Ameren Illinois (AILLI) 10-Q 11 May 20 2020 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2020	Apr. 30, 2020
Entity Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2020
Document Transition Report	false
Entity File Number	1-14756
Entity Registrant Name	Ameren Corporation
Entity Tax Identification Number	43-1723446
Entity Incorporation, State or Country Code	MO
Entity Address, Address Line One	1901 Chouteau Avenue
Entity Address, City or Town	St. Louis
Entity Address, State or Province	MO
Entity Address, Postal Zip Code	63103
City Area Code	(314)
Local Phone Number	621-3222
Title of 12(b) Security	Common Stock, $0.01 par value per share
Trading Symbol	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		246,891,031
Entity Central Index Key	0001002910
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Amendment Flag	false
Union Electric Company
Entity Information [Line Items]
Entity File Number	1-2967
Entity Registrant Name	Union Electric Company
Entity Tax Identification Number	43-0559760
Entity Incorporation, State or Country Code	MO
Entity Address, Address Line One	1901 Chouteau Avenue
Entity Address, City or Town	St. Louis
Entity Address, State or Province	MO
Entity Address, Postal Zip Code	63103
City Area Code	(314)
Local Phone Number	621-3222
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		102,123,834
Entity Central Index Key	0000100826
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Amendment Flag	false
Ameren Illinois Company
Entity Information [Line Items]
Entity File Number	1-3672
Entity Registrant Name	Ameren Illinois Company
Entity Tax Identification Number	37-0211380
Entity Incorporation, State or Country Code	IL
Entity Address, Address Line One	10 Executive Drive
Entity Address, City or Town	Collinsville
Entity Address, State or Province	IL
Entity Address, Postal Zip Code	62234
City Area Code	(618)
Local Phone Number	343-8150
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		25,452,373
Entity Central Index Key	0000018654
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Amendment Flag	false

Consolidated Statement of Incom

Consolidated Statement of Income (Loss) and Comprehensive Income (Loss) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,440	$ 1,556
Operating Expenses:
Fuel	140	160
Purchased Power	134	156
Natural gas purchased for resale	107	161
Other operations and maintenance	438	417
Depreciation and amortization	255	248
Taxes other than income taxes	125	126
Total operating expenses	1,199	1,268
Operating Income	241	288
Other Income (Expense), Net	21	29
Interest Charges	93	97
Income Before Income Taxes	169	220
Income Taxes	21	27
Net income	148	193
Less: Net Income Attributable to Noncontrolling Interests	2	2
Net income attributable to Ameren common shareholders	146	191
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	1	1
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	149	194
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	2	2
Comprehensive Income	$ 147	$ 192
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic and Diluted	$ 0.59	$ 0.78
Average Common Shares Outstanding - Basic (in shares)	246.4	244.9
Weighted Average Number of Shares Outstanding, Diluted (in shares)	248.1	246.4
Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,120	$ 1,182
Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	320	374
Union Electric Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	680	758
Operating Expenses:
Fuel	140	160
Purchased Power	39	51
Natural gas purchased for resale	18	27
Other operations and maintenance	239	224
Depreciation and amortization	139	140
Taxes other than income taxes	79	77
Total operating expenses	654	679
Operating Income	26	79
Other Income (Expense), Net	4	12
Interest Charges	40	47
Income Before Income Taxes	(10)	44
Income Taxes	(1)	4
Net income	(9)	40
Preferred Stock Dividends	1	1
Net Income (Loss) Attributable to Parent	(10)	39
Union Electric Company | Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	631	704
Union Electric Company | Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	49	54
Ameren Illinois Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	723	762
Operating Expenses:
Purchased Power	98	105
Natural gas purchased for resale	89	134
Other operations and maintenance	199	191
Depreciation and amortization	107	101
Taxes other than income taxes	42	45
Total operating expenses	535	576
Operating Income	188	186
Other Income (Expense), Net	11	11
Interest Charges	39	37
Income Before Income Taxes	160	160
Income Taxes	39	39
Net income	121	121
Preferred Stock Dividends	1	1
Net Income (Loss) Attributable to Parent	120	120
Ameren Illinois Company | Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	452	442
Ameren Illinois Company | Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 271	$ 320

Consolidated Statement of Inc_2

Consolidated Statement of Income (Loss) and Comprehensive Income (Loss) (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Statement [Abstract]
Pension and other postretirement benefit plan activity, tax expense (benefit)	$ 0	$ 0

Consolidated Balance Sheet

Consolidated Balance Sheet - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Current Assets:
Cash and cash equivalents	$ 42	$ 16
Accounts receivable - trade (less allowance for doubtful accounts)	456	393
Unbilled revenue	212	278
Miscellaneous accounts receivable	65	63
Inventories	471	494
Current regulatory assets	91	69
Other current assets	127	118
Total current assets	1,464	1,431
Property, Plant, and Equipment, Net	24,678	24,376
Investments and Other Assets:
Nuclear decommissioning trust fund	742	847
Goodwill	411	411
Regulatory assets	1,092	992
Other assets	885	876
Total investments and other assets	3,130	3,126
TOTAL ASSETS	29,272	28,933
Current Liabilities:
Current maturities of long-term debt	357	442
Short-term Debt	615	440
Accounts and wages payable	544	874
Current regulatory liabilities	189	164
Other current liabilities	662	585
Total current liabilities	2,367	2,505
Long-term Debt, Net	9,378	8,915
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	2,948	2,919
Regulatory liabilities	4,842	4,887
Asset retirement obligations	631	638
Pension and other postretirement benefits	397	401
Other deferred credits and liabilities	482	467
Total deferred credits and other liabilities	9,300	9,312
Commitments and Contingencies
Stockholders' Equity:
Common Stock	2	2
Other paid-in capital	5,695	5,694
Retained earnings	2,404	2,380
Accumulated other comprehensive income (loss)	(16)	(17)
Shareholder's equity	8,085	8,059
Noncontrolling Interest	142	142
Total equity	8,227	8,201
TOTAL LIABILITIES AND EQUITY	29,272	28,933
Union Electric Company
Current Assets:
Cash and cash equivalents	3	9
Accounts receivable - trade (less allowance for doubtful accounts)	174	164
Accounts receivable - affiliates	34	30
Unbilled revenue	117	139
Miscellaneous accounts receivable	43	33
Inventories	391	373
Other current assets	96	66
Total current assets	858	814
Property, Plant, and Equipment, Net	12,731	12,635
Investments and Other Assets:
Nuclear decommissioning trust fund	742	847
Regulatory assets	305	285
Other assets	357	356
Total investments and other assets	1,404	1,488
TOTAL ASSETS	14,993	14,937
Current Liabilities:
Current maturities of long-term debt	7	92
Short-term Debt	130	234
Accounts and wages payable	238	465
Accounts payable - affiliates	48	52
Taxes accrued	64	24
Interest accrued	46	48
Current asset retirement obligation	53	53
Current regulatory liabilities	80	62
Other current liabilities	128	96
Total current liabilities	794	1,126
Long-term Debt, Net	4,560	4,098
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,627	1,612
Regulatory liabilities	2,855	2,937
Asset retirement obligations	627	634
Pension and other postretirement benefits	139	141
Other deferred credits and liabilities	52	40
Total deferred credits and other liabilities	5,300	5,364
Commitments and Contingencies
Stockholders' Equity:
Common Stock	511	511
Other paid-in capital	2,027	2,027
Preferred stock	80	80
Retained earnings	1,721	1,731
Shareholder's equity	4,339	4,349
TOTAL LIABILITIES AND EQUITY	14,993	14,937
Ameren Illinois Company
Current Assets:
Cash and cash equivalents	6	0
Accounts receivable - trade (less allowance for doubtful accounts)	268	215
Accounts receivable - affiliates	15	28
Unbilled revenue	95	139
Miscellaneous accounts receivable	19	25
Inventories	80	121
Current regulatory assets	57	57
Other current assets	31	29
Total current assets	571	614
Property, Plant, and Equipment, Net	10,280	10,083
Investments and Other Assets:
Goodwill	411	411
Regulatory assets	771	694
Other assets	400	383
Total investments and other assets	1,582	1,488
TOTAL ASSETS	12,433	12,185
Current Liabilities:
Short-term Debt	60	53
Accounts and wages payable	246	299
Accounts payable - affiliates	84	82
Customer deposits	80	77
Current environmental remediation	52	42
Current regulatory liabilities	90	84
Other current liabilities	195	207
Total current liabilities	807	844
Long-term Debt, Net	3,575	3,575
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,251	1,224
Regulatory liabilities	1,884	1,849
Pension and other postretirement benefits	212	214
Environmental remediation	75	87
Other deferred credits and liabilities	277	260
Total deferred credits and other liabilities	3,699	3,634
Commitments and Contingencies
Stockholders' Equity:
Common Stock	0	0
Other paid-in capital	2,288	2,188
Preferred stock	62	62
Retained earnings	2,002	1,882
Shareholder's equity	4,352	4,132
TOTAL LIABILITIES AND EQUITY	$ 12,433	$ 12,185

Consolidated Balance Sheet (Par

Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Accounts receivable - trade allowance for doubtful accounts	$ 19	$ 17
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	400,000,000	400,000,000
Common Stock, Shares, Outstanding	246,900,000	246,200,000
Union Electric Company
Accounts receivable - trade allowance for doubtful accounts	$ 8	$ 7
Common stock, par value (in dollars per share)	$ 5	$ 5
Common stock, shares authorized (in shares)	150,000,000	150,000,000
Common Stock, Shares, Outstanding	102,100,000	102,100,000
Ameren Illinois Company
Accounts receivable - trade allowance for doubtful accounts	$ 11	$ 10
Common stock, no par value (in dollars per share)	$ 0	$ 0
Common stock, shares authorized (in shares)	45,000,000	45,000,000
Common Stock, Shares, Outstanding	25,500,000	25,500,000

Consolidated Statement of Cash

Consolidated Statement of Cash Flows - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash Flows From Operating Activities:
Net income	$ 148	$ 193
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	263	245
Amortization of nuclear fuel	23	23
Amortization of debt issuance costs and premium/discounts	5	5
Deferred income taxes and investment tax credits, net	23	32
Allowance for equity funds used during construction	(4)	(6)
Stock-based compensation costs	6	6
Other	17	(8)
Changes in assets and liabilities:
Receivables	(5)	4
Inventories	23	81
Accounts and wages payable	(221)	(213)
Taxes accrued	47	28
Regulatory assets and liabilities	(14)	26
Assets, other	(3)	(14)
Liabilities, other	(18)	(11)
Pension and other postretirement benefits	0	(4)
Net cash provided by operating activities	290	387
Cash Flows From Investing Activities:
Capital expenditures	(636)	(544)
Nuclear fuel expenditures	(35)	(21)
Purchases of securities – nuclear decommissioning trust fund	(96)	(39)
Sales and maturities of securities – nuclear decommissioning trust fund	81	36
Other	2	1
Net cash used in investing activities	(684)	(567)
Cash Flows From Financing Activities:
Dividends on common stock	(122)	(116)
Dividends paid to noncontrolling interest holders	(2)	(2)
Short-term debt, net	175	202
Maturities of long-term debt	(85)	(329)
Issuances of long-term debt	465	450
Issuances of common stock	13	19
Employee payroll taxes related to stock-based compensation	(20)	(29)
Debt issuance costs	(3)	(4)
Net cash provided by financing activities	421	191
Net change in cash, cash equivalents, and restricted cash	27	11
Cash, cash equivalents, and restricted cash at beginning of year	176	107
Cash, cash equivalents, and restricted cash at end of period	203	118
Union Electric Company
Cash Flows From Operating Activities:
Net income	(9)	40
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	148	138
Amortization of nuclear fuel	23	23
Amortization of debt issuance costs and premium/discounts	2	2
Deferred income taxes and investment tax credits, net	(5)	(1)
Allowance for equity funds used during construction	(2)	(4)
Other	2	2
Changes in assets and liabilities:
Receivables	(3)	56
Inventories	(18)	29
Accounts and wages payable	(172)	(167)
Taxes accrued	55	44
Regulatory assets and liabilities	16	11
Assets, other	2	(16)
Liabilities, other	0	(4)
Pension and other postretirement benefits	2	(1)
Net cash provided by operating activities	41	152
Cash Flows From Investing Activities:
Capital expenditures	(278)	(240)
Nuclear fuel expenditures	(35)	(21)
Purchases of securities – nuclear decommissioning trust fund	(96)	(39)
Sales and maturities of securities – nuclear decommissioning trust fund	81	36
Net cash used in investing activities	(328)	(264)
Cash Flows From Financing Activities:
Dividends on preferred stock	(1)	(1)
Short-term debt, net	(104)	0
Maturities of long-term debt	(85)	(329)
Issuances of long-term debt	465	450
Debt issuance costs	(3)	(4)
Net cash provided by financing activities	272	116
Net change in cash, cash equivalents, and restricted cash	(15)	4
Cash, cash equivalents, and restricted cash at beginning of year	39	8
Cash, cash equivalents, and restricted cash at end of period	24	12
Ameren Illinois Company
Cash Flows From Operating Activities:
Net income	121	121
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	106	100
Amortization of debt issuance costs and premium/discounts	2	3
Deferred income taxes and investment tax credits, net	22	24
Allowance for equity funds used during construction	(2)	(2)
Other	(2)	(1)
Changes in assets and liabilities:
Receivables	(6)	(56)
Inventories	41	52
Accounts and wages payable	(20)	(34)
Taxes accrued	16	12
Regulatory assets and liabilities	(28)	18
Assets, other	(4)	3
Liabilities, other	(14)	(12)
Pension and other postretirement benefits	(2)	(3)
Net cash provided by operating activities	232	227
Cash Flows From Investing Activities:
Capital expenditures	(324)	(267)
Other	1	0
Net cash used in investing activities	(323)	(267)
Cash Flows From Financing Activities:
Dividends on preferred stock	(1)	(1)
Short-term debt, net	7	54
Capital contribution from parent	100	0
Net cash provided by financing activities	106	53
Net change in cash, cash equivalents, and restricted cash	15	13
Cash, cash equivalents, and restricted cash at beginning of year	125	80
Cash, cash equivalents, and restricted cash at end of period	$ 140	$ 93

Consolidated Statement of Stock

Consolidated Statement of Stockholders' Equity - USD ($) $ in Millions	Total	Common Stock	Other Paid-in Capital	Retained Earnings	Deferred Retirement Benefit Costs	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interests	Total Ameren Corporation Shareholders' Equity	Union Electric Company	Union Electric CompanyCommon Stock	Union Electric CompanyOther Paid-in Capital	Union Electric CompanyPreferred Stock	Union Electric CompanyRetained Earnings	Ameren Illinois Company	Ameren Illinois CompanyCommon Stock	Ameren Illinois CompanyOther Paid-in Capital	Ameren Illinois CompanyPreferred Stock	Ameren Illinois CompanyRetained Earnings
Beginning of year at Dec. 31, 2018			$ 5,627	$ 2,024	$ (22)		$ 142						$ 1,735			$ 2,173		$ 1,539
Shares issued under the DRPlus and 401(k) plan			19
Stock-based compensation activity			(21)
Capital contribution from parent														$ 0		0
Net income	$ 193								$ 40				40	121				121
Net income attributable to Ameren common shareholders	191			191
Common stock dividends				(116)
Preferred stock dividends													(1)					(1)
Change in deferred retirement benefit costs	1				1
Net income attributable to noncontrolling interest holders	$ (2)						2
Dividends paid to noncontrolling interest holders							(2)
Common stock shares outstanding at beginning of year at Dec. 31, 2018	244,500,000
Shares issued under the DRPlus and 401(k) plan	300,000
Shares issued for stock-based compensation	800,000
Common stock shares outstanding at end of period at Mar. 31, 2019	245,600,000
End of period at Mar. 31, 2019	$ 7,847	$ 2	5,625	2,099	(21)	$ (21)	142			$ 511	$ 1,903	$ 80	1,774		$ 0	2,173	$ 62	1,659
Dividends per common share (in dollars per share)	$ 0.4750
Shareholders' equity, end of year at Mar. 31, 2019								$ 7,705	4,268					3,894
Beginning of year 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			13
Stock-based compensation activity			(12)
Capital contribution from parent														(100)		100
Net income	148								$ (9)				(9)	$ 121				121
Net income attributable to Ameren common shareholders	146			146
Common stock dividends				(122)
Preferred stock dividends													(1)					(1)
Change in deferred retirement benefit costs	1				1
Net income attributable to noncontrolling interest holders	$ (2)						2
Dividends paid to noncontrolling interest holders							(2)
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	200,000
Shares issued for stock-based compensation	500,000
Common stock shares outstanding at end of period at Mar. 31, 2020	246,900,000								102,100,000					25,500,000
End of period at Mar. 31, 2020	$ 8,227	$ 2	$ 5,695	$ 2,404	$ (16)	$ (16)	$ 142			$ 511	$ 2,027	$ 80	$ 1,721		$ 0	$ 2,288	$ 62	$ 2,002
Dividends per common share (in dollars per share)	$ 0.4950
Shareholders' equity, end of year at Mar. 31, 2020	$ 8,085							$ 8,085	$ 4,339					$ 4,352

