Consolidated Statement Of Stockholders' Equity - USD ($) shares in Millions, $ 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, 2016			$ 5,556	$ 1,568	$ (23)		$ 142				$ 1,828		$ 1,671			$ 2,005		$ 967
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Value, New Issues			0
Stock-based compensation activity			(16)
Capital contribution from parent									$ 30		30			$ 8		8
Net income	$ 529								326				326	271				271
Net Income (Loss) Available to Common Stockholders, Basic	523			523
Common stock dividends				(431)									(362)
Preferred stock dividends													(3)					(3)
Change in deferred retirement benefit costs	(5)				5
Net income attributable to noncontrolling interest holder	$ 6						6
Dividends paid to noncontrolling interest holders							(6)
Beginning of year (shares) at Dec. 31, 2016	242.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Shares, New Issues	0.0
Stock Issued During Period, Shares, Other	0.0
End of year (shares) at Dec. 31, 2017	242.6
End of year at Dec. 31, 2017	$ 7,326	$ 2	5,540	1,660	(18)	$ (18)	142			$ 511	1,858	$ 80	1,632		$ 0	2,013	$ 62	1,235
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Dividends, Per Share, Cash Paid	$ 1.7775
Stockholders' equity, end of year at Dec. 31, 2017								$ 7,184	4,081					3,310
Increase (Decrease) in Stockholders' Equity [Roll Forward]
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)
Preferred stock dividends													(3)					(3)
Change in deferred retirement benefit costs	4				(4)
Net income attributable to noncontrolling interest holder	$ 6						6
Dividends paid to noncontrolling interest holders							(6)
Stock Issued During Period, Shares, New Issues	1.2
Stock Issued During Period, Shares, Other	0.7
End of year (shares) at Dec. 31, 2018	244.5								102.1					25.5
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							7,631	$ 4,229					$ 3,774
Increase (Decrease) in Stockholders' Equity [Roll Forward]
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)
Preferred stock dividends													(3)					(3)
Change in deferred retirement benefit costs	(5)				5
Net income attributable to noncontrolling interest holder	$ 6						6
Dividends paid to noncontrolling interest holders							(6)
Stock Issued During Period, Shares, New Issues	0.9
Stock Issued During Period, Shares, Other	0.8
End of year (shares) at Dec. 31, 2019	246.2								102.1					25.5
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