				$ 45
ICC Staff recommended rate increase (decrease)		$ (49)
Pending Rate Case \| FEJA energy-efficiency rider \| Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 7
Subsequent Event \| Pending Rate Case \| Natural Gas
Rate And Regulatory Matters [Line Items]
ICC Staff recommended rate increase (decrease)	$ 69
ICC Staff recommended Return on Equity, Percentage	9.30%
ICC Staff recommended Equity Capital Structure, Percentage	50.40%
ICC Staff recommended Rate Base	$ 2,100

Rate And Regulatory Matters (Narrative-Federal) (Detail) - USD ($) $ in Millions	1 Months Ended		3 Months Ended	9 Months Ended
	May 31, 2020	Nov. 30, 2019	Jun. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Rate And Regulatory Matters [Line Items]
Lower revenue due to transmission formula rate revisions			$ 2
Income recognized due to higher FERC-allowed base ROE			13
Current regulatory liabilities				$ 96	$ 164
Incentive adder cap on FERC allowed return on equity				2.50%
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Income recognized due to higher FERC-allowed base ROE			$ 7
Current regulatory liabilities				$ 47	$ 84
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%	10.02%	10.32%
Customer Requested Rate on Equity				9.15%
Current regulatory liabilities				$ 15
Incentive adder to FERC allowed base return on common equity				0.50%
Midwest Independent Transmission System Operator, Inc \| Final Rate Order \| Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities				$ 7
Midwest Independent Transmission System Operator, Inc \| Notice of Proposed Rulemaking
Rate And Regulatory Matters [Line Items]
Incentive adder to FERC allowed base return on common equity				1.00%
Maximum \| Midwest Independent Transmission System Operator, Inc \| Final Rate Order
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage				10.82%

Short-Term Debt And Liquidity (Narrative) (Detail) $ in Billions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019	Sep. 30, 2020 USD ($)	Sep. 30, 2019
Credit Agreements
Short-term Debt [Line Items]
Net Liquidity Available	$ 2.0		$ 2.0
Actual debt-to-capital ratio	0.55		0.55
Utilities
Short-term Debt [Line Items]
Short-term Debt, Weighted Average Interest Rate, over Time	0.10%	2.40%	0.81%	2.67%
Union Electric Company \| Missouri Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.48		0.48
Ameren Illinois Company \| Illinois Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.45		0.45

Short-Term Debt and Liquidity (Short-Term Debt outstanding) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Short-term Debt [Line Items]
Short-term debt	$ 272	$ 440
Union Electric Company
Short-term Debt [Line Items]
Short-term debt	0	234
Ameren Illinois Company
Short-term Debt [Line Items]
Short-term debt	242	53
Commercial Paper
Short-term Debt [Line Items]
Short-term debt	272	440
Commercial Paper \| Ameren (parent)
Short-term Debt [Line Items]
Short-term debt	30	153
Commercial Paper \| Union Electric Company
Short-term Debt [Line Items]
Short-term debt	0	234
Commercial Paper \| Ameren Illinois Company
Short-term Debt [Line Items]
Short-term debt	$ 242	$ 53

Short-Term Debt and Liquidity (Short-Term Debt Activity) (Details) - Commercial Paper - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 257	$ 821
Weighted-average interest rate	1.70%	2.68%
Short-term Debt, Maximum Amount Outstanding During Period	$ 908	$ 1,113
Peak interest rate	5.05%	5.00%
Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 62	$ 532
Weighted-average interest rate	2.04%	2.70%
Short-term Debt, Maximum Amount Outstanding During Period	$ 425	$ 651
Peak interest rate	3.30%	3.80%
Union Electric Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 142	$ 141
Weighted-average interest rate	1.76%	2.73%
Short-term Debt, Maximum Amount Outstanding During Period	$ 573	$ 549
Peak interest rate	5.05%	2.97%
Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 53	$ 147
Weighted-average interest rate	1.10%	2.58%
Short-term Debt, Maximum Amount Outstanding During Period	$ 243	$ 310
Peak interest rate	3.40%	5.00%

Long-Term debt and Equity Financings (Narrative) (Details) shares in Millions	1 Months Ended	3 Months Ended			9 Months Ended
	Oct. 31, 2020 USD ($)	Sep. 30, 2020 USD ($) shares	Mar. 31, 2020 USD ($) shares	Sep. 30, 2019 shares	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($) shares	Oct. 30, 2020 USD ($) numberOfWindGenerationFacilities	Apr. 30, 2020 USD ($)	Aug. 05, 2019 shares
Long-Term Debt And Equity Financings [Line Items]
Stock Issued During Period, Shares, New Issues \| shares		0.1		0.2	0.5	0.7
Issuance of common stock		$ 10,000,000			$ 37,000,000	$ 54,000,000
Stock Issued During Period, Shares, Other \| shares		0.0	0.5	0.0	0.5	0.8
Issuance of common stock for stock-based compensation			$ 38,000,000		$ 38,000,000	$ 54,000,000
Forward Contract Indexed to Issuer's Equity, Indexed Shares (in shares) \| shares									7.5
Forward Contract Indexed to Issuer's Equity, Settlement Alternatives, Cash, at Fair Value		$ 543,000,000			543,000,000
Period End Net Cash Settlement Price		$ 54,000,000			$ 54,000,000
Period End Net Share Settlement Price (in shares) \| shares		0.7			0.7
Ameren (parent) \| Unsecured Debt \| Senior Unsecured Notes 3.50% Due 2031
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount								$ 800,000,000
Debt Instrument, Interest Rate, Stated Percentage								3.50%
Proceeds from Issuance of Debt					$ 793,000,000
Ameren (parent) \| Unsecured Debt \| Senior Unsecured Notes270 due 2020 \| Subsequent Event
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount							$ 350,000,000
Debt Instrument, Interest Rate, Stated Percentage							2.70%
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Issuance of common stock for stock-based compensation					0	0
Union Electric Company \| Secured Debt \| First Mortgage Bonds, 2.95%, Due 2030 - $465 Issuance
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount			$ 465,000,000
Debt Instrument, Interest Rate, Stated Percentage			2.95%
Proceeds from Issuance of Secured Debt					462,000,000
Union Electric Company \| Secured Debt \| Senior Secured Notes, 5.00%, Due 2020
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount		$ 85,000,000			$ 85,000,000
Debt Instrument, Interest Rate, Stated Percentage		5.00%			5.00%
Union Electric Company \| Secured Debt \| First Mortgage Bonds, 2.625%, Due 2051 - $550 Issuance [Member] \| Subsequent Event
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount							$ 550,000,000
Debt Instrument, Interest Rate, Stated Percentage							2.625%
Proceeds from Issuance of Secured Debt	$ 543,000,000
Number Of Wind Generation Facilities \| numberOfWindGenerationFacilities							2
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					$ 350,000,000	$ 0