Summary Of Significant Accounti

Summary Of Significant Accounting Policies	3 Months Ended
	Mar. 31, 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 is a rapidly evolving situation. While the COVID-19 pandemic did not have a material impact on our results of operations, financial position, or liquidity for the three months ended March 31, 2020, it may adversely affect our results of operations, financial position, or liquidity in subsequent periods. The effect will depend on the severity and longevity of the COVID-19 pandemic and the resulting impact on business, economic, and capital market conditions. As a result of the COVID-19 pandemic, measures have been taken by local, state, and federal governments, such as travel bans, quarantines, and shelter-in place orders. On March 21, 2020, a shelter-in-place order for the state of Illinois became effective and will remain in effect until at least May 30, 2020. Similar orders became effective for Saint Louis City and County on March 23, 2020, and the state of Missouri on April 6, 2020. The state of Missouri order was effective through May 3, 2020, while Saint Louis City and County are expected to begin easing restrictions on May 18, 2020. These orders generally preclude or limit the operation of businesses that are deemed nonessential. Ameren's business operations are deemed essential and are not directly impacted by the shelter-in-place orders. As a result of the COVID-19 pandemic, economic activity has been disrupted in the service territories of Ameren Missouri and Ameren Illinois. It has also caused disruptions in the capital markets, which could adversely affect our ability to access these markets on reasonable terms and when needed. These disruptions could continue for a prolonged period of time or become more severe. On March 13, 2020, and March 16, 2020, Ameren Illinois and Ameren Missouri, respectively, suspended customer disconnections for non-payment and began to waive late fees. Regarding bad debt expense, Ameren Illinois' electric distribution and natural gas distribution businesses have bad debt riders, which would provide for recovery of increased bad debt expense. However, Ameren Missouri’s earnings are exposed to potential increases in future bad debt expense, which could result in incremental accounts receivable write-offs in future periods as Ameren Missouri does not have a bad debt rider or regulatory tracking mechanism. Our customers’ ability to pay for our services may be adversely affected by the COVID-19 pandemic. A reduction in collections from our tariff-based revenues could reduce our cash from operations and cause an adverse impact to our liquidity. 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, temporary suspension of the payment of the employer portion of Social Security taxes, and additional funding for customer energy assistance, among other things. Ameren has implemented certain provisions of the act, and is currently evaluating other provisions of the act. As of March 31, 2020, there was no material impact to Ameren’s, Ameren Missouri’s, and Ameren Illinois’ 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. Ameren Missouri and Ameren Illinois have no subsidiaries. All tabular dollar amounts are in millions, unless otherwise indicated. Our accounting policies conform to GAAP. Our financial statements reflect all adjustments (which include normal, recurring adjustments) that are necessary, in our opinion, for a fair statement of our results. The preparation of financial statements in conformity with GAAP requires management to make certain estimates and assumptions. Such estimates and assumptions affect reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the dates of financial statements, and the reported amounts of revenues and expenses during the reported periods. Actual results could differ from those estimates. The results of operations of an interim period may not give a true indication of results that may be expected for a full year. These financial statements should be read in conjunction with the financial statements and accompanying notes included in the Form 10-K. Variable Interest Entities As of March 31, 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 March 31, 2020 , the maximum exposure to loss related to these variable interest entities was approximately $15 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 March 31, 2020 , and December 31, 2019 , Ameren had unconsolidated variable interests as a limited partner in various equity method investments, totaling $31 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 March 31, 2020 , the maximum exposure to loss related to these variable interests is limited to the investment in these partnerships of $31 million plus associated outstanding funding commitments of $34 million . Company-owned Life Insurance Ameren and Ameren Illinois have company-owned life insurance, which is recorded at the net cash surrender value. The net cash surrender value is the amount that can be realized under the insurance policies at the balance sheet date. As of March 31, 2020 , the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $245 million ( December 31, 2019 – $264 million ) and $118 million ( December 31, 2019 – $123 million ), respectively, while total borrowings against the policies were $109 million ( December 31, 2019 – $114 million ) at both Ameren and Ameren Illinois. Ameren and Ameren Illinois have the right to offset the borrowings against the cash surrender value of the policies and, consequently, present the net asset in “Other assets” on their respective balance sheets. The cash surrender value decreased during the three months ended March 31, 2020 , primarily because of a decrease in the market value of underlying investments. 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 recently issued authoritative accounting guidance relating to defined benefit plan disclosures. 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

Rate And Regulatory Matters	3 Months Ended
	Mar. 31, 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 2019 Electric Service Regulatory Rate Review 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. 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. These changes will result in 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 . An estimated $70 million would have otherwise been deferred under the PISA. A stipulation and agreement approved by the MoPSC’s March 2020 order states that the net impact of the revenue and expense changes noted above reflect 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 for the three months ended March 31, 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 have begun construction activities. In 2020, the developers of the wind generation facilities received notices from the wind turbine supplier, 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. Ameren Missouri and the developers continue to monitor the impact to each project schedule. To date, neither developer has reported to Ameren Missouri that the projects will not be completed in 2020. Ameren Missouri expects the up-to 400 -megawatt project to be placed in-service by the end of 2020. However, at this time, due to manufacturing, shipping, and other supply chain issues, and based on Ameren Missouri’s discussions with the developer, Ameren Missouri expects that a portion of the up-to 300 -megawatt project, representing approximately $100 million of investment, could be placed in-service in the first quarter of 2021. The build-transfer agreements include provisions for the event in which any portion of either project is completed after 2020. In such an event, according to the terms of the agreements, Ameren Missouri would pay a reduced contract price on the portion of the project completed after 2020, to account for risks associated with qualifying for production tax credits, subject to an obligation to later pay such price differential should Ameren Missouri be entitled to receive production tax credits. MEEIA As a result of MoPSC orders issued in September 2017, October 2018, and January 2019 related to performance incentives for the MEEIA 2013 and MEEIA 2016 programs, and in accordance with revenue recognition guidance, Ameren Missouri recognized revenues of $20 million during the first quarter of 2019. Ameren Missouri did not recognize revenues related to MEEIA performance incentives during the first quarter of 2020. Illinois Electric Distribution Service Rates In April 2020, Ameren Illinois filed its annual electric distribution service formula rate update to establish the revenue requirement to be used for 2021 rates with the ICC. Pending ICC approval, this update filing will result in a $45 million decrease in Ameren Illinois’ electric distribution service rates, beginning in January 2021. 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. An ICC decision in this proceeding is expected by December 2020. 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 by $102 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% allowed 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. 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 a request with the ICC for a reconciliation hearing to determine the accuracy and prudence of natural gas infrastructure 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 capital investments made during 2018. Ameren Illinois’ 2018 QIP rate recovery under review by the ICC is within the rate increase limitations allowed by law. 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, natural gas, water, and sewer utilities to suspend disconnections for customer non-payment and waive late fees, on an interim basis, effective March 18, 2020, and for as long as the public health emergency related to the COVID-19 pandemic remains in effect for the state of Illinois. At this time, the state of Illinois’ public health emergency remains in effect until May 30, 2020. The order also requires utilities to design and implement, upon ICC approval and on a temporary basis, more flexible credit and collection practices. In March 2020, Ameren Illinois filed a response to the ICC order stating their compliance with the suspension of disconnections and late fees for electric distribution and natural gas customers, and proposing more flexible credit and collection practices, including longer deferred payment arrangements for customers that fall behind on bill payments. In April 2020, similar to other utilities in Illinois, Ameren Illinois also requested approval to recover forgone late fees related to natural gas service through its existing bad debt rider and the ability to defer, as a regulatory asset, costs incurred related to the COVID-19 pandemic. Recovery of electric distribution forgone late fees and costs incurred related to the COVID-19 pandemic are included in Ameren Illinois’ electric distribution formula rates. In April 2020, the ICC staff recommended extending the suspension of disconnections and late fees for 60 days beyond when the state of Illinois’ public health emergency has ended. The ICC is under no deadline to issue an order in this proceeding. Federal Ameren Illinois Transmission Formula Rate Revisions In February 2020, MISO, on behalf of Ameren Illinois, filed a request with the FERC to revise Ameren Illinois’ transmission formula rate calculation with respect to calculation inputs for materials and supplies. In May 2020, the FERC issued an order approving the revisions prospectively. In addition, the FERC noted that the FERC staff should review historical rate recovery in connection with an ongoing FERC audit. At this time, Ameren and Ameren Illinois are evaluating this order, but do not expect the impact to be material on their 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% 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. As of March 31, 2020 , Ameren and Ameren Illinois had recorded current regulatory liabilities of $40 million and $23 million , respectively, to reflect the expected refunds, including interest, associated with the reduced ROEs in the November 2019 order in the November 2013 complaint case. The reduction in the FERC-allowed base ROE from 10.32% to 9.88% is not material to Ameren Missouri’s results of operations, financial position, or liquidity. In December 2019, Ameren and 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. The FERC has not ruled on the merits of the rehearing requests and is under no deadline to do so. The allowed base ROE for the 15-month period related to the February 2015 complaint case was 12.38% . Each 50 basis point reduction in the allowed base ROE for this 15-month period would reduce Ameren’s and Ameren Illinois’ net income by an estimated $11 million and $6 million , respectively. In March 2019, the FERC issued separate Notices of Inquiry regarding its allowed base ROE policy and its transmission incentives policy. Initial comments were due by June 2019, and reply comments were due by late August 2019. 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 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 improved parameters for awarding incentives, while limiting the overall incentives to a cap of 250 basis points, among other things. Initial comments are due by July 2020. Ameren is unable to predict the ultimate impact of the Notices of Inquiry or the Notice of Proposed Rulemaking at this time.

Short-Term Debt And Liquidity

Short-Term Debt And Liquidity	3 Months Ended
	Mar. 31, 2020
Debt Disclosure [Abstract]
SHORT-TERM DEBT AND LIQUIDITY	SHORT-TERM DEBT AND LIQUIDITY The liquidity needs of the Ameren Companies are typically supported through the use of available cash, drawings under committed credit agreements, commercial paper issuances, and, in the case of Ameren Missouri and Ameren Illinois, short-term affiliate borrowings. See Note 4 – Short-term Debt and Liquidity under Part II, Item 8, in the Form 10-K for a description of our indebtedness provisions and other covenants as well as a description of money pool arrangements. Credit Agreements 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 issue letters of credit. As of March 31, 2020 , based on credit facility borrowings, commercial paper outstanding, and letters of credit issued under the Credit Agreements, along with cash and cash equivalents, the net liquidity available to Ameren (parent), Ameren Missouri, and Ameren Illinois, collectively, was $1.7 billion . The Ameren Companies were in compliance with the covenants in their Credit Agreements as of March 31, 2020 . As of March 31, 2020 , the ratios of consolidated indebtedness to consolidated total capitalization, calculated in accordance with the provisions of the Credit Agreements, were 55% , 51% , and 46% for Ameren, Ameren Missouri, and Ameren Illinois, respectively. The following table presents the credit facility borrowings and commercial paper outstanding, net of issuance discounts, as of March 31, 2020 , and December 31, 2019 : March 31, 2020 December 31, 2019 Credit Facility Borrowings Commercial Paper Total Short-term Debt Credit Facility Borrowings Commercial Paper Total Short-term Debt Ameren (parent) $ 275 $ 150 $ 425 $ — $ 153 $ 153 Ameren Missouri 130 — 130 — 234 234 Ameren Illinois 60 — 60 — 53 53 Ameren consolidated $ 465 $ 150 $ 615 $ — $ 440 $ 440 The following table summarizes the borrowing activity and relevant interest rates under the Credit Agreements for the three months ended March 31, 2020 . There were no borrowings under the Credit Agreements for the three months ended March 31, 2019 . Ameren (parent) Ameren Missouri Ameren Illinois Total Missouri Credit Agreement Average daily credit facility borrowings outstanding during the period $ 1 $ 12 $ — $ 13 Weighted-average interest rate 2.06 % 2.36 % — % 2.33 % Peak credit facility borrowings during the period (a) $ 100 $ 130 $ — $ 230 Peak interest rate 2.06 % 3.33 % — % 3.33 % Illinois Credit Agreement Average daily credit facility borrowings outstanding during the period $ 2 $ — $ 5 $ 7 Weighted-average interest rate 2.06 % — % 2.05 % 2.06 % Peak credit facility borrowings during the period (a) $ 175 $ — $ 60 $ 235 Peak interest rate 2.06 % — % 2.05 % 2.06 % (a) The timing of peak credit facility borrowings varies by company. Therefore, the sum of individual company peak amounts may not equal the Ameren consolidated peak credit facility borrowings for the period. Commercial Paper The following table summarizes the borrowing activity and relevant interest rates under Ameren (parent)’s, Ameren Missouri’s, and Ameren Illinois’ commercial paper programs for the three months ended March 31, 2020 and 2019: Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2020 Average daily commercial paper outstanding at par value $ 154 $ 383 $ 71 $ 608 Weighted-average interest rate 1.94 % 1.84 % 1.99 % 1.88 % Peak commercial paper during period at par value (a) $ 225 $ 521 $ 137 $ 854 Peak interest rate 3.30 % 5.05 % (b) 3.40 % 5.05 % (b) 2019 Average daily commercial paper outstanding at par value $ 480 $ 246 $ 89 $ 815 Weighted-average interest rate 2.87 % 2.84 % 2.76 % 2.85 % Peak commercial paper during period at par value (a) $ 618 $ 549 $ 130 $ 1,113 Peak interest rate 3.10 % 2.97 % 2.90 % 3.10 % (a) The timing of peak outstanding commercial paper issuances varies by company. Therefore, the sum of individual company peak amounts may not equal the Ameren consolidated peak commercial paper issuances for the period. (b) In the first quarter of 2020, Ameren Missouri’s peak interest rate was affected by temporary disruptions in the commercial paper market. Money Pools Ameren has money pool agreements with and among its subsidiaries to coordinate and provide for certain short-term cash and working capital requirements. The average interest rate for borrowings under the money pool for the three months ended March 31, 2020 , was 1.93% ( 2019 – 2.87% ). See Note 8 – Related-party Transactions for the amount of interest income and expense from the money pool arrangements recorded by the Ameren Companies for the three months ended March 31, 2020 and 2019 .

Long-Term Debt And Equity Finan

Long-Term Debt And Equity Financings	3 Months Ended
	Mar. 31, 2020
Long-Term Debt And Equity Financings [Abstract]
LONG-TERM DEBT AND EQUITY FINANCINGS	LONG-TERM DEBT AND EQUITY FINANCINGS Ameren For the three months ended March 31, 2020 , Ameren issued a total of 0.2 million shares of common stock under its DRPlus and 401(k) plan, and received proceeds of $13 million . 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 March 31, 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 $552 million . The forward sale agreement could also have been settled at March 31, 2020 , with delivery of approximately $ 2 million or less than 0.1 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, 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 will be used to fund the repayment of Ameren’s 2.70% senior unsecured notes due November 2020. 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. Indenture Provisions and Other Covenants See Note 5 – Long-Term Debt and Equity Financings under Part II, Item 8, in the Form 10-K for a description of our indenture provisions and other covenants, as well as restrictions on the payment of dividends. At March 31, 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 March 31, 2020 , none of the Ameren Companies had any significant off-balance-sheet financing arrangements, other than the forward sale agreement relating to common stock, variable interest entities, letters of credit, and Ameren (parent) guarantee arrangements on behalf of its subsidiaries. See Note 1 – Summary of Significant Accounting Policies for further detail concerning variable interest entities.

Other Income and Expenses

Other Income and Expenses	3 Months Ended
	Mar. 31, 2020
Other Nonoperating Income (Expense) [Abstract]
OTHER INCOME AND EXPENSES	OTHER INCOME, NET The following table presents the components of “Other Income, Net” in the Ameren Companies’ statements of income for the three months ended March 31, 2020 and 2019 : Three Months 2020 2019 Ameren: Allowance for equity funds used during construction $ 4 $ 6 Interest income on industrial development revenue bonds 6 6 Other interest income 1 2 Non-service cost components of net periodic benefit income (a) 23 22 Miscellaneous income 2 2 Donations (13 ) (b) (6 ) Miscellaneous expense (2 ) (3 ) Total Other Income, Net $ 21 $ 29 Ameren Missouri: Allowance for equity funds used during construction $ 2 $ 4 Interest income on industrial development revenue bonds 6 6 Non-service cost components of net periodic benefit income (a) 5 4 Miscellaneous income 1 1 Donations (8 ) (b) (2 ) Miscellaneous expense (2 ) (1 ) Total Other Income, Net $ 4 $ 12 Ameren Illinois: Allowance for equity funds used during construction $ 2 $ 2 Interest income 1 2 Non-service cost components of net periodic benefit income 13 12 Miscellaneous income 1 1 Donations (4 ) (4 ) Miscellaneous expense (2 ) (2 ) Total Other Income, Net $ 11 $ 11 (a) For the three months ended March 31, 2020 and 2019 , the non-service cost components of net periodic benefit income were partially offset by a deferral of $6 million and $7 million , respectively, due to a regulatory tracking mechanism for the difference between the level of such costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. (b) Includes $8 million pursuant to Ameren Missouri’s March 2020 electric rate order. See Note 2 – Rate and Regulatory Matters for additional information.

