Consolidated Statement Of Stockholders' Equity - USD ($) $ in Millions	Total	Common Stock	Other Paid-In Capital	Retained Earnings	Deferred Retirement Benefit Costs	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interest	Total Ameren Corporation Stockholders' Equity	Union Electric Company	Union Electric Company Common Stock	Union Electric Company Other Paid-In Capital	Union Electric Company Preferred Stock Not Subject To Mandatory Redemption	Union Electric Company Retained Earnings	Ameren Illinois Company	Ameren Illinois Company Common Stock	Ameren Illinois Company Other Paid-In Capital	Ameren Illinois Company Preferred Stock Not Subject To Mandatory Redemption	Ameren Illinois Company Retained Earnings
Beginning of year at Dec. 31, 2017		$ 2	$ 5,540	$ 1,660	$ (18)		$ 142				$ 1,858		$ 1,632			$ 2,013		$ 1,235
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Settlement of forward sale agreement through common shares issuance		0	0
Stock Issued During Period, Value, New Issues			74
Stock-based compensation activity			13
Capital contribution from parent									$ 45		45			$ 160		160
Net income	$ 821								481				481	307				307
Net Income (Loss) Available to Common Stockholders, Basic	815			815
Common stock dividends				(451)									(375)					0
Preferred stock dividends													(3)					(3)
Change in deferred retirement benefit costs	4				(4)
Net income attributable to noncontrolling interest holders	$ 6						6
Dividends paid to noncontrolling interest holders							(6)
Beginning of year (shares) at Dec. 31, 2017	242,600,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Shares, New Issues	1,200,000
Stock Issued During Period, Shares, Other	700,000
End of year (shares) at Dec. 31, 2018	244,500,000
End of year at Dec. 31, 2018	$ 7,773	2	5,627	2,024	(22)	$ (22)	142			$ 511	1,903	$ 80	1,735		$ 0	2,173	$ 62	1,539
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Dividends, Per Share, Cash Paid	$ 1.8475
Stockholders' equity, end of year at Dec. 31, 2018								$ 7,631	4,229					3,774
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under forward sale agreement	0
Settlement of forward sale agreement through common shares issuance		0	0
Stock Issued During Period, Value, New Issues			68
Stock-based compensation activity			(1)
Capital contribution from parent									124		124			15		15
Net income	$ 834								$ 429				429	$ 346				346
Net Income (Loss) Available to Common Stockholders, Basic	828			828
Common stock dividends				(472)									(430)					0
Preferred stock dividends													(3)					(3)
Change in deferred retirement benefit costs	(5)				5
Net income attributable to noncontrolling interest holders	$ 6						6
Dividends paid to noncontrolling interest holders							(6)
Stock Issued During Period, Shares, New Issues	900,000
Stock Issued During Period, Shares, Other	800,000
End of year (shares) at Dec. 31, 2019	246,200,000								102,100,000					25,500,000
End of year at Dec. 31, 2019	$ 8,201	2	5,694	2,380	(17)	(17)	142			511	2,027	80	1,731		0	2,188	62	1,882
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Dividends, Per Share, Cash Paid	$ 1.9200
Stockholders' equity, end of year at Dec. 31, 2019	$ 8,059							8,059	$ 4,349					$ 4,132
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under forward sale agreement	0
Settlement of forward sale agreement through common shares issuance		1	424
Stock Issued During Period, Value, New Issues			51
Stock-based compensation activity			10
Capital contribution from parent									491		491			464		464
Net income	$ 877								$ 439				439	$ 382				382
Net Income (Loss) Available to Common Stockholders, Basic	871			871
Common stock dividends				(494)									(66)					(9)
Preferred stock dividends													(3)					(3)
Change in deferred retirement benefit costs	(16)				16
Net income attributable to noncontrolling interest holders	$ 6						6
Dividends paid to noncontrolling interest holders							(6)
Stock Issued During Period, Shares, New Issues	700,000
Stock Issued During Period, Shares, Other	500,000
End of year (shares) at Dec. 31, 2020	253,300,000								102,100,000					25,500,000
End of year at Dec. 31, 2020	$ 9,080	$ 3	$ 6,179	$ 2,757	$ (1)	$ (1)	$ 142			$ 511	$ 2,518	$ 80	$ 2,101		$ 0	$ 2,652	$ 62	$ 2,252
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Dividends, Per Share, Cash Paid	$ 2.0000
Stockholders' equity, end of year at Dec. 31, 2020	$ 8,938							$ 8,938	$ 5,210					$ 4,966
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under forward sale agreement	5,900,000