Other Income, Net (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
Other Income, Net (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 12	$ 7	$ 25	$ 20
Interest income on industrial development revenue bonds	7	6	19	19
Other interest income	0	2	2	6
Non-service cost components of net periodic benefit income	32	23	85	67
Miscellaneous income	1	2	10	6
Donations	0	(1)	(14)	(8)
Miscellaneous expense	(4)	(5)	(10)	(11)
Total Other Income, Net	48	34	117	99
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	7	6	15	14
Interest income on industrial development revenue bonds	7	6	19	19
Non-service cost components of net periodic benefit income	13	4	32	13
Miscellaneous income	1	2	3	4
Donations	0	(1)	(9)	(3)
Miscellaneous expense	(2)	(2)	(5)	(4)
Total Other Income, Net	26	15	55	43
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	(3)	7	(1)	22
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	5	1	10	6
Other interest income	0	1	2	5
Non-service cost components of net periodic benefit income	12	12	36	36
Miscellaneous income	1	1	6	3
Donations	0	0	(5)	(5)
Miscellaneous expense	(1)	(2)	(4)	(6)
Total Other Income, Net	$ 17	$ 13	45	$ 39
Final Rate Order \| Electricity \| Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Donations			$ (8)

Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) lb in Thousands, gal in Millions, MWh in Millions, MMBTU in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020 MMBTU MWh lb gal	Dec. 31, 2019 MMBTU MWh lb gal
Fuel Oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume \| gal	44	58
Natural Gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MMBTU	155	156
Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MWh	13	12
Uranium
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass \| lb	365	565
Union Electric Company \| Fuel Oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume \| gal	44	58
Union Electric Company \| Natural Gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MMBTU	32	20
Union Electric Company \| Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MWh	6	5
Union Electric Company \| Uranium
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass \| lb	365	565
Ameren Illinois Company \| Fuel Oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume \| gal	0	0
Ameren Illinois Company \| Natural Gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MMBTU	123	136
Ameren Illinois Company \| Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MWh	7	7
Ameren Illinois Company \| Uranium
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass \| lb	0	0

Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Derivative [Line Items]
Derivative assets	$ 33	$ 26
Derivative liabilities	246	255
Fuel Oils \| Other Current Assets
Derivative [Line Items]
Derivative assets	2	4
Fuel Oils \| Other Assets
Derivative [Line Items]
Derivative assets	0	2
Fuel Oils \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	14	4
Fuel Oils \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	6	3
Natural Gas \| Other Current Assets
Derivative [Line Items]
Derivative assets	12	3
Natural Gas \| Other Assets
Derivative [Line Items]
Derivative assets	6	1
Natural Gas \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	2	13
Natural Gas \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	1	7
Power \| Other Current Assets
Derivative [Line Items]
Derivative assets	12	14
Power \| Other Assets
Derivative [Line Items]
Derivative assets	1	2
Power \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	22	19
Power \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	201	208
Uranium \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Union Electric Company
Derivative [Line Items]
Derivative assets	19	22
Derivative liabilities	30	13
Union Electric Company \| Fuel Oils \| Other Current Assets
Derivative [Line Items]
Derivative assets	2	4
Union Electric Company \| Fuel Oils \| Other Assets
Derivative [Line Items]
Derivative assets	0	2
Union Electric Company \| Fuel Oils \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	14	4
Union Electric Company \| Fuel Oils \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	6	3
Union Electric Company \| Natural Gas \| Other Current Assets
Derivative [Line Items]
Derivative assets	2	0
Union Electric Company \| Natural Gas \| Other Assets
Derivative [Line Items]
Derivative assets	2	0
Union Electric Company \| Natural Gas \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Union Electric Company \| Natural Gas \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Union Electric Company \| Power \| Other Current Assets
Derivative [Line Items]
Derivative assets	12	14
Union Electric Company \| Power \| Other Assets
Derivative [Line Items]
Derivative assets	1	2
Union Electric Company \| Power \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	6	2
Union Electric Company \| Power \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	4	1
Union Electric Company \| Uranium \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	14	4
Derivative liabilities	216	242
Ameren Illinois Company \| Fuel Oils \| Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company \| Fuel Oils \| Other Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company \| Fuel Oils \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company \| Fuel Oils \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company \| Natural Gas \| Other Current Assets
Derivative [Line Items]
Derivative assets	10	3
Ameren Illinois Company \| Natural Gas \| Other Assets
Derivative [Line Items]
Derivative assets	4	1
Ameren Illinois Company \| Natural Gas \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	2	12
Ameren Illinois Company \| Natural Gas \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	1	6
Ameren Illinois Company \| Power \| Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company \| Power \| Other Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company \| Power \| Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	16	17
Ameren Illinois Company \| Power \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	197	207
Ameren Illinois Company \| Uranium \| Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	$ 0	$ 0

Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	$ 898	$ 842
Assets fair value	931	868
Excluded receivables, payables, and accrued income, net	6	5
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	604	569
Assets fair value	605	569
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	294	273
Assets fair value	307	276
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Assets fair value	19	23
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	33	26
Derivative liabilities	246	255
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	1	0
Derivative liabilities	21	4
Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	13	3
Derivative liabilities	2	16
Commodity Contract \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	19	23
Derivative liabilities	223	235
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	898	842
Assets fair value	917	864
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	604	569
Assets fair value	605	569
Union Electric Company \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	294	273
Assets fair value	298	275
Union Electric Company \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Assets fair value	14	20
Union Electric Company \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	19	22
Derivative liabilities	30	13
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	1	0
Derivative liabilities	21	1
Union Electric Company \| Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	2
Derivative liabilities	0	4
Union Electric Company \| Commodity Contract \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	14	20
Derivative liabilities	9	8
Union Electric Company \| Fuel Oils \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	6
Derivative liabilities	20	7
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	0	0
Derivative liabilities	13	1
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	2	6
Derivative liabilities	7	6
Union Electric Company \| Natural Gas \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	0
Derivative liabilities	0	2
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	0
Derivative liabilities	0	2
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	13	16
Derivative liabilities	10	3
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	1	0
Derivative liabilities	8	0
Union Electric Company \| Power \| Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	2
Derivative liabilities	0	2
Union Electric Company \| Power \| Commodity Contract \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	12	14
Derivative liabilities	2	1
Union Electric Company \| Uranium \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	1
Union Electric Company \| Uranium \| Commodity Contract \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Union Electric Company \| Uranium \| Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Union Electric Company \| Uranium \| Commodity Contract \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	1
Union Electric Company \| Equity Securities \| U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	604	569
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	604	569
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	112	107
Union Electric Company \| Debt Securities \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	116	93
Union Electric Company \| Debt Securities \| Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	66	73
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	112	107
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	116	93
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	66	73
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	216	242
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	3
Ameren Illinois Company \| Commodity Contract \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	2	12
Ameren Illinois Company \| Commodity Contract \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	214	227
Ameren Illinois Company \| Natural Gas \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	14	4
Derivative liabilities	3	18
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	3
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	9	1
Derivative liabilities	2	12
Ameren Illinois Company \| Natural Gas \| Commodity Contract \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	5	3
Derivative liabilities	1	3
Ameren Illinois Company \| Power \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	213	224
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	$ 213	$ 224

Fair Value Measurements (Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level Three In The Fair Value Hierarchy) (Detail) - Power - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	$ (203)	$ (196)	$ (203)	$ (196)	$ (213)	$ (211)	$ (176)	$ (183)
Included in regulatory assets/liabilities	10	(21)	16	(20)
Settlements, assets		3		9
Settlement, liabilities	0		(8)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	0	(2)	0	(2)
Change in unrealized gains (losses) related to assets/liabilities held at period end	9	(21)	5	(23)
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	10	8	10	8	16	13	15	0
Included in regulatory assets/liabilities	(2)	(4)	18	12
Settlement, liabilities	(4)	(1)	(21)	(2)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	0	(2)	0	(2)
Change in unrealized gains (losses) related to assets/liabilities held at period end	(2)	(4)	6	8
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	(213)	(204)	(213)	(204)	$ (229)	$ (224)	$ (191)	$ (183)
Included in regulatory assets/liabilities	12	(17)	(2)	(32)
Settlements, assets	4	4	13	11
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0	0	0
Change in unrealized gains (losses) related to assets/liabilities held at period end	$ 11	$ (17)	$ (1)	$ (31)

Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) - Power $ in Millions	Sep. 30, 2020 USD ($) $ / MWh $ / MMBTU	Dec. 31, 2019 USD ($) $ / MWh $ / MMBTU
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets \| $	$ 12	$ 14
Derivative liabilities \| $	$ (215)	$ (225)
Commodity Forward Price \| Discounted Cash Flow \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	20	22
Commodity Forward Price \| Discounted Cash Flow \| Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	36	34
Commodity Forward Price \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	28	25
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	1	0
Nodal Basis \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	(2)	(2)
Commodity Future Price \| Discounted Cash Flow \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input \| $ / MMBTU	3	(1)
Commodity Future Price \| Discounted Cash Flow \| Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input \| $ / MMBTU	4	0
Commodity Future Price \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input \| $ / MMBTU	3	0

Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Financial Assets and Liabilities) (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 168	$ 176	$ 155	$ 107
Short-term debt	272	440
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	10	39	14	8
Advances to money pool	5	0
Short-term debt	0	234
Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	147	125	$ 117	$ 80
Short-term debt	242	53
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	168	176
Available-for-sale Securities and Held-to-maturity Securities	263	263
Short-term debt	272	440
Long-term debt (including current portion)	10,529	9,357
Debt Issuance Costs, Net	77	72
Carrying Amount \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	10	39
Advances to money pool	5
Available-for-sale Securities and Held-to-maturity Securities	263	263
Short-term debt		234
Long-term debt (including current portion)	4,568	4,190
Debt Issuance Costs, Net	32	30
Carrying Amount \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	147	125
Short-term debt	242	53
Long-term debt (including current portion)	3,576	3,575
Debt Issuance Costs, Net	33	34
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	168	176
Investments, Fair Value Disclosure	263	263
Short-term Debt, Fair Value	272	440
Long-term Debt, Fair Value	12,412	10,441
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	168	176
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	263	263
Short-term Debt, Fair Value	272	440
Long-term Debt, Fair Value	11,891	9,957
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	521	484
Fair Value \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	10	39
Advances to money pool	5
Investments, Fair Value Disclosure	263	263
Short-term Debt, Fair Value		234
Long-term Debt, Fair Value	5,316	4,772
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	10	39
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	5
Investments, Fair Value Disclosure	263	263
Short-term Debt, Fair Value		234
Long-term Debt, Fair Value	5,316	4,772
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	147	125
Short-term Debt, Fair Value	242	53
Long-term Debt, Fair Value	4,431	4,019
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	147	125
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	0	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	242	53
Long-term Debt, Fair Value	4,431	4,019
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	0
Long-term Debt, Fair Value	$ 0	$ 0

Related Party Transactions (Narrative) (Details) - Ameren Illinois Company $ in Millions	1 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Apr. 30, 2020 USD ($)	Sep. 30, 2020 MWh $ / MWh
April 2020 Procurement \| Ameren Illinois Capacity Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Energy Supply Agreements Amount		$ 2
September 2020 Procurement \| Ameren Illinois Capacity Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Energy Supply Agreements Amount	$ 1
September 2020 Procurement \| Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Related Party Long Term Contract For Purchase of Electric Power \| MWh			204,800
Related Party Long Term Contract For Purchase of Electric Power Rate \| $ / MWh			31