Derivative Financial Instrument

Derivative Financial Instruments	3 Months Ended
	Mar. 31, 2020
Derivative Instruments and Hedging Activities Disclosure [Abstract]
DERIVATIVE FINANCIAL INSTRUMENTS	DERIVATIVE FINANCIAL INSTRUMENTS We use derivatives to manage the risk of changes in market prices for natural gas, 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 March 31, 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 March 31, 2020 , and December 31, 2019 . As of March 31, 2020 , these contracts extended through October 2023, March 2024, May 2032 and March 2023 for fuel oils, natural gas, power and uranium, respectively. Quantity (in millions, except as indicated) 2020 2019 Commodity Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils (in gallons) 62 — 62 58 — 58 Natural gas (in mmbtu) 20 147 167 20 136 156 Power (in megawatthours) 5 7 12 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 March 31, 2020 , and December 31, 2019 : March 31, 2020 December 31, 2019 Balance Sheet Location Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils Other current assets $ 3 $ — $ 3 $ 4 $ — $ 4 Other assets 1 — 1 2 — 2 Natural gas Other current assets — 2 2 — 3 3 Other assets — 2 2 — 1 1 Power Other current assets 18 — 18 14 — 14 Other assets 4 — 4 2 — 2 Total assets $ 26 $ 4 $ 30 $ 22 $ 4 $ 26 Fuel oils Other current liabilities $ 19 $ — $ 19 $ 4 $ — $ 4 Other deferred credits and liabilities 12 — 12 3 — 3 Natural gas Other current liabilities 3 17 20 1 12 13 Other deferred credits and liabilities — 5 5 1 6 7 Power Other current liabilities 2 18 20 2 17 19 Other deferred credits and liabilities 1 223 224 1 207 208 Uranium Other deferred credits and liabilities 1 — 1 1 — 1 Total liabilities $ 38 $ 263 $ 301 $ 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 March 31, 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 March 31, 2020 , if counterparty groups were to fail completely to perform on contracts, the Ameren Companies’ maximum exposure related to derivative assets would have been immaterial with or without consideration of the application of master netting arrangements or similar agreements and collateral held. Certain of our derivative instruments contain collateral provisions tied to the Ameren Companies’ credit ratings. If our credit ratings were downgraded below investment grade, or if a counterparty with reasonable grounds for uncertainty regarding our ability to satisfy an obligation requested adequate assurance of performance, additional collateral postings might be required. The additional collateral required is the net liability position allowed under master netting arrangements or similar agreements, assuming (1) the credit risk-related contingent features underlying these arrangements were triggered and (2) those counterparties with rights to do so requested collateral. As of March 31, 2020 , the aggregate fair value of derivative instruments with credit risk-related contingent features in a gross liability position, the cash collateral posted, and the aggregate amount of additional collateral that counterparties could require were each immaterial to Ameren, Ameren Missouri, and Ameren Illinois.

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 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 months ended March 31, 2020 or 2019 . At March 31, 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 March 31, 2020 , and December 31, 2019 : March 31, 2020 December 31, 2019 Level 1 Level 2 Level 3 Total Level 1 Level 2 Level 3 Total Assets: Ameren Missouri Derivative assets – commodity contracts: Fuel oils $ — $ — $ 4 $ 4 $ — $ — $ 6 $ 6 Power 4 — 18 22 — 2 14 16 Total derivative assets – commodity contracts $ 4 $ — $ 22 $ 26 $ — $ 2 $ 20 $ 22 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 457 $ — $ — $ 457 $ 569 $ — $ — $ 569 Debt securities: U.S. Treasury and agency securities — 105 — 105 — 107 — 107 Corporate bonds — 96 — 96 — 93 — 93 Other — 68 — 68 — 73 — 73 Total nuclear decommissioning trust fund $ 457 $ 269 $ — $ 726 (a) $ 569 $ 273 $ — $ 842 (a) Total Ameren Missouri $ 461 $ 269 $ 22 $ 752 $ 569 $ 275 $ 20 $ 864 Ameren Illinois Derivative assets – commodity contracts: Natural gas $ — $ 1 $ 3 $ 4 $ — $ 1 $ 3 $ 4 March 31, 2020 December 31, 2019 Level 1 Level 2 Level 3 Total Level 1 Level 2 Level 3 Total Ameren Derivative assets – commodity contracts (b) $ 4 $ 1 $ 25 $ 30 $ — $ 3 $ 23 $ 26 Nuclear decommissioning trust fund (c) 457 269 — 726 (a) 569 273 — 842 (a) Total Ameren $ 461 $ 270 $ 25 $ 756 $ 569 $ 276 $ 23 $ 868 Liabilities: Ameren Missouri Derivative liabilities – commodity contracts: Fuel oils $ 18 $ — $ 13 $ 31 $ 1 $ — $ 6 $ 7 Natural gas — 3 — 3 — 2 — 2 Power 2 — 1 3 — 2 1 3 Uranium — — 1 1 — — 1 1 Total Ameren Missouri $ 20 $ 3 $ 15 $ 38 $ 1 $ 4 $ 8 $ 13 Ameren Illinois Derivative liabilities – commodity contracts: Natural gas $ 2 $ 15 $ 5 $ 22 $ 3 $ 12 $ 3 $ 18 Power — — 241 241 — — 224 224 Total Ameren Illinois $ 2 $ 15 $ 246 $ 263 $ 3 $ 12 $ 227 $ 242 Ameren Derivative liabilities – commodity contracts (b) $ 22 $ 18 $ 261 $ 301 $ 4 $ 16 $ 235 $ 255 (a) Balance excludes $16 million and $5 million of cash and cash equivalents, receivables, payables, and accrued income, net, for March 31, 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 months ended March 31, 2020 and 2019 : 2020 2019 Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren For the three months ended March 31: Beginning balance at January 1 $ 13 $ (224 ) $ (211 ) $ — $ (183 ) $ (183 ) Realized and unrealized gains/(losses) included in regulatory assets/liabilities 11 (21 ) (10 ) — (4 ) (4 ) Settlements (7 ) 4 (3 ) — 3 3 Ending balance at March 31 $ 17 $ (241 ) $ (224 ) $ — $ (184 ) $ (184 ) Change in unrealized gains/(losses) related to assets/liabilities held at March 31 $ 10 $ (21 ) $ (11 ) $ — $ (4 ) $ (4 ) All gains or losses related to our Level 3 derivative commodity contracts are expected to be recovered or returned through customer rates; therefore, there is no impact to either net income or other comprehensive income resulting from changes in the fair value of these instruments. The following table describes the valuation techniques and significant unobservable inputs utilized for the fair value of our Level 3 power derivative contract assets and liabilities as of March 31, 2020 , and December 31, 2019 : Fair Value Weighted Average (b) Commodity Assets Liabilities Valuation Technique(s) Unobservable Input (a) Range 2020 Power (c) $ 18 $ (242) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) 16 – 33 25 Nodal basis ($/MWh) (6) – 0 (2) Trend rate (%) 2 – 3 2 2019 Power (d) $ 14 $ (225) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) 22 – 34 25 Nodal basis ($/MWh) (6) – 0 (2) Trend rate (%) (1) – 0 0 (a) Generally, significant increases (decreases) in these inputs in isolation would result in a significantly higher (lower) fair value measurement. (b) Unobservable inputs were weighted by relative fair value (c) Valuations through 2029 use visible forward prices adjusted for nodal-to-hub basis differentials. Valuations beyond 2029 use a trend rate factor and are similarly adjusted for nodal-to-hub basis differentials. (d) Valuations through 2028 use visible forward prices adjusted for nodal-to-hub basis differentials. Valuations beyond 2028 use a trend rate factor and are similarly adjusted for nodal-to-hub basis differentials. The following table sets forth, by level within the fair value hierarchy, the carrying amount and fair value of financial assets and liabilities disclosed, but not recorded, at fair value as of March 31, 2020 , and December 31, 2019 : March 31, 2020 Carrying Amount Fair Value Level 1 Level 2 Level 3 Total Ameren: Cash, cash equivalents, and restricted cash $ 203 $ 203 $ — $ — $ 203 Investments in industrial development revenue bonds (a) 263 — 263 — 263 Short-term debt 615 — 615 — 615 Long-term debt (including current portion) (a) 9,735 (b) — 10,189 449 (c) 10,638 Ameren Missouri: Cash, cash equivalents, and restricted cash $ 24 $ 24 $ — $ — $ 24 Investments in industrial development revenue bonds (a) 263 — 263 — 263 Short-term debt 130 — 130 — 130 Long-term debt (including current portion) (a) 4,567 (b) — 5,098 — 5,098 Ameren Illinois: Cash, cash equivalents, and restricted cash $ 140 $ 140 $ — $ — $ 140 Short-term debt 60 — 60 — 60 Long-term debt (including current portion) 3,575 (b) — 3,945 — 3,945 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 March 31, 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 $73 million , $32 million , and $34 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of March 31, 2020 . Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $72 million , $30 million , and $34 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2019 . (c) The Level 3 fair value amount consists of ATXI’s senior unsecured notes.

Related Party Transactions

Related Party Transactions	3 Months Ended
	Mar. 31, 2020
Related Party Transactions [Abstract]
RELATED PARTY TRANSACTIONS	RELATED-PARTY TRANSACTIONS In the normal course of business, Ameren Missouri and Ameren Illinois have engaged in, and may in the future engage in, affiliate transactions. These transactions primarily consist of natural gas and power purchases and sales, services received or rendered, and borrowings and lendings. Transactions between Ameren’s subsidiaries are reported as affiliate transactions on their individual financial statements, but those transactions are eliminated in consolidation for Ameren’s consolidated financial statements. For a discussion of material related-party agreements and money pool arrangements, see Note 13 – Related-party Transactions and Note 4 – Short-term Debt and Liquidity under Part II, Item 8, of the Form 10-K. Capacity Supply Agreement In April 2020, Ameren Illinois conducted a procurement event, administered by the IPA, to acquire capacity. Ameren Missouri was among the winning suppliers in this event. 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. Tax Allocation Agreement See Note 1 – Summary of Significant Accounting Policies under Part II, Item 8, of the Form 10-K for a discussion of the tax allocation agreement. The following table presents the affiliate balances related to income taxes for Ameren Missouri and Ameren Illinois as of March 31, 2020 , and December 31, 2019 : March 31, 2020 December 31, 2019 Ameren Missouri Ameren Illinois Ameren Missouri Ameren Illinois Income taxes payable to parent (a) $ 12 $ 48 $ 15 $ 43 Income taxes receivable from parent (b) 16 1 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 months ended March 31, 2020 and 2019 : Three Months Agreement Income Statement Line Item Ameren Ameren Ameren Missouri power supply Operating Revenues 2020 $ 3 $ (a) agreements with Ameren Illinois 2019 (b) (a) Ameren Missouri and Ameren Illinois Operating Revenues 2020 $ 7 $ 1 rent and facility services 2019 7 1 Ameren Missouri and Ameren Illinois Operating Revenues 2020 $ (b) $ (b) miscellaneous support services 2019 (b) (b) Total Operating Revenues 2020 $ 10 $ 1 2019 7 1 Ameren Illinois power supply Purchased Power 2020 $ (a) $ 3 agreements with Ameren Missouri 2019 (a) (b) Ameren Illinois transmission Purchased Power 2020 $ (a) $ (b) services with ATXI 2019 (a) (b) Total Purchased Power 2020 $ (a) $ 3 2019 (a) (b) Ameren Missouri and Ameren Illinois Other Operations and Maintenance 2020 $ (b) $ 1 rent and facility services 2019 (b) 1 Ameren Services support services Other Operations and Maintenance 2020 $ 35 $ 33 agreement 2019 32 30 Total Other Operations and 2020 $ 35 $ 34 Maintenance 2019 32 31 Money pool borrowings (advances) (Interest Charges)/Other Income, Net 2020 $ (b) $ (b) 2019 — — (a) Not applicable. (b) Amount less than $1 million.

