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	Ameren Illinois Company Deferred Retirement Benefit Costs	Ameren Illinois Company Accumulated Other Comprehensive Income (Loss)
Beginning of year at Dec. 31, 2015			$ 5,616	$ 1,331	$ (3)		$ 142				$ 1,822		$ 1,669			$ 2,005		$ 825	$ 5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Value, New Issues			0
Stock-based compensation activity			(60)
Proceeds from Contributions from Parent									$ 44		6			$ 0		0
Net income	$ 659								360				360	255				255
Net Income (Loss) Available to Common Stockholders, Basic	653			653
Common stock dividends				(416)									(355)					(110)
Preferred stock dividends													(3)					(3)
Change in deferred retirement benefit costs	20				(20)									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, 2015	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, 2016	242.6
End of year at Dec. 31, 2016	$ 7,245	$ 2	5,556	1,568	(23)	$ (23)	142			$ 511	1,828	$ 80	1,671		$ 0	2,005	$ 62	967	0	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Dividends, Per Share, Cash Paid	$ 1.715
Stockholders' equity, end of year at Dec. 31, 2016								$ 7,103	4,090					3,034
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Value, New Issues			0
Stock-based compensation activity			(16)
Proceeds from Contributions 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)					0
Preferred stock dividends													(3)					(3)
Change in deferred retirement benefit costs	(5)				5									$ 0						0
Net income attributable to noncontrolling interest holder	$ 6						6
Dividends paid to noncontrolling interest holders							(6)
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								102.1					25.5
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	0	0
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							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
Proceeds from Contributions 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)									$ 0						0
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	$ 0	$ 0
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