Related Party Transactions (Schedule of Affiliate Receivables and Payables) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Union Electric Company
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	$ 70	$ 52
Accounts Receivable, Related Parties, Current	12	30
Union Electric Company \| Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	38	15
Union Electric Company \| Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	0	15
Ameren Illinois Company
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	43	82
Accounts Receivable, Related Parties, Current	15	28
Ameren Illinois Company \| Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	9	43
Ameren Illinois Company \| Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	$ 6	$ 17

Related Party Transactions (Effects of Related-party Transactions on the Statement of Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Union Electric Company \| Ameren Missouri Power Supply Agreements with Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues	$ 5	$ 1	$ 11	$ 3
Union Electric Company \| Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	6	7	19	20
Operating Expenses	1	1	1	1
Union Electric Company \| Ameren Missouri and Ameren Illinois Miscellaneous Support Services
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	1
Union Electric Company \| Total Related Party Operating Revenues
Related Party Transaction [Line Items]
Operating Revenues	11	9	31	24
Union Electric Company \| Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	36	34	103	98
Union Electric Company \| Total Related Party Other Operations and Maintenance
Related Party Transaction [Line Items]
Operating Expenses	36	34	103	99
Union Electric Company \| Money pool borrowings (advances)
Related Party Transaction [Line Items]
Interest Charges (Income)	1	1	1	1
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	2
Operating Expenses	1	1	3	4
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois Miscellaneous Support Services
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	1
Ameren Illinois Company \| Ameren Missouri Software Licensing with Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues		19		19
Related Party Transaction, Amounts of Transaction		24
Capitalized Computer Software, Net		5		5
Ameren Illinois Company \| Total Related Party Operating Revenues
Related Party Transaction [Line Items]
Operating Revenues	1	20	2	22
Ameren Illinois Company \| Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Operating Expenses	5	1	11	3
Ameren Illinois Company \| Ameren Illinois Transmission Services With ATXI
Related Party Transaction [Line Items]
Operating Expenses	1	1	1	1
Ameren Illinois Company \| Purchased Power
Related Party Transaction [Line Items]
Operating Expenses	5	2	12	4
Ameren Illinois Company \| Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	34	30	98	91
Ameren Illinois Company \| Total Related Party Other Operations and Maintenance
Related Party Transaction [Line Items]
Operating Expenses	35	31	101	95
Ameren Illinois Company \| Money pool borrowings (advances)
Related Party Transaction [Line Items]
Interest Charges (Income)	$ 1	1	$ 1	$ 1
Ameren Illinois Electric Distribution \| Ameren Missouri Software Licensing with Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues		14
Ameren Illinois Natural Gas \| Ameren Missouri Software Licensing with Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues		$ 5

Commitments And Contingencies (Other Obligations) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($) MWh
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	$ 245
2021	672
2022	402
2023	274
2024	169
Thereafter	231
Total unrecorded unconditional purchase obligation	1,993
Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	94
2021	242
2022	193
2023	113
2024	94
Thereafter	55
Total unrecorded unconditional purchase obligation	791
Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	52
2021	161
2022	108
2023	69
2024	28
Thereafter	73
Total unrecorded unconditional purchase obligation	491
Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	3
2021	57
2022	12
2023	45
2024	20
Thereafter	21
Total unrecorded unconditional purchase obligation	158
Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	57
2021	156
2022	63
2023	21
2024	4
Thereafter	0
Total unrecorded unconditional purchase obligation	$ 301
Amount of Megawatts \| MWh	102
Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	$ 0
2021	3
2022	3
2023	3
2024	3
Thereafter	21
Total unrecorded unconditional purchase obligation	33
Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	39
2021	53
2022	23
2023	23
2024	20
Thereafter	61
Total unrecorded unconditional purchase obligation	219
Renewable Energy Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Total unrecorded unconditional purchase obligation	36
Zero Emission Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Total unrecorded unconditional purchase obligation	42
Union Electric Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	131
2021	391
2022	269
2023	216
2024	150
Thereafter	143
Total unrecorded unconditional purchase obligation	1,300
Union Electric Company \| Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	94
2021	242
2022	193
2023	113
2024	94
Thereafter	55
Total unrecorded unconditional purchase obligation	791
Union Electric Company \| Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	12
2021	43
2022	38
2023	32
2024	13
Thereafter	22
Total unrecorded unconditional purchase obligation	160
Union Electric Company \| Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	3
2021	57
2022	12
2023	45
2024	20
Thereafter	21
Total unrecorded unconditional purchase obligation	158
Union Electric Company \| Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	0
2021	0
2022	0
2023	0
2024	0
Thereafter	0
Total unrecorded unconditional purchase obligation	$ 0
Amount of Megawatts \| MWh	102
Union Electric Company \| Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	$ 0
2021	3
2022	3
2023	3
2024	3
Thereafter	21
Total unrecorded unconditional purchase obligation	33
Union Electric Company \| Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	22
2021	46
2022	23
2023	23
2024	20
Thereafter	24
Total unrecorded unconditional purchase obligation	158
Ameren Illinois Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	111
2021	276
2022	133
2023	58
2024	19
Thereafter	51
Total unrecorded unconditional purchase obligation	648
Ameren Illinois Company \| Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	0
2021	0
2022	0
2023	0
2024	0
Thereafter	0
Total unrecorded unconditional purchase obligation	0
Ameren Illinois Company \| Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	40
2021	118
2022	70
2023	37
2024	15
Thereafter	51
Total unrecorded unconditional purchase obligation	331
Ameren Illinois Company \| Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	0
2021	0
2022	0
2023	0
2024	0
Thereafter	0
Total unrecorded unconditional purchase obligation	0
Ameren Illinois Company \| Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	57
2021	156
2022	63
2023	21
2024	4
Thereafter	0
Total unrecorded unconditional purchase obligation	301
Ameren Illinois Company \| Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	0
2021	0
2022	0
2023	0
2024	0
Thereafter	0
Total unrecorded unconditional purchase obligation	0
Ameren Illinois Company \| Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	14
2021	2
2022	0
2023	0
2024	0
Thereafter	0
Total unrecorded unconditional purchase obligation	16
Ameren Illinois Company \| Renewable Energy Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Total unrecorded unconditional purchase obligation	36
Ameren Illinois Company \| Zero Emission Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Total unrecorded unconditional purchase obligation	$ 42