Commitments And Contingencies

Commitments And Contingencies	3 Months Ended
	Mar. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	COMMITMENTS AND CONTINGENCIES We are involved in legal, tax, and regulatory proceedings before various courts, regulatory commissions, authorities, and governmental agencies with respect to matters that arise in the ordinary course of business, some of which involve substantial amounts of money. We believe that the final disposition of these proceedings, except as otherwise disclosed in the notes to our financial statements in this report and in the Form 10-K, will not have a material adverse effect on our results of operations, financial position, or liquidity. Reference is made to Note 1 – Summary of Significant Accounting Policies, Note 2 – Rate and Regulatory Matters, Note 13 – Related-party Transactions, and Note 14 – Commitments and Contingencies under Part II, Item 8, of the Form 10-K. See also Note 1 – Summary of Significant Accounting Policies, Note 2 – Rate and Regulatory Matters, Note 8 – Related-party Transactions, and Note 10 – Callaway Energy Center of this report. Other Obligations To supply a portion of the fuel requirements of Ameren Missouri’s energy centers, Ameren Missouri has entered into various long-term commitments for the procurement of coal, natural gas, nuclear fuel, and methane gas. Ameren Missouri and Ameren Illinois also have entered into various long-term commitments for purchased power and natural gas for distribution. The table below presents our estimated minimum fuel, purchased power, and other commitments at March 31, 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 March 31, 2020 . Coal Natural Gas (a) Nuclear Fuel Purchased Power (b)(c) Methane Gas Other Total Ameren: 2020 $ 242 $ 150 $ 35 $ 85 (d) $ 2 $ 55 $ 569 2021 219 129 57 51 3 40 499 2022 185 69 11 13 3 24 305 2023 105 39 44 3 3 24 218 2024 94 13 15 — 3 23 148 Thereafter 55 43 16 — 24 59 197 Total $ 900 $ 443 $ 178 $ 152 $ 38 $ 225 $ 1,936 Ameren Missouri: 2020 $ 242 $ 32 $ 35 $ — $ 2 $ 43 $ 354 2021 219 26 57 — 3 31 336 2022 185 15 11 — 3 23 237 2023 105 13 44 — 3 24 189 2024 94 6 15 — 3 23 141 Thereafter 55 19 16 — 24 26 140 Total $ 900 $ 111 $ 178 $ — $ 38 $ 170 $ 1,397 Ameren Illinois: 2020 $ — $ 118 $ — $ 85 (d) $ — $ 5 $ 208 2021 — 103 — 51 — 4 158 2022 — 54 — 13 — — 67 2023 — 26 — 3 — — 29 2024 — 7 — — — — 7 Thereafter — 24 — — — — 24 Total $ — $ 332 $ — $ 152 $ — $ 9 $ 493 (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 $15 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. The amounts above reflect Ameren Illinois’ commitment to acquire approximately $11 million of zero emission credits through May 2020. In April 2020, Ameren Illinois conducted procurement events, administered by the IPA, to purchase energy products and capacity through May 2023. In the April 2020 procurement events, Ameren Illinois contracted to purchase 3,550,800 megawatthours of energy products for $92 million from June 2020 through May 2023 and 617 megawatts of capacity for $4 million from June 2021 through May 2023. See Note 8 – Related-party Transactions for additional information regarding the capacity agreement between Ameren Missouri and Ameren Illinois as a result of the April 2020 capacity procurement event. Environmental Matters We are subject to various environmental laws, including statutes and regulations, enforced by federal, state, and local authorities. The development and operation of electric generation, transmission, and distribution facilities and natural gas storage, transmission, and distribution facilities can trigger compliance obligations with respect to environmental laws. These laws address emissions, discharges to water, water intake, impacts to air, land, and water, and chemical and waste handling. Complex and lengthy processes are required to obtain and renew approvals, permits, and licenses for new, existing or modified facilities. Additionally, the use and handling of various chemicals or hazardous materials require release prevention plans and emergency response procedures. The EPA has promulgated environmental regulations that have a significant impact on the electric utility industry. Over time, compliance with these regulations could be costly for Ameren Missouri, which operates coal-fired power plants. 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 SO 2 , particulate matter, NO x , mercury, toxic metals, and acid gases, and CO 2 emissions from new power plants. Water intake and discharges from power plants are regulated under the Clean Water Act. Such regulation could require modifications to water intake structures or more stringent limitations on wastewater discharges at Ameren Missouri’s energy centers, either of which could result in significant capital expenditures. The management and disposal of coal ash is regulated under the CCR rule, which will require the closure of surface impoundments and the installations of dry ash handling systems at several of Ameren Missouri’s energy centers. The individual or combined effects of existing environmental regulations could result in significant capital expenditures, increased operating costs, or the closure or alteration of operations at some of Ameren Missouri’s energy centers. Ameren and Ameren Missouri expect that such compliance costs would be recoverable through rates, subject to MoPSC prudence review, but the timing of costs and their recovery could be subject to regulatory lag. Ameren and Ameren Missouri estimate that they will need to make capital expenditures of $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 expenditures required by the CCR regulations, by the Clean Water Act rule applicable to cooling water intake structures at existing power plants, and by effluent limitation guidelines applicable to steam electric generating units, all of which are discussed below. 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 whether the EPA will substantially revise regulatory obligations, exactly which compliance strategies will be used and their ultimate cost, among other things. The following sections describe the more significant environmental laws and rules and environmental enforcement and remediation matters that affect or could affect our operations. The EPA has initiated an administrative review of several regulations and proposed amendments to regulations and guidelines, including to the effluent limitation guidelines and the CCR Rule, which could ultimately result in the revision of all or part of such rules. Clean Air Act Federal and state laws, including CSAPR, regulate emissions of SO 2 and NO x through the reduction of emissions at their source and the use and retirement of emission allowances. The first phase of the CSAPR emission reduction requirements became effective in 2015. The second phase of emission reduction requirements, which were revised by the EPA in 2016, became effective in 2017; additional emission reduction requirements may apply in subsequent years. To achieve compliance with the CSAPR, Ameren Missouri burns low-sulfur coal, operates two scrubbers at its Sioux Energy Center, and optimizes other existing air pollution control equipment. Ameren Missouri expects to incur additional costs to lower its emissions at one or more of its energy centers to comply with the CSAPR in future years. These higher costs are expected to be recovered from customers through the FAC or higher base rates. CO 2 Emissions Standards In September 2019, the EPA’s Affordable Clean Energy Rule, which establishes emission guidelines for states to follow in developing plans to limit CO 2 emissions from coal-fired electric generating units, became effective. 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 are evaluating the impact of the adoption and implementation of the Affordable Clean Energy Rule and, along with other stakeholders, will be working with the state of Missouri to develop the compliance plan submitted to the EPA. At this time, we 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. Additionally, in October 2019, following a request by Ameren Missouri, the district court stayed implementation of the majority of its order’s requirements while the case is appealed. Ameren Missouri believes the district court both misinterpreted and misapplied the law in its ruling. We are unable to predict the ultimate resolution of this matter. Ameren Missouri expects to file its brief in the appeal in late May 2020. Based on anticipated scheduling, the court is expected to hear oral arguments in 2020; however, it 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 In July 2018, the United States Court of Appeals for the Second Circuit upheld the EPA’s Section 316(b) Rule applicable to cooling water intake structures at existing power plants. The rule requires a case-by-case evaluation and plan for reducing 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 by Ameren Missouri during the permit renewal process of each energy center’s water discharge permit, which is expected to be completed by 2023. In 2015, the EPA issued a rule to revise the effluent limitation guidelines applicable to steam electric generating units. These guidelines established national standards for water discharges that are based on the effectiveness of available control technology. The EPA’s 2015 rule prohibits effluent discharges of certain waste streams and imposes more stringent limitations on certain water discharges from power plants. In September 2017, the EPA published a rule that postponed the compliance dates by two years for the limitations applicable to two specific waste streams so that it could potentially revise those standards. To meet the requirements of the guidelines, Ameren Missouri is constructing wastewater treatment facilities and dry ash handling systems at three of its energy centers and is scheduled to complete the projects in 2020. 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 its surface impoundments, with the last of such closures scheduled for 2023. The EPA issued revisions to the CCR rule in July 2018, proposed additional revisions in July and November 2019, and indicated that additional revisions to the CCR rule are likely. Ameren and Ameren Missouri have AROs of $137 million recorded on their respective balance sheets as of March 31, 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 March 31, 2020 , Ameren Illinois has remediated the majority of the 44 former MGP sites in Illinois it owned or for which it was otherwise responsible. Ameren Illinois estimates it could substantially conclude remediation efforts at the remaining sites by 2023. The ICC allows Ameren Illinois to recover such remediation and related litigation costs from its electric and natural gas utility customers through environmental cost riders. Costs are subject to annual prudence review by the ICC. As of March 31, 2020 , Ameren Illinois estimated the remaining obligation related to these former MGP sites at $126 million to $213 million . Ameren and Ameren Illinois recorded a liability of $126 million to represent the estimated minimum obligation for these sites, as no other amount within the range was a better estimate. The scope of the remediation activities at these former MGP sites may increase as remediation efforts continue. Considerable uncertainty remains in these estimates because many site-specific factors can influence the ultimate actual costs, including unanticipated underground structures, the degree to which groundwater is encountered, regulatory changes, local ordinances, and site accessibility. The actual costs and timing of completion may vary substantially from these estimates. Our operations or those of our predecessor companies involve the use of, disposal of, and, in appropriate circumstances, the cleanup of substances regulated under environmental laws. We are unable to determine whether such practices will result in future environmental commitments or will affect our results of operations, financial position, or liquidity.

Callaway Energy Center

Callaway Energy Center	3 Months Ended
	Mar. 31, 2020
Nuclear Waste Matters [Abstract]
CALLAWAY ENERGY CENTER	CALLAWAY ENERGY CENTER See Note 9 – Callaway Energy Center under Part II, Item 8, of the Form 10-K for information regarding spent nuclear fuel recovery, recovery of decommissioning costs, and the nuclear decommissioning trust fund. The fair value of the trust fund for Ameren Missouri’s Callaway Energy Center is reported as “Nuclear decommissioning trust fund” in Ameren’s and Ameren Missouri’s balance sheets. This amount is legally restricted and may be used only to fund the costs of nuclear decommissioning. Changes in the fair value of the trust fund are recorded as an increase or decrease to the nuclear decommissioning trust fund, with an offsetting adjustment to the related regulatory liability. Insurance The following table presents insurance coverage at Ameren Missouri’s Callaway Energy Center at April 1, 2020 : Type and Source of Coverage Most Recent 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

Retirement Benefits	3 Months Ended
	Mar. 31, 2020
Defined Benefit Plan [Abstract]
RETIREMENT BENEFITS	RETIREMENT BENEFITS The following table presents the components of the net periodic benefit cost (income) incurred for Ameren’s pension and postretirement benefit plans for the three months ended March 31, 2020 and 2019 : Pension Benefits Postretirement Benefits Three Months Three Months 2020 2019 2020 2019 Service cost (a) $ 27 $ 22 $ 4 $ 4 Non-service cost components: Interest cost 43 47 10 11 Expected return on plan assets (73 ) (69 ) (20 ) (19 ) Amortization of: Prior service benefit — — (1 ) (1 ) Actuarial loss (gain) 14 6 (2 ) (4 ) Total non-service cost components (b) $ (16 ) $ (16 ) $ (13 ) $ (13 ) Net periodic benefit cost (income) $ 11 $ 6 $ (9 ) $ (9 ) (a) Service cost, net of capitalization, is reflected in “Operating Expenses – Other operations and maintenance” on Ameren’s statement of income. (b) Non-service cost components are reflected in “Other Income, Net” on Ameren’s statement of income. See Note 5 – Other Income, Net for additional information. Ameren Missouri and Ameren Illinois are responsible for their respective shares of Ameren’s pension and postretirement costs. The following table presents the respective share of net periodic pension and other postretirement benefit costs (income) incurred for the three months ended March 31, 2020 and 2019 : Pension Benefits Postretirement Benefits Three Months Three Months 2020 2019 2020 2019 Ameren Missouri (a) $ 4 $ 1 $ (1 ) $ (2 ) Ameren Illinois 7 5 (8 ) (7 ) Ameren (a) $ 11 $ 6 $ (9 ) $ (9 ) (a) Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. Funding Ameren expects to make annual contributions of approximately $5 million to $45 million in each year through 2024, with aggregate estimated contributions of $115 million , based on its assumptions at March 31, 2020 , its investment performance in 2020, and its pension funding policy. This is an increase from the aggregate estimated contributions of $70 million at December 31, 2019 , due to year-to-date performance of Ameren’s pension and other postretirement benefit plan assets in 2020.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 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 months ended March 31, 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 excess deferred taxes (9) (7) (15) (12) (3) (3) Depreciation differences (1) — — — (1) — Amortization of deferred investment tax credit — (1) (1) (1) — — State tax 6 6 3 4 7 7 Stock-based compensation (5) (7) — — — — Other permanent items — — — (3) — — Effective income tax rate 12% 12% 8% 9% 24% 25%

Supplemental Information

Supplemental Information	3 Months Ended
	Mar. 31, 2020
Supplemental Information [Abstract]
Supplemental Information	SUPPLEMENTAL INFORMATION Cash, Cash Equivalents, and Restricted Cash The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets and the statements of cash flows as of March 31, 2020 , and December 31, 2019 : March 31, 2020 December 31, 2019 Ameren Ameren Ameren Ameren Ameren Ameren Cash and cash equivalents $ 42 $ 3 $ 6 $ 16 $ 9 $ — Restricted cash included in “Other current assets” 16 4 6 14 4 5 Restricted cash included in “Other assets” 128 — 128 120 — 120 Restricted cash included in “Nuclear decommissioning trust fund” 17 17 — 26 26 — Total cash, cash equivalents, and restricted cash $ 203 $ 24 $ 140 $ 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 March 31, 2020 , and December 31, 2019 , “Other current liabilities” on Ameren’s and Ameren Illinois’ balance sheets included payables for purchased receivables of $33 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 months ended March 31, 2020 and 2019 : 2020 2019 Ameren Ameren Ameren (a) Ameren Ameren Ameren (a) Balance at January 1 $ 17 $ 7 $ 10 $ 18 $ 7 $ 11 Bad debt expense 3 2 1 3 1 2 Net write-offs (1 ) (1 ) — (2 ) (1 ) (1 ) Balance at March 31 $ 19 $ 8 $ 11 $ 19 $ 7 $ 12 (a) Ameren Illinois has a rate-adjustment mechanism that allows it to recover the difference between its actual net bad debt write-offs under GAAP and the amount of net bad debt write-offs included in its base rates. Supplemental Cash Flow Information The following table provides noncash financing and investing activity excluded from the statements of cash flows for the three months ended March 31, 2020 and 2019 : March 31, 2020 March 31, 2019 Ameren Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Investing Accrued capital expenditures $ 235 $ 97 $ 127 $ 208 $ 92 $ 106 Accrued nuclear fuel expenditures 7 7 — — — — Net realized and unrealized gain (loss) – nuclear decommissioning trust fund (111 ) (111 ) — 64 64 — Financing 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 three months ended March 31, 2020 : Ameren Missouri Ameren Illinois Ameren Balance at December 31, 2019 $ 687 (a) $ 4 (b) $ 691 (a) Liabilities settled (14 ) — (14 ) Accretion 7 (c) — 7 (c) Balance at March 31, 2020 $ 680 (a) $ 4 (b) $ 684 (a) (a) Balance included $53 million in “Other current liabilities” on the balance sheet as of both December 31, 2019 , and March 31, 2020 . (b) Included in “Other deferred credits and liabilities” on the balance sheet. (c) Accretion expense attributable to Ameren Missouri was recorded as a decrease to regulatory liabilities. Stock-based Compensation 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. For the three months ended March 31, 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 March 31, 2020 , and December 31, 2019 , “Other current liabilities” and “Other deferred credits and liabilities” on Ameren’s balance sheet included deferred compensation obligations of $85 million and $86 million , respectively, recorded at the present value of future benefits to be paid. Operating Revenues As of March 31, 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 months ended March 31, 2020 and 2019 : Three Months 2020 2019 Ameren Missouri $ 30 $ 31 Ameren Illinois 35 39 Ameren $ 65 $ 70 Earnings per Share The following table reconciles the basic weighted-average number of common shares outstanding to the diluted weighted-average number of common shares outstanding for the three months ended March 31, 2020 and 2019 : Three Months 2020 2019 Weighted-average Common Shares Outstanding – Basic 246.4 244.9 Assumed settlement of performance share units and restricted stock units 1.1 1.5 Dilutive effect of forward sale agreement 0.6 — Weighted-average Common Shares Outstanding – Diluted (a) 248.1 246.4 (a) There were no potentially dilutive securities excluded from the earnings per diluted share calculations for the three months ended March 31, 2020 and 2019 .

Segment Information

Segment Information	3 Months Ended
	Mar. 31, 2020
Segment Reporting [Abstract]
SEGMENT INFORMATION	SEGMENT INFORMATION The following tables present revenues, net income (loss) attributable to common shareholders, and capital expenditures by segment at Ameren and Ameren Illinois for the three months ended March 31, 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 $ 670 $ 389 $ 271 $ 110 $ — $ — $ 1,440 Intersegment revenues 10 1 — 13 — (24 ) — Net income (loss) attributable to Ameren common shareholders (10 ) 37 55 47 (a) 17 — 146 Capital expenditures 278 123 61 170 3 1 636 Three Months 2019: External revenues $ 751 $ 386 $ 320 $ 99 $ — $ — $ 1,556 Intersegment revenues 7 1 — 15 — (23 ) — Net income attributable to Ameren common shareholders 39 36 57 44 (a) 15 — 191 Capital expenditures 240 124 51 121 10 (2 ) 544 (a) Ameren Transmission earnings reflect an allocation of financing costs from Ameren (parent). Ameren Illinois Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Ameren Illinois Three Months 2020: External revenues $ 390 $ 271 $ 62 $ — $ 723 Intersegment revenues — — 12 (a) (12 ) — Net income available to common shareholder 37 55 28 — 120 Capital expenditures 123 61 140 — 324 Three Months 2019: External revenues $ 387 $ 320 $ 55 $ — $ 762 Intersegment revenues — — 15 (a) (15 ) — Net income available to common shareholder 36 57 27 — 120 Capital expenditures 124 51 92 — 267 (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 months ended March 31, 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 $ 297 $ 220 $ — $ — $ — $ 517 Commercial 221 126 — — — 347 Industrial 53 35 — — — 88 Other 60 9 — 123 (24 ) 168 Total electric revenues $ 631 $ 390 $ — $ 123 $ (24 ) $ 1,120 Residential $ 33 $ — $ 213 $ — $ — $ 246 Commercial 13 — 54 — — 67 Industrial 1 — 3 — — 4 Other 2 — 1 — — 3 Total natural gas revenues $ 49 $ — $ 271 $ — $ — $ 320 Total revenues (a) $ 680 $ 390 $ 271 $ 123 $ (24 ) $ 1,440 Three Months 2019: Residential $ 312 $ 217 $ — $ — $ — $ 529 Commercial 239 123 — — — 362 Industrial 55 34 — — — 89 Other 98 13 — 114 (23 ) 202 Total electric revenues $ 704 $ 387 $ — $ 114 $ (23 ) $ 1,182 Residential $ 38 $ — $ 246 $ — $ — $ 284 Commercial 16 — 65 — — 81 Industrial 2 — 4 — — 6 Other (2 ) — 5 — — 3 Total natural gas revenues $ 54 $ — $ 320 $ — $ — $ 374 Total revenues (a) $ 758 $ 387 $ 320 $ 114 $ (23 ) $ 1,556 (a) The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the three months ended March 31, 2020 and 2019 : Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Ameren Three Months 2020: Revenues from alternative revenue programs $ (3 ) $ 46 $ 11 $ 12 $ 66 Other revenues not from contracts with customers 8 1 1 — 10 Three Months 2019: Revenues from alternative revenue programs $ 15 $ 22 $ (3 ) $ (5 ) $ 29 Other revenues not from contracts with customers 5 3 1 — 9 Ameren Illinois Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Ameren Illinois Three Months 2020: Residential $ 220 $ 213 $ — $ — $ 433 Commercial 126 54 — — 180 Industrial 35 3 — — 38 Other 9 1 74 (12 ) 72 Total revenues (a) $ 390 $ 271 $ 74 $ (12 ) $ 723 Three Months 2019: Residential $ 217 $ 246 $ — $ — $ 463 Commercial 123 65 — — 188 Industrial 34 4 — — 38 Other 13 5 70 (15 ) 73 Total revenues (a) $ 387 $ 320 $ 70 $ (15 ) $ 762 (a) The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the Ameren Illinois segments for the three months ended March 31, 2020 and 2019 : Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Ameren Illinois Three Months 2020: Revenues from alternative revenue programs $ 46 $ 11 $ 10 $ 67 Other revenues not from contracts with customers 1 1 — 2 Three Months 2019: Revenues from alternative revenue programs $ 22 $ (3 ) $ (5 ) $ 14 Other revenues not from contracts with customers 3 1 — 4

Summary Of Significant Accoun_2

Summary Of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 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. Ameren Missouri and Ameren Illinois have no subsidiaries. All tabular dollar amounts are in millions, unless otherwise indicated. Our accounting policies conform to GAAP. Our financial statements reflect all adjustments (which include normal, recurring adjustments) that are necessary, in our opinion, for a fair statement of our results. The preparation of financial statements in conformity with GAAP requires management to make certain estimates and assumptions. Such estimates and assumptions affect reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the dates of financial statements, and the reported amounts of revenues and expenses during the reported periods. Actual results could differ from those estimates. The results of operations of an interim period may not give a true indication of results that may be expected for a full year. These financial statements should be read in conjunction with the financial statements and accompanying notes included in the Form 10-K.
Consolidation, Variable Interest Entity, Policy	Variable Interest Entities As of March 31, 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 March 31, 2020 , the maximum exposure to loss related to these variable interest entities was approximately $15 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 March 31, 2020 , and December 31, 2019 , Ameren had unconsolidated variable interests as a limited partner in various equity method investments, totaling $31 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 March 31, 2020 , the maximum exposure to loss related to these variable interests is limited to the investment in these partnerships of $31 million plus associated outstanding funding commitments of $34 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 March 31, 2020 , the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $245 million ( December 31, 2019 – $264 million ) and $118 million ( December 31, 2019 – $123 million ), respectively, while total borrowings against the policies were $109 million ( December 31, 2019 – $114 million ) at both Ameren and Ameren Illinois. Ameren and Ameren Illinois have the right to offset the borrowings against the cash surrender value of the policies and, consequently, present the net asset in “Other assets” on their respective balance sheets. The cash surrender value decreased during the three months ended March 31, 2020 , primarily because of a decrease in the market value of underlying investments.
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 recently issued authoritative accounting guidance relating to defined benefit plan disclosures. 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.
Deferred Compensation	Deferred Compensation As of March 31, 2020 , and December 31, 2019 , “Other current liabilities” and “Other deferred credits and liabilities” on Ameren’s balance sheet included deferred compensation obligations of $85 million and $86 million , respectively, recorded at the present value of future benefits to be paid.
Revenue from Contract with Customer	Operating Revenues As of March 31, 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 Taxes

Short-Term Debt and Liquidity (

Short-Term Debt and Liquidity (Tables)	3 Months Ended
	Mar. 31, 2020
Debt Disclosure [Abstract]
Schedule of Short-term Debt	The following table presents the credit facility borrowings and commercial paper outstanding, net of issuance discounts, as of March 31, 2020 , and December 31, 2019 : March 31, 2020 December 31, 2019 Credit Facility Borrowings Commercial Paper Total Short-term Debt Credit Facility Borrowings Commercial Paper Total Short-term Debt Ameren (parent) $ 275 $ 150 $ 425 $ — $ 153 $ 153 Ameren Missouri 130 — 130 — 234 234 Ameren Illinois 60 — 60 — 53 53 Ameren consolidated $ 465 $ 150 $ 615 $ — $ 440 $ 440 The following table summarizes the borrowing activity and relevant interest rates under the Credit Agreements for the three months ended March 31, 2020 . There were no borrowings under the Credit Agreements for the three months ended March 31, 2019 . Ameren (parent) Ameren Missouri Ameren Illinois Total Missouri Credit Agreement Average daily credit facility borrowings outstanding during the period $ 1 $ 12 $ — $ 13 Weighted-average interest rate 2.06 % 2.36 % — % 2.33 % Peak credit facility borrowings during the period (a) $ 100 $ 130 $ — $ 230 Peak interest rate 2.06 % 3.33 % — % 3.33 % Illinois Credit Agreement Average daily credit facility borrowings outstanding during the period $ 2 $ — $ 5 $ 7 Weighted-average interest rate 2.06 % — % 2.05 % 2.06 % Peak credit facility borrowings during the period (a) $ 175 $ — $ 60 $ 235 Peak interest rate 2.06 % — % 2.05 % 2.06 % (a) The timing of peak credit facility borrowings varies by company. Therefore, the sum of individual company peak amounts may not equal the Ameren consolidated peak credit facility borrowings for the period. Commercial Paper The following table summarizes the borrowing activity and relevant interest rates under Ameren (parent)’s, Ameren Missouri’s, and Ameren Illinois’ commercial paper programs for the three months ended March 31, 2020 and 2019: Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2020 Average daily commercial paper outstanding at par value $ 154 $ 383 $ 71 $ 608 Weighted-average interest rate 1.94 % 1.84 % 1.99 % 1.88 % Peak commercial paper during period at par value (a) $ 225 $ 521 $ 137 $ 854 Peak interest rate 3.30 % 5.05 % (b) 3.40 % 5.05 % (b) 2019 Average daily commercial paper outstanding at par value $ 480 $ 246 $ 89 $ 815 Weighted-average interest rate 2.87 % 2.84 % 2.76 % 2.85 % Peak commercial paper during period at par value (a) $ 618 $ 549 $ 130 $ 1,113 Peak interest rate 3.10 % 2.97 % 2.90 % 3.10 % (a) The timing of peak outstanding commercial paper issuances varies by company. Therefore, the sum of individual company peak amounts may not equal the Ameren consolidated peak commercial paper issuances for the period.

Other Income and Expenses (Tabl

Other Income and Expenses (Tables)	3 Months Ended
	Mar. 31, 2020
Other Nonoperating Income (Expense) [Abstract]
Other Income And Expenses	The following table presents the components of “Other Income, Net” in the Ameren Companies’ statements of income for the three months ended March 31, 2020 and 2019 : Three Months 2020 2019 Ameren: Allowance for equity funds used during construction $ 4 $ 6 Interest income on industrial development revenue bonds 6 6 Other interest income 1 2 Non-service cost components of net periodic benefit income (a) 23 22 Miscellaneous income 2 2 Donations (13 ) (b) (6 ) Miscellaneous expense (2 ) (3 ) Total Other Income, Net $ 21 $ 29 Ameren Missouri: Allowance for equity funds used during construction $ 2 $ 4 Interest income on industrial development revenue bonds 6 6 Non-service cost components of net periodic benefit income (a) 5 4 Miscellaneous income 1 1 Donations (8 ) (b) (2 ) Miscellaneous expense (2 ) (1 ) Total Other Income, Net $ 4 $ 12 Ameren Illinois: Allowance for equity funds used during construction $ 2 $ 2 Interest income 1 2 Non-service cost components of net periodic benefit income 13 12 Miscellaneous income 1 1 Donations (4 ) (4 ) Miscellaneous expense (2 ) (2 ) Total Other Income, Net $ 11 $ 11 (a) For the three months ended March 31, 2020 and 2019 , the non-service cost components of net periodic benefit income were partially offset by a deferral of $6 million and $7 million , respectively, due to a regulatory tracking mechanism for the difference between the level of such costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. (b) Includes $8 million pursuant to Ameren Missouri’s March 2020 electric rate order. See Note 2 – Rate and Regulatory Matters for additional information.

Derivative Financial Instrume_2

Derivative Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 2020 , and December 31, 2019 . As of March 31, 2020 , these contracts extended through October 2023, March 2024, May 2032 and March 2023 for fuel oils, natural gas, power and uranium, respectively. Quantity (in millions, except as indicated) 2020 2019 Commodity Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils (in gallons) 62 — 62 58 — 58 Natural gas (in mmbtu) 20 147 167 20 136 156 Power (in megawatthours) 5 7 12 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 March 31, 2020 , and December 31, 2019 : March 31, 2020 December 31, 2019 Balance Sheet Location Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils Other current assets $ 3 $ — $ 3 $ 4 $ — $ 4 Other assets 1 — 1 2 — 2 Natural gas Other current assets — 2 2 — 3 3 Other assets — 2 2 — 1 1 Power Other current assets 18 — 18 14 — 14 Other assets 4 — 4 2 — 2 Total assets $ 26 $ 4 $ 30 $ 22 $ 4 $ 26 Fuel oils Other current liabilities $ 19 $ — $ 19 $ 4 $ — $ 4 Other deferred credits and liabilities 12 — 12 3 — 3 Natural gas Other current liabilities 3 17 20 1 12 13 Other deferred credits and liabilities — 5 5 1 6 7 Power Other current liabilities 2 18 20 2 17 19 Other deferred credits and liabilities 1 223 224 1 207 208 Uranium Other deferred credits and liabilities 1 — 1 1 — 1 Total liabilities $ 38 $ 263 $ 301 $ 13 $ 242 $ 255

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2020
Fair Value Disclosures [Abstract]
Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis	The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of March 31, 2020 , and December 31, 2019 : March 31, 2020 December 31, 2019 Level 1 Level 2 Level 3 Total Level 1 Level 2 Level 3 Total Assets: Ameren Missouri Derivative assets – commodity contracts: Fuel oils $ — $ — $ 4 $ 4 $ — $ — $ 6 $ 6 Power 4 — 18 22 — 2 14 16 Total derivative assets – commodity contracts $ 4 $ — $ 22 $ 26 $ — $ 2 $ 20 $ 22 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 457 $ — $ — $ 457 $ 569 $ — $ — $ 569 Debt securities: U.S. Treasury and agency securities — 105 — 105 — 107 — 107 Corporate bonds — 96 — 96 — 93 — 93 Other — 68 — 68 — 73 — 73 Total nuclear decommissioning trust fund $ 457 $ 269 $ — $ 726 (a) $ 569 $ 273 $ — $ 842 (a) Total Ameren Missouri $ 461 $ 269 $ 22 $ 752 $ 569 $ 275 $ 20 $ 864 Ameren Illinois Derivative assets – commodity contracts: Natural gas $ — $ 1 $ 3 $ 4 $ — $ 1 $ 3 $ 4 March 31, 2020 December 31, 2019 Level 1 Level 2 Level 3 Total Level 1 Level 2 Level 3 Total Ameren Derivative assets – commodity contracts (b) $ 4 $ 1 $ 25 $ 30 $ — $ 3 $ 23 $ 26 Nuclear decommissioning trust fund (c) 457 269 — 726 (a) 569 273 — 842 (a) Total Ameren $ 461 $ 270 $ 25 $ 756 $ 569 $ 276 $ 23 $ 868 Liabilities: Ameren Missouri Derivative liabilities – commodity contracts: Fuel oils $ 18 $ — $ 13 $ 31 $ 1 $ — $ 6 $ 7 Natural gas — 3 — 3 — 2 — 2 Power 2 — 1 3 — 2 1 3 Uranium — — 1 1 — — 1 1 Total Ameren Missouri $ 20 $ 3 $ 15 $ 38 $ 1 $ 4 $ 8 $ 13 Ameren Illinois Derivative liabilities – commodity contracts: Natural gas $ 2 $ 15 $ 5 $ 22 $ 3 $ 12 $ 3 $ 18 Power — — 241 241 — — 224 224 Total Ameren Illinois $ 2 $ 15 $ 246 $ 263 $ 3 $ 12 $ 227 $ 242 Ameren Derivative liabilities – commodity contracts (b) $ 22 $ 18 $ 261 $ 301 $ 4 $ 16 $ 235 $ 255 (a) Balance excludes $16 million and $5 million of cash and cash equivalents, receivables, payables, and accrued income, net, for March 31, 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 months ended March 31, 2020 and 2019 : 2020 2019 Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren For the three months ended March 31: Beginning balance at January 1 $ 13 $ (224 ) $ (211 ) $ — $ (183 ) $ (183 ) Realized and unrealized gains/(losses) included in regulatory assets/liabilities 11 (21 ) (10 ) — (4 ) (4 ) Settlements (7 ) 4 (3 ) — 3 3 Ending balance at March 31 $ 17 $ (241 ) $ (224 ) $ — $ (184 ) $ (184 ) Change in unrealized gains/(losses) related to assets/liabilities held at March 31 $ 10 $ (21 ) $ (11 ) $ — $ (4 ) $ (4 )
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis, Valuation Techniques	The following table describes the valuation techniques and significant unobservable inputs utilized for the fair value of our Level 3 power derivative contract assets and liabilities as of March 31, 2020 , and December 31, 2019 : Fair Value Weighted Average (b) Commodity Assets Liabilities Valuation Technique(s) Unobservable Input (a) Range 2020 Power (c) $ 18 $ (242) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) 16 – 33 25 Nodal basis ($/MWh) (6) – 0 (2) Trend rate (%) 2 – 3 2 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, by level within the fair value hierarchy, the carrying amount and fair value of financial assets and liabilities disclosed, but not recorded, at fair value as of March 31, 2020 , and December 31, 2019 : March 31, 2020 Carrying Amount Fair Value Level 1 Level 2 Level 3 Total Ameren: Cash, cash equivalents, and restricted cash $ 203 $ 203 $ — $ — $ 203 Investments in industrial development revenue bonds (a) 263 — 263 — 263 Short-term debt 615 — 615 — 615 Long-term debt (including current portion) (a) 9,735 (b) — 10,189 449 (c) 10,638 Ameren Missouri: Cash, cash equivalents, and restricted cash $ 24 $ 24 $ — $ — $ 24 Investments in industrial development revenue bonds (a) 263 — 263 — 263 Short-term debt 130 — 130 — 130 Long-term debt (including current portion) (a) 4,567 (b) — 5,098 — 5,098 Ameren Illinois: Cash, cash equivalents, and restricted cash $ 140 $ 140 $ — $ — $ 140 Short-term debt 60 — 60 — 60 Long-term debt (including current portion) 3,575 (b) — 3,945 — 3,945 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 March 31, 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 $73 million , $32 million , and $34 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of March 31, 2020 . Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $72 million , $30 million , and $34 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2019 . (c) The Level 3 fair value amount consists of ATXI’s senior unsecured notes.

Related Party Transactions (Tab

Related Party Transactions (Tables)	3 Months Ended
	Mar. 31, 2020
Related Party Transactions [Abstract]
Schedule of Affiliate Receivables and Payables	The following table presents the affiliate balances related to income taxes for Ameren Missouri and Ameren Illinois as of March 31, 2020 , and December 31, 2019 : March 31, 2020 December 31, 2019 Ameren Missouri Ameren Illinois Ameren Missouri Ameren Illinois Income taxes payable to parent (a) $ 12 $ 48 $ 15 $ 43 Income taxes receivable from parent (b) 16 1 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 months ended March 31, 2020 and 2019 : Three Months Agreement Income Statement Line Item Ameren Ameren Ameren Missouri power supply Operating Revenues 2020 $ 3 $ (a) agreements with Ameren Illinois 2019 (b) (a) Ameren Missouri and Ameren Illinois Operating Revenues 2020 $ 7 $ 1 rent and facility services 2019 7 1 Ameren Missouri and Ameren Illinois Operating Revenues 2020 $ (b) $ (b) miscellaneous support services 2019 (b) (b) Total Operating Revenues 2020 $ 10 $ 1 2019 7 1 Ameren Illinois power supply Purchased Power 2020 $ (a) $ 3 agreements with Ameren Missouri 2019 (a) (b) Ameren Illinois transmission Purchased Power 2020 $ (a) $ (b) services with ATXI 2019 (a) (b) Total Purchased Power 2020 $ (a) $ 3 2019 (a) (b) Ameren Missouri and Ameren Illinois Other Operations and Maintenance 2020 $ (b) $ 1 rent and facility services 2019 (b) 1 Ameren Services support services Other Operations and Maintenance 2020 $ 35 $ 33 agreement 2019 32 30 Total Other Operations and 2020 $ 35 $ 34 Maintenance 2019 32 31 Money pool borrowings (advances) (Interest Charges)/Other Income, Net 2020 $ (b) $ (b) 2019 — — (a) Not applicable. (b) Amount less than $1 million.

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Long-term Purchase Commitment	The table below presents our estimated minimum fuel, purchased power, and other commitments at March 31, 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 March 31, 2020 . Coal Natural Gas (a) Nuclear Fuel Purchased Power (b)(c) Methane Gas Other Total Ameren: 2020 $ 242 $ 150 $ 35 $ 85 (d) $ 2 $ 55 $ 569 2021 219 129 57 51 3 40 499 2022 185 69 11 13 3 24 305 2023 105 39 44 3 3 24 218 2024 94 13 15 — 3 23 148 Thereafter 55 43 16 — 24 59 197 Total $ 900 $ 443 $ 178 $ 152 $ 38 $ 225 $ 1,936 Ameren Missouri: 2020 $ 242 $ 32 $ 35 $ — $ 2 $ 43 $ 354 2021 219 26 57 — 3 31 336 2022 185 15 11 — 3 23 237 2023 105 13 44 — 3 24 189 2024 94 6 15 — 3 23 141 Thereafter 55 19 16 — 24 26 140 Total $ 900 $ 111 $ 178 $ — $ 38 $ 170 $ 1,397 Ameren Illinois: 2020 $ — $ 118 $ — $ 85 (d) $ — $ 5 $ 208 2021 — 103 — 51 — 4 158 2022 — 54 — 13 — — 67 2023 — 26 — 3 — — 29 2024 — 7 — — — — 7 Thereafter — 24 — — — — 24 Total $ — $ 332 $ — $ 152 $ — $ 9 $ 493 (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 $15 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. The amounts above reflect Ameren Illinois’ commitment to acquire approximately $11 million of zero emission credits through May 2020. In April 2020, Ameren Illinois conducted procurement events, administered by the IPA, to purchase energy products and capacity through May 2023. In the April 2020 procurement events, Ameren Illinois contracted to purchase 3,550,800 megawatthours of energy products for $92 million from June 2020 through May 2023 and 617 megawatts of capacity for $4 million from June 2021 through May 2023. See Note 8 – Related-party Transactions for additional information regarding the capacity agreement between Ameren Missouri and Ameren Illinois as a result of the April 2020 capacity procurement event.