Commitments And Contingencies (Environmental Matters) (Detail) $ in Millions	Sep. 30, 2020 USD ($) center scrubber site	Dec. 31, 2019 USD ($)
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 728	$ 691
Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	200
Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 250
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 Closing Surface Impoundments \| center	3
Asset Retirement Obligation	$ 723	687
Number of energy centers \| center	4
Union Electric Company \| Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 200
Union Electric Company \| Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	250
Ameren Illinois Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	5	$ 4
Coal Combustion Residuals Estimate \| Union Electric Company \| Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	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	125
Manufactured Gas Plant
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 106
Manufactured Gas Plant \| Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites \| site	44
Accrual for environmental loss contingencies	$ 106
Manufactured Gas Plant \| Ameren Illinois Company \| Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	106
Manufactured Gas Plant \| Ameren Illinois Company \| Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	178
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	125
New CCR Rules Estimate \| Union Electric Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 125

Callaway Energy Center (Insurance Disclosure) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($)
Nuclear Waste Matters [Line Items]
Decommissioning Cost	$ 7.0
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
Public Liability And Nuclear Worker Liability - American Nuclear Insurers
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	$ 450.0
Insurance Maximum Coverage per Incident	0.0
Public Liability And Nuclear Worker Liability - Pool Participation
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	13,348.0
Insurance Maximum Coverage per Incident	138.0
Threshold Amount For Retrospective Insurance Assessment For Covered Loss Under Public Liability And Nuclear Worker Liability Insurance Policy	450.0
Maximum Annual Payment Per Incident At Licensed Commercial Nuclear Reactor	21.0
Public Liability
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	13,798.0
Insurance Maximum Coverage per Incident	138.0
Property Damage - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	3,200.0
Insurance Maximum Coverage per Incident	25.0
Replacement Power - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	490.0
Insurance Maximum Coverage per Incident	7.0
Amount Of Weekly Indemnity Coverage Commencing Twelve Weeks After Power Outage	4.5
Amount Of Additional Weekly Indemnity Coverage Commencing After Initial Indemnity Coverage	3.6
Amount Of Weekly Indemnity Coverage Thereafter Not Exceeding Policy Limit	490.0
Sub-Limit Of Amount Of Weekly Indemnity Coverage Thereafter Not Exceeding Policy Limit For Non-Nuclear Events	328.0
Radiation Event
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	2,700.0
Aggregate Nuclear Power Industry Insurance Policy Limit For Losses From Terrorist Attacks Within Twelve Month Period	3,200.0
Non-Radiation Event
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	2,300.0
Aggregate Nuclear Power Industry Insurance Policy Limit For Losses From Terrorist Attacks Within Twelve Month Period	1,800.0
Property Damage European Mutual Association for Nuclear Insurance
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	$ 490.0

Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	$ (32)	$ (23)	$ (85)	$ (67)
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	28	22	83	66
Interest cost	43	46	130	139
Expected return on plan assets	(73)	(69)	(218)	(207)
Prior service cost (benefit)	0	0	(1)	0
Actuarial loss (gain)	15	6	45	19
Total non-service cost components	(15)	(17)	(44)	(49)
Net periodic benefit cost (income)	13	5	39	17
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	4	4	14	13
Interest cost	10	11	29	32
Expected return on plan assets	(20)	(19)	(60)	(57)
Prior service cost (benefit)	(1)	(1)	(3)	(4)
Actuarial loss (gain)	(3)	(4)	(7)	(11)
Total non-service cost components	(14)	(13)	(41)	(40)
Net periodic benefit cost (income)	$ (10)	$ (9)	$ (27)	$ (27)

Retirement Benefits (Summary of Benefit Plan Costs Incurred) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 13	$ 5	$ 39	$ 17
Pension Plan \| Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	5	1	16	3
Pension Plan \| Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	8	5	24	15
Pension Plan \| Other Affiliated Entities And Intercompany Eliminations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	0	(1)	(1)	(1)
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(10)	(9)	(27)	(27)
Other Postretirement Benefit Plan, Defined Benefit \| Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(2)	(1)	(4)	(4)
Other Postretirement Benefit Plan, Defined Benefit \| Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(8)	(8)	(24)	(23)
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	$ 1	$ 0

Retirement Benefits (Narrative) (Details) $ in Millions	Dec. 31, 2019 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan estimated future employer contributions over next five years	$ 70

Income Taxes Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Taxes [Line Items]
Federal statutory corporate income tax rate	21.00%	21.00%	21.00%	21.00%
Amortization of deferred investment tax credit	(1.00%)	(1.00%)	0.00%	(1.00%)
Amortization of excess deferred taxes	(9.00%)	(7.00%)	(9.00%)	(7.00%)
Depreciation differences	0.00%	0.00%
Renewable and other tax credits	(1.00%)	0.00%	(1.00%)	0.00%
State tax	4.00%	6.00%	5.00%	6.00%
Stock-based compensation	1.00%	1.00%	(1.00%)	(1.00%)
Effective income tax rate	15.00%	20.00%	15.00%	18.00%
Union Electric Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate	21.00%	21.00%	21.00%	21.00%
Amortization of deferred investment tax credit	(1.00%)	(1.00%)	(1.00%)	(1.00%)
Amortization of excess deferred taxes	(16.00%)	(11.00%)	(16.00%)	(12.00%)
Depreciation differences	0.00%	0.00%
Renewable and other tax credits	(2.00%)	0.00%	(1.00%)	0.00%
State tax	4.00%	5.00%	3.00%	5.00%
Stock-based compensation	0.00%	0.00%	0.00%	0.00%
Effective income tax rate	6.00%	14.00%	6.00%	13.00%
Ameren Illinois Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate	21.00%	21.00%	21.00%	21.00%
Amortization of deferred investment tax credit	0.00%	0.00%	0.00%	0.00%
Amortization of excess deferred taxes	(3.00%)	(3.00%)	(3.00%)	(4.00%)
Depreciation differences	(1.00%)	0.00%
Renewable and other tax credits	0.00%	0.00%	0.00%	0.00%
State tax	7.00%	6.00%	7.00%	7.00%
Stock-based compensation	0.00%	0.00%	0.00%	0.00%
Effective income tax rate	24.00%	24.00%	25.00%	24.00%