Callaway Energy Center (Tables)

Callaway Energy Center (Tables)	3 Months Ended
	Mar. 31, 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 April 1, 2020 : Type and Source of Coverage Most Recent 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)

Retirement Benefits (Tables)	3 Months Ended
	Mar. 31, 2020
Defined Benefit Plan [Abstract]
Components Of Net Periodic Benefit Cost	The following table presents the components of the net periodic benefit cost (income) incurred for Ameren’s pension and postretirement benefit plans for the three months ended March 31, 2020 and 2019 : Pension Benefits Postretirement Benefits Three Months Three Months 2020 2019 2020 2019 Service cost (a) $ 27 $ 22 $ 4 $ 4 Non-service cost components: Interest cost 43 47 10 11 Expected return on plan assets (73 ) (69 ) (20 ) (19 ) Amortization of: Prior service benefit — — (1 ) (1 ) Actuarial loss (gain) 14 6 (2 ) (4 ) Total non-service cost components (b) $ (16 ) $ (16 ) $ (13 ) $ (13 ) Net periodic benefit cost (income) $ 11 $ 6 $ (9 ) $ (9 ) (a) Service cost, net of capitalization, is reflected in “Operating Expenses – Other operations and maintenance” on Ameren’s statement of income. (b) Non-service cost components are reflected in “Other Income, Net” on Ameren’s statement of income. See Note 5 – Other Income, Net for additional information.
Summary Of Benefit Plan Costs Incurred	Ameren Missouri and Ameren Illinois are responsible for their respective shares of Ameren’s pension and postretirement costs. The following table presents the respective share of net periodic pension and other postretirement benefit costs (income) incurred for the three months ended March 31, 2020 and 2019 : Pension Benefits Postretirement Benefits Three Months Three Months 2020 2019 2020 2019 Ameren Missouri (a) $ 4 $ 1 $ (1 ) $ (2 ) Ameren Illinois 7 5 (8 ) (7 ) Ameren (a) $ 11 $ 6 $ (9 ) $ (9 ) (a) Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates.

Income Taxes Income Taxes (Tabl

Income Taxes Income Taxes (Tables)	3 Months Ended
	Mar. 31, 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 months ended March 31, 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 excess deferred taxes (9) (7) (15) (12) (3) (3) Depreciation differences (1) — — — (1) — Amortization of deferred investment tax credit — (1) (1) (1) — — State tax 6 6 3 4 7 7 Stock-based compensation (5) (7) — — — — Other permanent items — — — (3) — — Effective income tax rate 12% 12% 8% 9% 24% 25%

Supplemental Information (Table

Supplemental Information (Tables)	3 Months Ended
	Mar. 31, 2020
Supplemental Information [Abstract]
Schedule of Cash and Cash Equivalents Including Restricted Cash	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets and the statements of cash flows as of March 31, 2020 , and December 31, 2019 : March 31, 2020 December 31, 2019 Ameren Ameren Ameren Ameren Ameren Ameren Cash and cash equivalents $ 42 $ 3 $ 6 $ 16 $ 9 $ — Restricted cash included in “Other current assets” 16 4 6 14 4 5 Restricted cash included in “Other assets” 128 — 128 120 — 120 Restricted cash included in “Nuclear decommissioning trust fund” 17 17 — 26 26 — Total cash, cash equivalents, and restricted cash $ 203 $ 24 $ 140 $ 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 months ended March 31, 2020 and 2019 : 2020 2019 Ameren Ameren Ameren (a) Ameren Ameren Ameren (a) Balance at January 1 $ 17 $ 7 $ 10 $ 18 $ 7 $ 11 Bad debt expense 3 2 1 3 1 2 Net write-offs (1 ) (1 ) — (2 ) (1 ) (1 ) Balance at March 31 $ 19 $ 8 $ 11 $ 19 $ 7 $ 12 (a) Ameren Illinois has a rate-adjustment mechanism that allows it to recover the difference between its actual net bad debt write-offs under GAAP and the amount of net bad debt write-offs included in its base rates.
Schedule of Cash Flow, Supplemental Disclosures	Supplemental Cash Flow Information The following table provides noncash financing and investing activity excluded from the statements of cash flows for the three months ended March 31, 2020 and 2019 : March 31, 2020 March 31, 2019 Ameren Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Investing Accrued capital expenditures $ 235 $ 97 $ 127 $ 208 $ 92 $ 106 Accrued nuclear fuel expenditures 7 7 — — — — Net realized and unrealized gain (loss) – nuclear decommissioning trust fund (111 ) (111 ) — 64 64 — Financing 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 three months ended March 31, 2020 : Ameren Missouri Ameren Illinois Ameren Balance at December 31, 2019 $ 687 (a) $ 4 (b) $ 691 (a) Liabilities settled (14 ) — (14 ) Accretion 7 (c) — 7 (c) Balance at March 31, 2020 $ 680 (a) $ 4 (b) $ 684 (a) (a) Balance included $53 million in “Other current liabilities” on the balance sheet as of both December 31, 2019 , and March 31, 2020 . (b) Included in “Other deferred credits and liabilities” on the balance sheet. (c) Accretion expense attributable to Ameren Missouri was recorded as a decrease to regulatory liabilities.
Schedule of excise taxes	The following table presents the excise taxes recorded on a gross basis in “Operating Revenues – Electric,” “Operating Revenues – Natural gas” and “Operating Expenses – Taxes other than income taxes” on the statements of income for the three months ended March 31, 2020 and 2019 : Three Months 2020 2019 Ameren Missouri $ 30 $ 31 Ameren Illinois 35 39 Ameren $ 65 $ 70
Schedule of Earnings Per Share, Basic and Diluted	The following table reconciles the basic weighted-average number of common shares outstanding to the diluted weighted-average number of common shares outstanding for the three months ended March 31, 2020 and 2019 : Three Months 2020 2019 Weighted-average Common Shares Outstanding – Basic 246.4 244.9 Assumed settlement of performance share units and restricted stock units 1.1 1.5 Dilutive effect of forward sale agreement 0.6 — Weighted-average Common Shares Outstanding – Diluted (a) 248.1 246.4 (a) There were no potentially dilutive securities excluded from the earnings per diluted share calculations for the three months ended March 31, 2020 and 2019 .

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
	Mar. 31, 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 months ended March 31, 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 $ 670 $ 389 $ 271 $ 110 $ — $ — $ 1,440 Intersegment revenues 10 1 — 13 — (24 ) — Net income (loss) attributable to Ameren common shareholders (10 ) 37 55 47 (a) 17 — 146 Capital expenditures 278 123 61 170 3 1 636 Three Months 2019: External revenues $ 751 $ 386 $ 320 $ 99 $ — $ — $ 1,556 Intersegment revenues 7 1 — 15 — (23 ) — Net income attributable to Ameren common shareholders 39 36 57 44 (a) 15 — 191 Capital expenditures 240 124 51 121 10 (2 ) 544 (a) Ameren Transmission earnings reflect an allocation of financing costs from Ameren (parent). Ameren Illinois Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Ameren Illinois Three Months 2020: External revenues $ 390 $ 271 $ 62 $ — $ 723 Intersegment revenues — — 12 (a) (12 ) — Net income available to common shareholder 37 55 28 — 120 Capital expenditures 123 61 140 — 324 Three Months 2019: External revenues $ 387 $ 320 $ 55 $ — $ 762 Intersegment revenues — — 15 (a) (15 ) — Net income available to common shareholder 36 57 27 — 120 Capital expenditures 124 51 92 — 267 (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 months ended March 31, 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 $ 297 $ 220 $ — $ — $ — $ 517 Commercial 221 126 — — — 347 Industrial 53 35 — — — 88 Other 60 9 — 123 (24 ) 168 Total electric revenues $ 631 $ 390 $ — $ 123 $ (24 ) $ 1,120 Residential $ 33 $ — $ 213 $ — $ — $ 246 Commercial 13 — 54 — — 67 Industrial 1 — 3 — — 4 Other 2 — 1 — — 3 Total natural gas revenues $ 49 $ — $ 271 $ — $ — $ 320 Total revenues (a) $ 680 $ 390 $ 271 $ 123 $ (24 ) $ 1,440 Three Months 2019: Residential $ 312 $ 217 $ — $ — $ — $ 529 Commercial 239 123 — — — 362 Industrial 55 34 — — — 89 Other 98 13 — 114 (23 ) 202 Total electric revenues $ 704 $ 387 $ — $ 114 $ (23 ) $ 1,182 Residential $ 38 $ — $ 246 $ — $ — $ 284 Commercial 16 — 65 — — 81 Industrial 2 — 4 — — 6 Other (2 ) — 5 — — 3 Total natural gas revenues $ 54 $ — $ 320 $ — $ — $ 374 Total revenues (a) $ 758 $ 387 $ 320 $ 114 $ (23 ) $ 1,556 (a) The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the three months ended March 31, 2020 and 2019 : Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Ameren Three Months 2020: Revenues from alternative revenue programs $ (3 ) $ 46 $ 11 $ 12 $ 66 Other revenues not from contracts with customers 8 1 1 — 10 Three Months 2019: Revenues from alternative revenue programs $ 15 $ 22 $ (3 ) $ (5 ) $ 29 Other revenues not from contracts with customers 5 3 1 — 9 Ameren Illinois Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Ameren Illinois Three Months 2020: Residential $ 220 $ 213 $ — $ — $ 433 Commercial 126 54 — — 180 Industrial 35 3 — — 38 Other 9 1 74 (12 ) 72 Total revenues (a) $ 390 $ 271 $ 74 $ (12 ) $ 723 Three Months 2019: Residential $ 217 $ 246 $ — $ — $ 463 Commercial 123 65 — — 188 Industrial 34 4 — — 38 Other 13 5 70 (15 ) 73 Total revenues (a) $ 387 $ 320 $ 70 $ (15 ) $ 762 (a) The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the Ameren Illinois segments for the three months ended March 31, 2020 and 2019 : Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Ameren Illinois Three Months 2020: Revenues from alternative revenue programs $ 46 $ 11 $ 10 $ 67 Other revenues not from contracts with customers 1 1 — 2 Three Months 2019: Revenues from alternative revenue programs $ 22 $ (3 ) $ (5 ) $ 14 Other revenues not from contracts with customers 3 1 — 4

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Basis Of Presentation And Significant Accounting Policies [Line Items]
Unrecorded Unconditional Purchase Obligation	$ 1,936
Cash Surrender Value of Life Insurance	245	$ 264
Corporate owned life insurance, borrowings	109	114
Ameren Illinois Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Unrecorded Unconditional Purchase Obligation	493
Cash Surrender Value of Life Insurance	118	123
Corporate owned life insurance, borrowings	109	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	15
Equity Method Investments	31	$ 28
Partnership Funding Commitment
Basis Of Presentation And Significant Accounting Policies [Line Items]
Unrecorded Unconditional Purchase Obligation	$ 34

Rate And Regulatory Matters (Na

Rate And Regulatory Matters (Narrative-Missouri) (Detail) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020USD ($)MWh	Mar. 31, 2019USD ($)	Dec. 31, 2019MWh	Dec. 31, 2018MWh
Rate And Regulatory Matters [Line Items]
Purchased Power	$ 134	$ 156
Other operations and maintenance	438	417
Income Taxes	21	27
Donations	13	6
Union Electric Company
Rate And Regulatory Matters [Line Items]
Purchased Power	39	51
Other operations and maintenance	239	224
Income Taxes	(1)	4
Donations	8	2
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
PISA deferrals	70
Donations	$ 8
Wind Generation Facility | Union Electric Company
Rate And Regulatory Matters [Line Items]
Amount of Megawatts | MWh	300		300	400
Project Placed In-Service Amount	$ 100
Estimated Capital Project Costs	$ 1,200
MEEIA 2013 & 2016 | Final Rate Order | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Revenues		$ 20
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 (_2

Rate And Regulatory Matters (Narrative-Illinois) (Detail) - Ameren Illinois Company - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Feb. 29, 2020	Jun. 30, 2020	Mar. 31, 2020
Rate And Regulatory Matters [Line Items]
Disconnection suspension extension period			60 days
Pending Rate Case | Natural Gas
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 102
Revenues	$ 46
Public Utilities, Requested Return on Equity, Percentage			10.50%
Public Utilities, Requested Equity Capital Structure, Percentage			54.10%
Rate Base			$ 2,100
Subsequent Event | Pending Rate Case | IEIMA | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 45

Rate And Regulatory Matters (_3

Rate And Regulatory Matters (Narrative-Federal) (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Nov. 30, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		$ 189	$ 164
Reduction to FERC allowed base return on common equity		0.50%
Incentive adder cap on FERC allowed return on equity		2.50%
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		$ 90	$ 84
Net Income (Loss) Attributable to Parent		120		$ 120
February 2015 FERC ROE Complaint Case
Rate And Regulatory Matters [Line Items]
Net Income (Loss) Attributable to Parent		11
February 2015 FERC ROE Complaint Case | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Net Income (Loss) Attributable to Parent		$ 6
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	9.88%	10.32%	9.88%
Customer Requested Rate on Equity		9.15%
Current regulatory liabilities		$ 40
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		$ 23
Midwest Independent Transmission System Operator, Inc | Notice of Proposed Rulemaking [Member]
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_2

Short-Term Debt And Liquidity (Narrative) (Detail) $ in Billions	3 Months Ended
	Mar. 31, 2020USD ($)	Mar. 31, 2019
Credit Agreements
Short-term Debt [Line Items]
Net Liquidity Available	$ 1.7
Actual debt-to-capital ratio	0.55
Utilities
Short-term Debt [Line Items]
Short-term Debt, Weighted Average Interest Rate, over Time	1.93%	2.87%
Union Electric Company | Missouri Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.51
Ameren Illinois Company | Illinois Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.46

Short-Term Debt and Liquidity_3

Short-Term Debt and Liquidity (Short-Term Debt outstanding) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Short-term Debt [Line Items]
Short-term Debt	$ 615	$ 440
Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt	425	153
Union Electric Company
Short-term Debt [Line Items]
Short-term Debt	130	234
Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt	60	53
Line of Credit
Short-term Debt [Line Items]
Short-term Debt	465	0
Line of Credit | Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt	275	0
Line of Credit | Union Electric Company
Short-term Debt [Line Items]
Short-term Debt	130	0
Line of Credit | Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt	60	0
Commercial Paper
Short-term Debt [Line Items]
Short-term Debt	150	440
Commercial Paper | Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt	150	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	$ 0	$ 53

Short-Term Debt and Liquidity_4

Short-Term Debt and Liquidity (Short-Term Debt Activity) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Line of Credit | Missouri Credit Agreement
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 13
Weighted-average interest rate	2.33%
Short-term Debt, Maximum Amount Outstanding During Period	$ 230
Peak interest rate	3.33%
Line of Credit | Missouri Credit Agreement | Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 1
Weighted-average interest rate	2.06%
Short-term Debt, Maximum Amount Outstanding During Period	$ 100
Peak interest rate	2.06%
Line of Credit | Missouri Credit Agreement | Union Electric Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 12
Weighted-average interest rate	2.36%
Short-term Debt, Maximum Amount Outstanding During Period	$ 130
Peak interest rate	3.33%
Line of Credit | Missouri Credit Agreement | Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 0
Weighted-average interest rate	0.00%
Short-term Debt, Maximum Amount Outstanding During Period	$ 0
Peak interest rate	0.00%
Line of Credit | Illinois Credit Agreement
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 7
Weighted-average interest rate	2.06%
Short-term Debt, Maximum Amount Outstanding During Period	$ 235
Peak interest rate	2.06%
Line of Credit | Illinois Credit Agreement | Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 2
Weighted-average interest rate	2.06%
Short-term Debt, Maximum Amount Outstanding During Period	$ 175
Peak interest rate	2.06%
Line of Credit | Illinois Credit Agreement | Union Electric Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 0
Weighted-average interest rate	0.00%
Short-term Debt, Maximum Amount Outstanding During Period	$ 0
Peak interest rate	0.00%
Line of Credit | Illinois Credit Agreement | Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 5
Weighted-average interest rate	2.05%
Short-term Debt, Maximum Amount Outstanding During Period	$ 60
Peak interest rate	2.05%
Commercial Paper
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 608	$ 815
Weighted-average interest rate	1.88%	2.85%
Short-term Debt, Maximum Amount Outstanding During Period	$ 854	$ 1,113
Peak interest rate	5.05%	3.10%
Commercial Paper | Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 154	$ 480
Weighted-average interest rate	1.94%	2.87%
Short-term Debt, Maximum Amount Outstanding During Period	$ 225	$ 618
Peak interest rate	3.30%	3.10%
Commercial Paper | Union Electric Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 383	$ 246
Weighted-average interest rate	1.84%	2.84%
Short-term Debt, Maximum Amount Outstanding During Period	$ 521	$ 549
Peak interest rate	5.05%	2.97%
Commercial Paper | Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 71	$ 89
Weighted-average interest rate	1.99%	2.76%
Short-term Debt, Maximum Amount Outstanding During Period	$ 137	$ 130
Peak interest rate	3.40%	2.90%

Long-Term debt and Equity Fin_2

Long-Term debt and Equity Financings (Narrative) (Details) - USD ($) shares in Millions	1 Months Ended	3 Months Ended
	Apr. 30, 2020	Mar. 31, 2020	Mar. 31, 2019	Aug. 05, 2019
Long-Term Debt And Equity Financings [Line Items]
Stock Issued During Period, Shares, New Issues		0.2	0.3
Issuance of common stock		$ 13,000,000	$ 19,000,000
Stock Issued During Period, Shares, Other		0.5	0.8
Issuance of common stock for stock-based compensation		$ 38,000,000	$ 54,000,000
Forward Contract Indexed to Issuer's Equity, Indexed Shares (in shares)				7.5
Forward Contract Indexed to Issuer's Equity, Settlement Alternatives, Cash, at Fair Value		552,000,000
Period End Net Cash Settlement Price		$ 2,000,000
Period End Net Share Settlement Price (in shares)		0.1
Ameren (parent) | Unsecured Debt [Member] | Senior Unsecured Notes270 due 2020
Long-Term Debt And Equity Financings [Line Items]
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
Debt Instrument, Interest Rate, Stated Percentage		5.00%
Subsequent Event | Unsecured Debt [Member] | 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

Other Income, Net (Detail)

Other Income, Net (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 4	$ 6
Interest income on industrial development revenue bonds	6	6
Other interest income	1	2
Non-service cost components of net periodic benefit income	23	22
Miscellaneous income	2	2
Donations	(13)	(6)
Miscellaneous expense	(2)	(3)
Total Other Income, Net	21	29
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	2	4
Interest income on industrial development revenue bonds	6	6
Non-service cost components of net periodic benefit income	5	4
Miscellaneous income	1	1
Donations	(8)	(2)
Miscellaneous expense	(2)	(1)
Total Other Income, Net	4	12
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	6	7
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	2	2
Other interest income	1	2
Non-service cost components of net periodic benefit income	13	12
Miscellaneous income	1	1
Donations	(4)	(4)
Miscellaneous expense	(2)	(2)
Total Other Income, Net	11	$ 11
Final Rate Order | Electricity | Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Donations	$ (8)

Derivative Financial Instrume_3

Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) lb in Thousands, gal in Millions, MWh in Millions, MMBTU in Millions	3 Months Ended
	Mar. 31, 2020MWhMMBTUlbgal	Mar. 31, 2019MWhMMBTUlbgal
Fuel Oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | gal	62	58
Natural Gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	167	156
Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	12	12
Uranium [Member]
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	62	58
Union Electric Company | Natural Gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	20	20
Union Electric Company | Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	5	5
Union Electric Company | Uranium [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass | lb	365	565
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	147	136
Ameren Illinois Company | Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	7	7

Derivative Financial Instrume_4

Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Derivative assets	$ 30	$ 26
Derivative liabilities	301	255
Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	3	4
Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	1	2
Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	19	4
Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	12	3
Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	2	3
Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	2	1
Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	20	13
Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	5	7
Power | Other Current Assets
Derivative [Line Items]
Derivative assets	18	14
Power | Other Assets
Derivative [Line Items]
Derivative assets	4	2
Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	20	19
Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	224	208
Uranium [Member] | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	1	1
Union Electric Company
Derivative [Line Items]
Derivative assets	26	22
Derivative liabilities	38	13
Union Electric Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	3	4
Union Electric Company | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	1	2
Union Electric Company | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	19	4
Union Electric Company | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	12	3
Union Electric Company | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Union Electric Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	0	0
Union Electric Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	3	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	18	14
Union Electric Company | Power | Other Assets
Derivative [Line Items]
Derivative assets	4	2
Union Electric Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	2	2
Union Electric Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	1	1
Union Electric Company | Uranium [Member] | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	1	1
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	4	4
Derivative liabilities	263	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	2	3
Ameren Illinois Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	2	1
Ameren Illinois Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	17	12
Ameren Illinois Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	5	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	18	17
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	223	207
Ameren Illinois Company | Uranium [Member] | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	$ 0	$ 0

Fair Value Measurements (Schedu

Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	$ 726	$ 842
Assets fair value	756	868
Excluded receivables, payables, and accrued income, net	16	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	457	569
Assets fair value	461	569
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	269	273
Assets fair value	270	276
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Assets fair value	25	23
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	30	26
Derivative liabilities	301	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	4	0
Derivative liabilities	22	4
Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	3
Derivative liabilities	18	16
Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	25	23
Derivative liabilities	261	235
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	726	842
Assets fair value	752	864
Excluded receivables, payables, and accrued income, net	8	5
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	457	569
Assets fair value	461	569
Union Electric Company | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	269	273
Assets fair value	269	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	22	20
Union Electric Company | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	26	22
Derivative liabilities	38	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	4	0
Derivative liabilities	20	1
Union Electric Company | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	2
Derivative liabilities	3	4
Union Electric Company | Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	22	20
Derivative liabilities	15	8
Union Electric Company | Uranium [Member] | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	1	1
Union Electric Company | Uranium [Member] | 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 [Member] | 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 [Member] | Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	1	1
Union Electric Company | Fuel Oils | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	6
Derivative liabilities	31	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	18	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	4	6
Derivative liabilities	13	6
Union Electric Company | Power | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	22	16
Derivative liabilities	3	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	4	0
Derivative liabilities	2	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	18	14
Derivative liabilities	1	1
Union Electric Company | Natural Gas | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	3	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 liabilities	0	0
Union Electric Company | Natural Gas | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	3	2
Union Electric Company | Natural Gas | Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Union Electric Company | Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	457	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	457	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	105	107
Union Electric Company | Debt Securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	96	93
Union Electric Company | Debt Securities | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	68	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	105	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	96	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	68	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	263	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	2	3
Ameren Illinois Company | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	15	12
Ameren Illinois Company | Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	246	227
Ameren Illinois Company | Power | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	241	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	241	224
Ameren Illinois Company | Natural Gas | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	4
Derivative liabilities	22	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	2	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	1	1
Derivative liabilities	15	12
Ameren Illinois Company | Natural Gas | Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	3
Derivative liabilities	$ 5	$ 3

Fair Value Measurements (Sche_2

Fair Value Measurements (Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level Three In The Fair Value Hierarchy) (Detail) - Power - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	$ (224)	$ (184)	$ (211)	$ (183)
Included in regulatory assets/liabilities	(10)	(4)
Settlements	(3)	3
Change in unrealized gains (losses) related to assets/liabilities held at period end	(11)	(4)
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	17	0	13	0
Included in regulatory assets/liabilities	11	0
Settlements	(7)	0
Change in unrealized gains (losses) related to assets/liabilities held at period end	10	0
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	(241)	(184)	$ (224)	$ (183)
Included in regulatory assets/liabilities	(21)	(4)
Settlements	4	3
Change in unrealized gains (losses) related to assets/liabilities held at period end	$ (21)	$ (4)

Fair Value Measurements (Sche_3

Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) - Power $ in Millions	Mar. 31, 2020USD ($)$ / MWh$ / MMBTU	Dec. 31, 2019USD ($)$ / MWh$ / MMBTU
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	$ 18	$ 14
Derivative liabilities | $	$ (242)	$ (225)
Commodity Forward Price | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	16	22
Commodity Forward Price | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	33	34
Commodity Forward Price | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	25	25
Nodal Basis | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	(6)	(6)
Nodal Basis | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0	0
Nodal Basis | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	(2)	(2)
Commodity Future Price | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU	2	(1)
Commodity Future Price | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU	3	0
Commodity Future Price | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU	2	0

Fair Value Measurements (Sche_4

Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Financial Assets and Liabilities) (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 203	$ 176	$ 118	$ 107
Short-term Debt	615	440
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	24	39	12	8
Short-term Debt	130	234
Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	140	125	$ 93	$ 80
Short-term Debt	60	53
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	203	176
Available-for-sale Securities and Held-to-maturity Securities	263	263
Short-term Debt	615	440
Long-term debt (including current portion)	9,735	9,357
Debt Issuance Costs, Net	73	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	24	39
Available-for-sale Securities and Held-to-maturity Securities	263	263
Short-term Debt	130	234
Long-term debt (including current portion)	4,567	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	140	125
Short-term Debt	60	53
Long-term debt (including current portion)	3,575	3,575
Debt Issuance Costs, Net	34	34
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	203	176
Investments, Fair Value Disclosure	263	263
Short-term Debt, Fair Value	615	440
Long-term Debt, Fair Value	10,638	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	203	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	615	440
Long-term Debt, Fair Value	10,189	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	449	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	24	39
Investments, Fair Value Disclosure	263	263
Short-term Debt, Fair Value	130	234
Long-term Debt, Fair Value	5,098	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	24	39
Investments, Fair Value Disclosure	0	0
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	0	0
Fair Value | Union Electric Company | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Investments, Fair Value Disclosure	263	263
Short-term Debt, Fair Value	130	234
Long-term Debt, Fair Value	5,098	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
Investments, Fair Value Disclosure	0	0
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	0	0
Fair Value | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	140	125
Short-term Debt, Fair Value	60	53
Long-term Debt, Fair Value	3,945	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	140	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	60	53
Long-term Debt, Fair Value	3,945	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 (Nar

Related Party Transactions (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2020USD ($)
Ameren Illinois Company | April 2020 Procurement | Ameren Illinois Capacity Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Energy Supply Agreements Amount	$ 2

Related Party Transactions (Sch

Related Party Transactions (Schedule of Affiliate Receivables and Payables) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Union Electric Company
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	$ 48	$ 52
Accounts Receivable, Related Parties, Current	34	30
Union Electric Company | Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	12	15
Union Electric Company | Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	16	15
Ameren Illinois Company
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	84	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	48	43
Ameren Illinois Company | Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	$ 1	$ 17

Related Party Transactions (Eff

Related Party Transactions (Effects of Related-party Transactions on the Statement of Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Union Electric Company | Ameren Missouri Power Supply Agreements with Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues	$ 3	$ 1
Union Electric Company | Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	7	7
Operating Expenses	1	1
Union Electric Company | Ameren Missouri and Ameren Illinois Miscellaneous Support Services
Related Party Transaction [Line Items]
Operating Revenues	1	1
Union Electric Company | Total Related Party Operating Revenues
Related Party Transaction [Line Items]
Operating Revenues	10	7
Union Electric Company | Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	35	32
Union Electric Company | Total Related Party Other Operations and Maintenance
Related Party Transaction [Line Items]
Operating Expenses	35	32
Union Electric Company | Money Pool Borrowings (Advances)
Related Party Transaction [Line Items]
Interest (Charges) Income	1	0
Ameren Illinois Company | Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	1	1
Operating Expenses	1	1
Ameren Illinois Company | Ameren Missouri and Ameren Illinois Miscellaneous Support Services
Related Party Transaction [Line Items]
Operating Revenues	1	1
Ameren Illinois Company | Total Related Party Operating Revenues
Related Party Transaction [Line Items]
Operating Revenues	1	1
Ameren Illinois Company | Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Operating Expenses	3	1
Ameren Illinois Company | Ameren Illinois Transmission Services With ATXI
Related Party Transaction [Line Items]
Operating Expenses	1	1
Ameren Illinois Company | Purchased Power
Related Party Transaction [Line Items]
Operating Expenses	3	1
Ameren Illinois Company | Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	33	30
Ameren Illinois Company | Total Related Party Other Operations and Maintenance
Related Party Transaction [Line Items]
Operating Expenses	34	31
Ameren Illinois Company | Money Pool Borrowings (Advances)
Related Party Transaction [Line Items]
Interest (Charges) Income	$ 1	$ 0

Commitments And Contingencies_2

Commitments And Contingencies (Other Obligations) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2020USD ($)MWh
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	$ 569
2021	499
2022	305
2023	218
2024	148
Thereafter	197
Total unrecorded unconditional purchase obligation	1,936
Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	242
2021	219
2022	185
2023	105
2024	94
Thereafter	55
Total unrecorded unconditional purchase obligation	900
Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	150
2021	129
2022	69
2023	39
2024	13
Thereafter	43
Total unrecorded unconditional purchase obligation	443
Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	35
2021	57
2022	11
2023	44
2024	15
Thereafter	16
Total unrecorded unconditional purchase obligation	178
Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	85
2021	51
2022	13
2023	3
2024	0
Thereafter	0
Total unrecorded unconditional purchase obligation	$ 152
Amount of Megawatts | MWh	102
Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	$ 2
2021	3
2022	3
2023	3
2024	3
Thereafter	24
Total unrecorded unconditional purchase obligation	38
Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	55
2021	40
2022	24
2023	24
2024	23
Thereafter	59
Total unrecorded unconditional purchase obligation	225
Renewable Energy Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Total unrecorded unconditional purchase obligation	15
Zero Emission Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Total unrecorded unconditional purchase obligation	11
Union Electric Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	354
2021	336
2022	237
2023	189
2024	141
Thereafter	140
Total unrecorded unconditional purchase obligation	1,397
Union Electric Company | Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	242
2021	219
2022	185
2023	105
2024	94
Thereafter	55
Total unrecorded unconditional purchase obligation	900
Union Electric Company | Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	32
2021	26
2022	15
2023	13
2024	6
Thereafter	19
Total unrecorded unconditional purchase obligation	111
Union Electric Company | Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	35
2021	57
2022	11
2023	44
2024	15
Thereafter	16
Total unrecorded unconditional purchase obligation	178
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	$ 2
2021	3
2022	3
2023	3
2024	3
Thereafter	24
Total unrecorded unconditional purchase obligation	38
Union Electric Company | Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	43
2021	31
2022	23
2023	24
2024	23
Thereafter	26
Total unrecorded unconditional purchase obligation	170
Ameren Illinois Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	208
2021	158
2022	67
2023	29
2024	7
Thereafter	24
Total unrecorded unconditional purchase obligation	493
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	118
2021	103
2022	54
2023	26
2024	7
Thereafter	24
Total unrecorded unconditional purchase obligation	332
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	85
2021	51
2022	13
2023	3
2024	0
Thereafter	0
Total unrecorded unconditional purchase obligation	152
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	5
2021	4
2022	0
2023	0
2024	0
Thereafter	0
Total unrecorded unconditional purchase obligation	9
Ameren Illinois Company | Renewable Energy Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Total unrecorded unconditional purchase obligation	15
Ameren Illinois Company | Zero Emission Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Total unrecorded unconditional purchase obligation	$ 11
Ameren Illinois Company | April 2020 Procurement
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Amount of Megawatthours | MWh	3,550,800
Long-term Purchase Commitment, Amount	$ 92
Ameren Illinois Company | April 2020 Procurement - Capacity
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Amount of Megawatts | MWh	617
Long-term Purchase Commitment, Amount	$ 4

Commitments And Contingencies_3

Commitments And Contingencies (Environmental Matters) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2020USD ($)waste_streamscrubbercentersite	Dec. 31, 2019USD ($)
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 684	$ 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
Postponement of EPA's 2015 Rule	two years
Waste Streams with Postponed Compliance Dates | waste_stream	2
Number of Energy Centers Constructing Wastewater Treatment Facilities | center	3
Asset Retirement Obligation	$ 680	687
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	4	$ 4
Coal Combustion Residuals Estimate | Union Electric Company | Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	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	$ 126
Manufactured Gas Plant | Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites | site	44
Accrual for environmental loss contingencies	$ 126
Manufactured Gas Plant | Ameren Illinois Company | Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	126
Manufactured Gas Plant | Ameren Illinois Company | Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	213
Rush Island Energy Center [Member] | Union Electric Company
Loss Contingencies [Line Items]
Estimated capital costs to comply with preliminary court order	1,000
Rush Island Energy Center [Member] | Union Electric Company | Minimum
Loss Contingencies [Line Items]
Estimated operations and maintenance costs to comply with preliminary court order	30
Rush Island Energy Center [Member] | 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	137
New CCR Rules Estimate | Union Electric Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 137