Supplemental Information (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Supplemental Information [Abstract]
Payables for purchased receivables	$ 36		$ 32
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Amount	8	$ 14
Deferred Compensation Liability, Classified, Noncurrent	$ 88		$ 86
January 1, 2020 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
September 18, 2020 Issuance
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	3 years
Performance Shares \| January 1, 2020 Issuance
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	294,320
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Granted in Period, Fair Value	$ 24
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, 2020 Issuance
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	252,370
Performance Shares \| Renewable generation and energy storage installation targets \| January 1, 2020 Issuance
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	41,950
Restricted Stock Units (RSUs) \| January 1, 2020 Issuance
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	132,307
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Granted in Period, Fair Value	$ 10
Restricted Stock Units (RSUs) \| September 18, 2020 Issuance
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	37,104
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Granted in Period, Fair Value	$ 3

Supplemental Information (Cash and Cash Equivalents) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	$ 6	$ 16
Restricted cash included in “Other current assets”	14	14
Restricted cash included in “Other assets”	142	120
Restricted cash included in “Nuclear decommissioning trust fund”	6	26
Total cash, cash equivalents, and restricted cash	168	176	$ 155	$ 107
Union Electric Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	0	9
Restricted cash included in “Other current assets”	4	4
Restricted cash included in “Other assets”	0	0
Restricted cash included in “Nuclear decommissioning trust fund”	6	26
Total cash, cash equivalents, and restricted cash	10	39	14	8
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”	5	5
Restricted cash included in “Other assets”	142	120
Restricted cash included in “Nuclear decommissioning trust fund”	0	0
Total cash, cash equivalents, and restricted cash	$ 147	$ 125	$ 117	$ 80

Supplemental Information (Allowance for Doubtful Accounts) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning of period	$ 25	$ 19	$ 17	$ 18
Bad debt expense	21	14	31	22
Net write-offs	(2)	(14)	(4)	(21)
End of period	44	19	44	19
Union Electric Company
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning of period	9	7	7	7
Bad debt expense	5	3	9	6
Net write-offs	0	(2)	(2)	(5)
End of period	14	8	14	8
Ameren Illinois Company
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning of period	16	12	10	11
Bad debt expense	16	11	22	16
Net write-offs	(2)	(12)	(2)	(16)
End of period	$ 30	$ 11	$ 30	$ 11

Supplemental Information (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2020	Sep. 30, 2020	Sep. 30, 2019
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures		$ 311	$ 273
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund		43	100
Issuance of common stock for stock-based compensation	$ 38	38	54
Debt Conversion, Original Debt, Amount		0	(17)
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures		115	138
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund		43	100
Issuance of common stock for stock-based compensation		0	0
Debt Conversion, Original Debt, Amount		0	0
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures		191	128
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund		0	0
Issuance of common stock for stock-based compensation		0	0
Debt Conversion, Original Debt, Amount		$ 0	$ (17)

Supplemental Information (Schedule of Asset Retirement Obligations) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	$ 691
Liabilities settled	(42)
Accretion	22
Change in estimates	57
Asset Retirement Obligation, Ending Balance	728
Other current liabilities	432	$ 437
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	687
Liabilities settled	(42)
Accretion	21
Change in estimates	57
Asset Retirement Obligation, Ending Balance	723
Other current liabilities	102	96
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	4
Liabilities settled	0
Accretion	1
Change in estimates	0
Asset Retirement Obligation, Ending Balance	5
Other current liabilities	234	249
Asset Retirement Obligation Balance
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Other current liabilities	53	53
Asset Retirement Obligation Balance \| Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Other current liabilities	$ 53	$ 53

Supplemental Information (Schedule Of Excise Taxes) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accounting Policies [Line Items]
Excise tax expense	$ 71	$ 76	$ 198	$ 209
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	45	49	111	118
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 26	$ 27	$ 87	$ 91

Supplemental Information (Earnings Per Share) (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share Reconciliation [Abstract]
Weighted-average Common Shares Outstanding – Basic	247,100,000	245,900,000	246,800,000	245,500,000
Assumed settlement of performance share units and restricted stock units	1,500,000	1,400,000	1,200,000	1,400,000
Dilutive effect of forward sale agreement	600,000	200,000	400,000	100,000
Weighted-average Common Shares Outstanding – Diluted	249,200,000	247,500,000	248,400,000	247,000,000.0
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount (in shares)	0	0	0	0

Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
External Revenues	$ 1,628	$ 1,659	$ 4,466	$ 4,594
Net income (loss) attributable to common shareholders	367	364	756	734
Capital expenditures	656	636	1,884	1,761
Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	1,628	1,640	4,466	4,575
Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	19	0	19
Ameren Illinois Electric Distribution \| Ameren Missouri Software Licensing with Ameren Illinois
Segment Reporting Information [Line Items]
Intersegment revenues		14		14
Ameren Illinois Natural Gas \| Ameren Missouri Software Licensing with Ameren Illinois
Segment Reporting Information [Line Items]
Intersegment revenues		5		5
Ameren Illinois Company
Segment Reporting Information [Line Items]
External Revenues	589	564	1,879	1,873
Net income (loss) attributable to common shareholders	77	65	280	247
Capital expenditures	370	344	1,031	900
Ameren Illinois Company \| Ameren Missouri Software Licensing with Ameren Illinois
Segment Reporting Information [Line Items]
Intersegment revenues		19		19
Ameren Illinois Company \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	589	564	1,879	1,873
Ameren Illinois Company \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0	0
Operating Segments \| Union Electric Company
Segment Reporting Information [Line Items]
External Revenues	1,001	1,059	2,473	2,615
Net income (loss) attributable to common shareholders	297	300	439	446
Capital expenditures	262	256	778	751
Operating Segments \| Union Electric Company \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	990	1,050	2,442	2,591
Operating Segments \| Union Electric Company \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	11	9	31	24
Operating Segments \| Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External Revenues	391	389	1,133	1,135
Net income (loss) attributable to common shareholders	34	32	107	105
Capital expenditures	129	139	391	390
Operating Segments \| Ameren Illinois Electric Distribution \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	390	374	1,131	1,118
Operating Segments \| Ameren Illinois Electric Distribution \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	1	15	2	17
Operating Segments \| Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
External Revenues	122	112	533	568
Net income (loss) attributable to common shareholders	2	(1)	66	57
Capital expenditures	81	113	221	241
Operating Segments \| Ameren Illinois Natural Gas \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	122	107	533	563
Operating Segments \| Ameren Illinois Natural Gas \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	5	0	5
Operating Segments \| Ameren Transmission
Segment Reporting Information [Line Items]
External Revenues	141	128	400	351
Net income (loss) attributable to common shareholders	62	53	168	139
Capital expenditures	185	129	490	377
Operating Segments \| Ameren Transmission \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	126	109	360	303
Operating Segments \| Ameren Transmission \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	15	19	40	48
Operating Segments \| Other
Segment Reporting Information [Line Items]
Net income (loss) attributable to common shareholders	(28)	(20)	(24)	(13)
Capital expenditures	2		4	3
Capital expenditures, transfers		(7)
Operating Segments \| Other \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Other \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0	0
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Net income (loss) attributable to common shareholders	34	32	107	105
Capital expenditures	129	139	391	390
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Electric Distribution \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	391	389	1,133	1,135
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Electric Distribution \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0	0
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
Net income (loss) attributable to common shareholders	2	(1)	66	57
Capital expenditures	81	113	221	241
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Natural Gas \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	122	112	533	568
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Natural Gas \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0	0
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Transmission
Segment Reporting Information [Line Items]
Net income (loss) attributable to common shareholders	41	34	107	85
Capital expenditures	160	92	419	269
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Transmission \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	76	63	213	170
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Transmission \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	15	18	39	47
Intersegment Eliminations
Segment Reporting Information [Line Items]
External Revenues	(27)	(29)	(73)	(75)
Net income (loss) attributable to common shareholders	0	0	0	0
Capital expenditures	(3)	6	0	(1)
Intersegment Eliminations \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	(27)	(29)	(73)	(75)
Intersegment Eliminations \| Ameren Illinois Company
Segment Reporting Information [Line Items]
External Revenues	(15)	(18)	(39)	(47)
Net income (loss) attributable to common shareholders	0	0	0	0
Capital expenditures	0	0	0	0
Intersegment Eliminations \| Ameren Illinois Company \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations \| Ameren Illinois Company \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	$ (15)	$ (18)	$ (39)	$ (47)