Callaway Energy Center (Insuran

Callaway Energy Center (Insurance Disclosure) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Apr. 01, 2020
Nuclear Waste Matters [Line Items]
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]
Maximum Coverages		$ 450
Maximum Assessments for Single Incidents		0
Public Liability And Nuclear Worker Liability - Pool Participation
Nuclear Waste Matters [Line Items]
Maximum Coverages		13,348
Maximum Assessments for Single Incidents		138
Threshold For Which A Retrospective Assessment For A Covered Loss Is Necessary	$ 450
Annual Payment In The Event Of An Incident At Any Licensed Commercial Reactor	21
Public Liability And Nuclear Worker Liability
Nuclear Waste Matters [Line Items]
Maximum Coverages		13,798
Maximum Assessments for Single Incidents		138
Property Damage - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Maximum Coverages		3,200
Maximum Assessments for Single Incidents		25
Replacement Power - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Maximum Coverages		490
Maximum Assessments for Single Incidents		7
Amount Of Weekly Indemnity Coverage Commencing Twelve Weeks After Power Outage	4.5
Amount Of Additional Weekly Indemnity Coverage Commencing After Initial Indemnity Coverage	3.6
Amount Of Weekly Indemnity Coverage Thereafter Not Exceeding Policy Limit	490
Sub-Limit Of Amount Of Weekly Indemnity Coverage Thereafter Not Exceeding Policy Limit For Non-Nuclear Events	328
Radiation Event
Nuclear Waste Matters [Line Items]
Maximum Coverages		2,700
Aggregate Nuclear Power Industry Insurance Policy Limit For Losses From Terrorist Attacks Within Twelve Month Period	3,200
Non-Radiation Event
Nuclear Waste Matters [Line Items]
Maximum Coverages		2,300
Aggregate Nuclear Power Industry Insurance Policy Limit For Losses From Terrorist Attacks Within Twelve Month Period	$ 1,800
Property Damage European Mutual Association for Nuclear Insurance
Nuclear Waste Matters [Line Items]
Maximum Coverages		$ 490

Retirement Benefits (Components

Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	$ (23)	$ (22)
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	27	22
Interest cost	43	47
Expected return on plan assets	(73)	(69)
Prior service cost (benefit)	0	0
Actuarial loss (gain)	14	6
Total non-service cost components	(16)	(16)
Net periodic benefit cost	11	6
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	4	4
Interest cost	10	11
Expected return on plan assets	(20)	(19)
Prior service cost (benefit)	(1)	(1)
Actuarial loss (gain)	(2)	(4)
Total non-service cost components	(13)	(13)
Net periodic benefit cost	(9)	(9)
Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(5)	(4)
Union Electric Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	4	1
Union Electric Company | Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(1)	(2)
Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(13)	(12)
Ameren Illinois Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	7	5
Ameren Illinois Company | Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ (8)	$ (7)

Retirement Benefits (Summary of

Retirement Benefits (Summary of Benefit Plan Costs Incurred) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 11	$ 6
Pension Plan | Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	4	1
Pension Plan | Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	7	5
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(9)	(9)
Other Postretirement Benefit Plan, Defined Benefit | Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(1)	(2)
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)	$ (7)

Retirement Benefits Retirement

Retirement Benefits Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan estimated future employer contributions over next five years	$ 115	$ 70
Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan estimated future employer contributions over next five years	5
Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan estimated future employer contributions over next five years	$ 45

Income Taxes Income Taxes (Sche

Income Taxes Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Taxes [Line Items]
Federal statutory corporate income tax rate	21.00%	21.00%
Amortization of excess deferred taxes	(9.00%)	(7.00%)
Depreciation differences	(1.00%)	0.00%
Amortization of deferred investment tax credit	0.00%	(1.00%)
State tax	6.00%	6.00%
Stock-based compensation	(5.00%)	(7.00%)
Other permanent items	0.00%	0.00%
Effective income tax rate	12.00%	12.00%
Union Electric Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate	21.00%	21.00%
Amortization of excess deferred taxes	(15.00%)	(12.00%)
Depreciation differences	0.00%	0.00%
Amortization of deferred investment tax credit	(1.00%)	(1.00%)
State tax	3.00%	4.00%
Stock-based compensation	0.00%	0.00%
Other permanent items	0.00%	(3.00%)
Effective income tax rate	8.00%	9.00%
Ameren Illinois Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate	21.00%	21.00%
Amortization of excess deferred taxes	(3.00%)	(3.00%)
Depreciation differences	(1.00%)	0.00%
Amortization of deferred investment tax credit	0.00%	0.00%
State tax	7.00%	7.00%
Stock-based compensation	0.00%	0.00%
Other permanent items	0.00%	0.00%
Effective income tax rate	24.00%	25.00%

Supplemental Information (Narra

Supplemental Information (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Supplemental Information [Abstract]
Payables for purchased receivables	$ 33		$ 32
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
Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Amount	$ 8	$ 14
Deferred Compensation Liability, Classified, Noncurrent	$ 85		$ 86
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	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
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	252,370
Performance Shares | Renewable generation and energy storage installation targets
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	41,950
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	132,307
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Granted in Period, Fair Value	$ 10

Supplemental Information (Cash

Supplemental Information (Cash and Cash Equivalents) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	$ 42	$ 16
Restricted cash included in “Other current assets”	16	14
Restricted cash included in “Other assets”	128	120
Restricted cash included in “Nuclear decommissioning trust fund”	17	26
Total cash, cash equivalents, and restricted cash	203	176	$ 118	$ 107
Union Electric Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	3	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”	17	26
Total cash, cash equivalents, and restricted cash	24	39	12	8
Ameren Illinois Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	6	0
Restricted cash included in “Other current assets”	6	5
Restricted cash included in “Other assets”	128	120
Restricted cash included in “Nuclear decommissioning trust fund”	0	0
Total cash, cash equivalents, and restricted cash	$ 140	$ 125	$ 93	$ 80

Supplemental Information Supple

Supplemental Information Supplemental Information (Allowance for Doubtful Accounts) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning balance	$ 17	$ 18
Bad debt expense	3	3
Net write-offs	(1)	(2)
Ending balance	19	19
Union Electric Company
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning balance	7	7
Bad debt expense	2	1
Net write-offs	(1)	(1)
Ending balance	8	7
Ameren Illinois Company
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning balance	10	11
Bad debt expense	1	2
Net write-offs	0	(1)
Ending balance	$ 11	$ 12

Supplemental Information (Suppl

Supplemental Information (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	$ 235	$ 208
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	(111)	64
Issuance of common stock for stock-based compensation	38	54
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	97	92
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	(111)	64
Issuance of common stock for stock-based compensation	0	0
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	127	106
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	0	0
Issuance of common stock for stock-based compensation	0	0
Nuclear Fuel
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	7	0
Nuclear Fuel | Union Electric Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	7	0
Nuclear Fuel | Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	$ 0	$ 0

Supplemental Information (Sched

Supplemental Information (Schedule of Asset Retirement Obligations) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	$ 684	$ 691
Liabilities settled	(14)
Accretion	7
Other current liabilities	662	585
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	680	687
Liabilities settled	(14)
Accretion	7
Other current liabilities	128	96
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	4	4
Liabilities settled	0
Accretion	0
Other current liabilities	195	207
Asset Retirement Obligation Balance [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Other current liabilities	53	53
Asset Retirement Obligation Balance [Member] | Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Other current liabilities	$ 53	$ 53

Supplemental Information (Sch_2

Supplemental Information (Schedule Of Excise Taxes) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accounting Policies [Line Items]
Excise tax expense	$ 65	$ 70
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	30	31
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 35	$ 39

Supplemental Information (Earni

Supplemental Information (Earnings Per Share) (Details) - shares	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share Reconciliation [Abstract]
Weighted-average Common Shares Outstanding – Basic	246,400,000	244,900,000
Assumed settlement of performance share units and restricted stock units	1,100,000	1,500,000
Dilutive effect of forward sale agreement	600,000	0
Weighted-average Common Shares Outstanding – Diluted	248,100,000	246,400,000
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount (in shares)	0	0

Segment Information (Schedule O

Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
External Revenues	$ 1,440	$ 1,556
Net income (loss) attributable to common shareholders	146	191
Capital expenditures	636	544
Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	1,440	1,556
Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0
Ameren Illinois Company
Segment Reporting Information [Line Items]
External Revenues	723	762
Net income (loss) attributable to common shareholders	120	120
Capital expenditures	324	267
Ameren Illinois Company | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	723	762
Ameren Illinois Company | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0
Operating Segments | Union Electric Company
Segment Reporting Information [Line Items]
External Revenues	680	758
Net income (loss) attributable to common shareholders	(10)	39
Capital expenditures	278	240
Operating Segments | Union Electric Company | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	670	751
Operating Segments | Union Electric Company | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	10	7
Operating Segments | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External Revenues	390	387
Net income (loss) attributable to common shareholders	37	36
Capital expenditures	123	124
Operating Segments | Ameren Illinois Electric Distribution | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	389	386
Operating Segments | Ameren Illinois Electric Distribution | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	1	1
Operating Segments | Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
External Revenues	271	320
Net income (loss) attributable to common shareholders	55	57
Capital expenditures	61	51
Operating Segments | Ameren Illinois Natural Gas | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	271	320
Operating Segments | Ameren Illinois Natural Gas | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0
Operating Segments | Ameren Transmission
Segment Reporting Information [Line Items]
External Revenues	123	114
Net income (loss) attributable to common shareholders	47	44
Capital expenditures	170	121
Operating Segments | Ameren Transmission | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	110	99
Operating Segments | Ameren Transmission | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	13	15
Operating Segments | Other
Segment Reporting Information [Line Items]
Net income (loss) attributable to common shareholders	17	15
Capital expenditures	3	10
Operating Segments | Other | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	0	0
Operating Segments | Other | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Net income (loss) attributable to common shareholders	37	36
Capital expenditures	123	124
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	390	387
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
Net income (loss) attributable to common shareholders	55	57
Capital expenditures	61	51
Operating Segments | Ameren Illinois Company | Ameren Illinois Natural Gas | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	271	320
Operating Segments | Ameren Illinois Company | Ameren Illinois Natural Gas | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Segment Reporting Information [Line Items]
Net income (loss) attributable to common shareholders	28	27
Capital expenditures	140	92
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	62	55
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	12	15
Intersegment Eliminations
Segment Reporting Information [Line Items]
External Revenues	(24)	(23)
Net income (loss) attributable to common shareholders	0	0
Capital expenditures	1	(2)
Intersegment Eliminations | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	0	0
Intersegment Eliminations | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	(24)	(23)
Intersegment Eliminations | Ameren Illinois Company
Segment Reporting Information [Line Items]
External Revenues	(12)	(15)
Net income (loss) attributable to common shareholders	0	0
Capital expenditures	0	0
Intersegment Eliminations | Ameren Illinois Company | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	0	0
Intersegment Eliminations | Ameren Illinois Company | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	$ (12)	$ (15)

Segment Information (Disaggrega

Segment Information (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
External Revenues	$ 1,440	$ 1,556
Revenues from alternative revenue programs	66	29
Other revenues not from contracts with customers	10	9
Electricity
Disaggregation of Revenue [Line Items]
External Revenues	1,120	1,182
Electricity | Residential
Disaggregation of Revenue [Line Items]
External Revenues	517	529
Electricity | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	347	362
Electricity | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	88	89
Electricity | Other
Disaggregation of Revenue [Line Items]
External Revenues	168	202
Natural Gas
Disaggregation of Revenue [Line Items]
External Revenues	320	374
Natural Gas | Residential
Disaggregation of Revenue [Line Items]
External Revenues	246	284
Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	67	81
Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	4	6
Natural Gas | Other
Disaggregation of Revenue [Line Items]
External Revenues	3	3
Ameren Illinois Company
Disaggregation of Revenue [Line Items]
External Revenues	723	762
Revenues from alternative revenue programs	67	14
Other revenues not from contracts with customers	2	4
Ameren Illinois Company | Residential
Disaggregation of Revenue [Line Items]
External Revenues	433	463
Ameren Illinois Company | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	180	188
Ameren Illinois Company | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	38	38
Ameren Illinois Company | Other
Disaggregation of Revenue [Line Items]
External Revenues	72	73
Ameren Illinois Company | Electricity
Disaggregation of Revenue [Line Items]
External Revenues	452	442
Ameren Illinois Company | Natural Gas
Disaggregation of Revenue [Line Items]
External Revenues	271	320
Operating Segments | Union Electric Company
Disaggregation of Revenue [Line Items]
External Revenues	680	758
Revenues from alternative revenue programs	(3)	15
Other revenues not from contracts with customers	8	5
Operating Segments | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External Revenues	390	387
Revenues from alternative revenue programs	46	22
Other revenues not from contracts with customers	1	3
Operating Segments | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
External Revenues	271	320
Revenues from alternative revenue programs	11	(3)
Other revenues not from contracts with customers	1	1
Operating Segments | Ameren Transmission
Disaggregation of Revenue [Line Items]
External Revenues	123	114
Revenues from alternative revenue programs	12	(5)
Other revenues not from contracts with customers	0	0
Operating Segments | Electricity | Union Electric Company
Disaggregation of Revenue [Line Items]
External Revenues	631	704
Operating Segments | Electricity | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
External Revenues	297	312
Operating Segments | Electricity | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	221	239
Operating Segments | Electricity | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	53	55
Operating Segments | Electricity | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
External Revenues	60	98
Operating Segments | Electricity | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External Revenues	390	387
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
External Revenues	220	217
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	126	123
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	35	34
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
External Revenues	9	13
Operating Segments | Electricity | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Operating Segments | Electricity | Ameren Illinois Natural Gas | Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Operating Segments | Electricity | Ameren Illinois Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Operating Segments | Electricity | Ameren Illinois Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Operating Segments | Electricity | Ameren Illinois Natural Gas | Other
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Operating Segments | Electricity | Ameren Transmission
Disaggregation of Revenue [Line Items]
External Revenues	123	114
Operating Segments | Electricity | Ameren Transmission | Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Operating Segments | Electricity | Ameren Transmission | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Operating Segments | Electricity | Ameren Transmission | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Operating Segments | Electricity | Ameren Transmission | Other
Disaggregation of Revenue [Line Items]
External Revenues	123	114
Operating Segments | Natural Gas | Union Electric Company
Disaggregation of Revenue [Line Items]
External Revenues	49	54
Operating Segments | Natural Gas | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
External Revenues	33	38
Operating Segments | Natural Gas | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	13	16
Operating Segments | Natural Gas | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	1	2
Operating Segments | Natural Gas | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
External Revenues	2	(2)
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Operating Segments | Natural Gas | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
External Revenues	271	320
Operating Segments | Natural Gas | Ameren Illinois Natural Gas | Residential
Disaggregation of Revenue [Line Items]
External Revenues	213	246
Operating Segments | Natural Gas | Ameren Illinois Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	54	65
Operating Segments | Natural Gas | Ameren Illinois Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	3	4
Operating Segments | Natural Gas | Ameren Illinois Natural Gas | Other
Disaggregation of Revenue [Line Items]
External Revenues	1	5
Operating Segments | Natural Gas | Ameren Transmission
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Operating Segments | Natural Gas | Ameren Transmission | Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Operating Segments | Natural Gas | Ameren Transmission | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Operating Segments | Natural Gas | Ameren Transmission | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Operating Segments | Natural Gas | Ameren Transmission | Other
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	46	22
Other revenues not from contracts with customers	1	3
Operating Segments | Ameren Illinois Company | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	11	(3)
Other revenues not from contracts with customers	1	1
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	10	(5)
Other revenues not from contracts with customers	0	0
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External Revenues	390	387
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
External Revenues	220	217
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	126	123
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	35	34
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
External Revenues	9	13
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
External Revenues	74	70
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission | Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission | Other
Disaggregation of Revenue [Line Items]
External Revenues	74	70
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
External Revenues	271	320
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Natural Gas | Residential
Disaggregation of Revenue [Line Items]
External Revenues	213	246
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	54	65
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	3	4
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Natural Gas | Other
Disaggregation of Revenue [Line Items]
External Revenues	1	5
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
External Revenues	(24)	(23)
Intersegment Eliminations | Electricity
Disaggregation of Revenue [Line Items]
External Revenues	(24)	(23)
Intersegment Eliminations | Electricity | Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Intersegment Eliminations | Electricity | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Intersegment Eliminations | Electricity | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Intersegment Eliminations | Electricity | Other
Disaggregation of Revenue [Line Items]
External Revenues	(24)	(23)
Intersegment Eliminations | Natural Gas
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Intersegment Eliminations | Natural Gas | Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Intersegment Eliminations | Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Intersegment Eliminations | Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Intersegment Eliminations | Natural Gas | Other
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Intersegment Eliminations | Ameren Illinois Company
Disaggregation of Revenue [Line Items]
External Revenues	(12)	(15)
Intersegment Eliminations | Ameren Illinois Company | Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Intersegment Eliminations | Ameren Illinois Company | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Intersegment Eliminations | Ameren Illinois Company | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0
Intersegment Eliminations | Ameren Illinois Company | Other
Disaggregation of Revenue [Line Items]
External Revenues	$ (12)	$ (15)