Segment Information (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
External Revenues	$ 1,628	$ 1,659	$ 4,466	$ 4,594
Revenues from alternative revenue programs	(102)	(130)	(16)	(93)
Other revenues not from contracts with customers	11	7	28	21
Ameren Illinois Electric Distribution \| Ameren Missouri Software Licensing with Ameren Illinois
Disaggregation of Revenue [Line Items]
Intersegment revenues		14		14
Ameren Illinois Natural Gas \| Ameren Missouri Software Licensing with Ameren Illinois
Disaggregation of Revenue [Line Items]
Intersegment revenues		5		5
Electricity
Disaggregation of Revenue [Line Items]
External Revenues	1,489	1,528	3,846	3,928
Electricity \| Residential
Disaggregation of Revenue [Line Items]
External Revenues	689	713	1,775	1,774
Electricity \| Commercial
Disaggregation of Revenue [Line Items]
External Revenues	470	517	1,193	1,313
Electricity \| Industrial
Disaggregation of Revenue [Line Items]
External Revenues	113	121	298	320
Electricity \| Other
Disaggregation of Revenue [Line Items]
External Revenues	217	177	580	521
Natural Gas
Disaggregation of Revenue [Line Items]
External Revenues	139	131	620	666
Natural Gas \| Residential
Disaggregation of Revenue [Line Items]
External Revenues	76	73	427	455
Natural Gas \| Commercial
Disaggregation of Revenue [Line Items]
External Revenues	21	21	114	129
Natural Gas \| Industrial
Disaggregation of Revenue [Line Items]
External Revenues	6	3	14	12
Natural Gas \| Other
Disaggregation of Revenue [Line Items]
External Revenues	36	34	65	70
Ameren Illinois Company
Disaggregation of Revenue [Line Items]
External Revenues	589	564	1,879	1,873
Revenues from alternative revenue programs	(102)	(156)	(13)	(135)
Other revenues not from contracts with customers	5	2	7	7
Ameren Illinois Company \| Ameren Missouri Software Licensing with Ameren Illinois
Disaggregation of Revenue [Line Items]
Intersegment revenues		19		19
Ameren Illinois Company \| Residential
Disaggregation of Revenue [Line Items]
External Revenues	302	289	1,039	1,039
Ameren Illinois Company \| Commercial
Disaggregation of Revenue [Line Items]
External Revenues	145	140	459	475
Ameren Illinois Company \| Industrial
Disaggregation of Revenue [Line Items]
External Revenues	31	29	102	103
Ameren Illinois Company \| Other
Disaggregation of Revenue [Line Items]
External Revenues	111	106	279	256
Ameren Illinois Company \| Electricity
Disaggregation of Revenue [Line Items]
External Revenues	467	452	1,346	1,305
Ameren Illinois Company \| Natural Gas
Disaggregation of Revenue [Line Items]
External Revenues	122	112	533	568
Operating Segments \| Union Electric Company
Disaggregation of Revenue [Line Items]
External Revenues	1,001	1,059	2,473	2,615
Revenues from alternative revenue programs	1	26	(8)	41
Other revenues not from contracts with customers	6	5	21	14
Operating Segments \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External Revenues	391	389	1,133	1,135
Revenues from alternative revenue programs	(110)	(145)	(59)	(111)
Other revenues not from contracts with customers	5	1	6	5
Operating Segments \| Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
External Revenues	122	112	533	568
Revenues from alternative revenue programs	3	1	17	2
Other revenues not from contracts with customers	0	1	1	2
Operating Segments \| Ameren Transmission
Disaggregation of Revenue [Line Items]
External Revenues	141	128	400	351
Revenues from alternative revenue programs	4	(12)	34	(25)
Other revenues not from contracts with customers	0	0	0	0
Operating Segments \| Electricity \| Union Electric Company
Disaggregation of Revenue [Line Items]
External Revenues	984	1,040	2,386	2,517
Operating Segments \| Electricity \| Union Electric Company \| Residential
Disaggregation of Revenue [Line Items]
External Revenues	455	489	1,111	1,134
Operating Segments \| Electricity \| Union Electric Company \| Commercial
Disaggregation of Revenue [Line Items]
External Revenues	343	394	828	943
Operating Segments \| Electricity \| Union Electric Company \| Industrial
Disaggregation of Revenue [Line Items]
External Revenues	87	94	207	226
Operating Segments \| Electricity \| Union Electric Company \| Other
Disaggregation of Revenue [Line Items]
External Revenues	99	63	240	214
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External Revenues	391	389	1,133	1,135
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution \| Residential
Disaggregation of Revenue [Line Items]
External Revenues	234	224	664	640
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution \| Commercial
Disaggregation of Revenue [Line Items]
External Revenues	127	123	365	370
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution \| Industrial
Disaggregation of Revenue [Line Items]
External Revenues	26	27	91	94
Operating Segments \| Electricity \| Ameren Illinois Electric Distribution \| Other
Disaggregation of Revenue [Line Items]
External Revenues	4	15	13	31
Operating Segments \| Electricity \| Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Electricity \| Ameren Illinois Natural Gas \| Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Electricity \| Ameren Illinois Natural Gas \| Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Electricity \| Ameren Illinois Natural Gas \| Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Electricity \| Ameren Illinois Natural Gas \| Other
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Electricity \| Ameren Transmission
Disaggregation of Revenue [Line Items]
External Revenues	141	128	400	351
Operating Segments \| Electricity \| Ameren Transmission \| Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Electricity \| Ameren Transmission \| Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Electricity \| Ameren Transmission \| Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Electricity \| Ameren Transmission \| Other
Disaggregation of Revenue [Line Items]
External Revenues	141	128	400	351
Operating Segments \| Natural Gas \| Union Electric Company
Disaggregation of Revenue [Line Items]
External Revenues	17	19	87	98
Operating Segments \| Natural Gas \| Union Electric Company \| Residential
Disaggregation of Revenue [Line Items]
External Revenues	8	8	52	56
Operating Segments \| Natural Gas \| Union Electric Company \| Commercial
Disaggregation of Revenue [Line Items]
External Revenues	3	4	20	24
Operating Segments \| Natural Gas \| Union Electric Company \| Industrial
Disaggregation of Revenue [Line Items]
External Revenues	1	1	3	3
Operating Segments \| Natural Gas \| Union Electric Company \| Other
Disaggregation of Revenue [Line Items]
External Revenues	5	6	12	15
Operating Segments \| Natural Gas \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Illinois Electric Distribution \| Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Illinois Electric Distribution \| Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Illinois Electric Distribution \| Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Illinois Electric Distribution \| Other
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
External Revenues	122	112	533	568
Operating Segments \| Natural Gas \| Ameren Illinois Natural Gas \| Residential
Disaggregation of Revenue [Line Items]
External Revenues	68	65	375	399
Operating Segments \| Natural Gas \| Ameren Illinois Natural Gas \| Commercial
Disaggregation of Revenue [Line Items]
External Revenues	18	17	94	105
Operating Segments \| Natural Gas \| Ameren Illinois Natural Gas \| Industrial
Disaggregation of Revenue [Line Items]
External Revenues	5	2	11	9
Operating Segments \| Natural Gas \| Ameren Illinois Natural Gas \| Other
Disaggregation of Revenue [Line Items]
External Revenues	31	28	53	55
Operating Segments \| Natural Gas \| Ameren Transmission
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Transmission \| Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Transmission \| Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Transmission \| Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Natural Gas \| Ameren Transmission \| Other
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(110)	(145)	(59)	(111)
Other revenues not from contracts with customers	5	1	6	5
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	3	1	17	2
Other revenues not from contracts with customers	0	1	1	2
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	5	(12)	29	(26)
Other revenues not from contracts with customers	0	0	0	0
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External Revenues	391	389	1,133	1,135
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution \| Residential
Disaggregation of Revenue [Line Items]
External Revenues	234	224	664	640
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution \| Commercial
Disaggregation of Revenue [Line Items]
External Revenues	127	123	365	370
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution \| Industrial
Disaggregation of Revenue [Line Items]
External Revenues	26	27	91	94
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Electric Distribution \| Other
Disaggregation of Revenue [Line Items]
External Revenues	4	15	13	31
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
External Revenues	91	81	252	217
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Transmission \| Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Transmission \| Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Transmission \| Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments \| Ameren Illinois Company \| Electricity \| Ameren Illinois Transmission \| Other
Disaggregation of Revenue [Line Items]
External Revenues	91	81	252	217
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
External Revenues	122	112	533	568
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Natural Gas \| Residential
Disaggregation of Revenue [Line Items]
External Revenues	68	65	375	399
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Natural Gas \| Commercial
Disaggregation of Revenue [Line Items]
External Revenues	18	17	94	105
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Natural Gas \| Industrial
Disaggregation of Revenue [Line Items]
External Revenues	5	2	11	9
Operating Segments \| Ameren Illinois Company \| Natural Gas \| Ameren Illinois Natural Gas \| Other
Disaggregation of Revenue [Line Items]
External Revenues	31	28	53	55
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
External Revenues	(27)	(29)	(73)	(75)
Intersegment Eliminations \| Electricity
Disaggregation of Revenue [Line Items]
External Revenues	(27)	(29)	(73)	(75)
Intersegment Eliminations \| Electricity \| Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations \| Electricity \| Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations \| Electricity \| Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations \| Electricity \| Other
Disaggregation of Revenue [Line Items]
External Revenues	(27)	(29)	(73)	(75)
Intersegment Eliminations \| Natural Gas
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations \| Natural Gas \| Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations \| Natural Gas \| Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations \| Natural Gas \| Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations \| Natural Gas \| Other
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations \| Ameren Illinois Company
Disaggregation of Revenue [Line Items]
External Revenues	(15)	(18)	(39)	(47)
Intersegment Eliminations \| Ameren Illinois Company \| Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations \| Ameren Illinois Company \| Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations \| Ameren Illinois Company \| Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations \| Ameren Illinois Company \| Other
Disaggregation of Revenue [Line Items]
External Revenues	$ (15)	$ (18)	$ (39)	$ (47)