Consolidated Statement of Stockholders' Equity 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, 2017			$ 5,540	$ 1,660	$ (18)		$ 142						$ 1,632			$ 2,013		$ 1,235
Stock Issued During Period, Value, New Issues			17
Stock-based compensation activity			(11)
Capital contribution from parent														$ 20		20
Net income	$ 153								$ 39				39	96				96
Net Income (Loss) Available to Common Stockholders, Basic	151			151
Common stock dividends				(111)									(50)
Preferred stock dividends													(1)					(1)
Stockholders' Equity, Other				1
Change in deferred retirement benefit costs	(1)				1
Net income attributable to noncontrolling interest holder	$ 2						2
Dividends paid to noncontrolling interest holders							(2)
Beginning of year (shares) at Dec. 31, 2017	242.6
Stock Issued During Period, Shares, New Issues	0.3
Stock Issued During Period, Shares, Other	0.7
End of year (shares) at Mar. 31, 2018	243.6
End of year at Mar. 31, 2018	$ 7,372	$ 2	5,546	1,699	(17)	$ (17)	142			$ 511	$ 1,858	$ 80	1,620		$ 0	2,033	$ 62	1,330
Dividends per Common Share (in dollars per share)	$ 0.4575
Stockholders' equity, end of year at Mar. 31, 2018								$ 7,230	4,069					3,425
Beginning of year at Dec. 31, 2018	$ 7,773		5,627	2,024	(22)		142						1,735			2,173		1,539
Stock Issued During Period, Value, New Issues			19
Stock-based compensation activity			(21)
Capital contribution from parent														0		0
Net income	193								$ 40				40	$ 121				121
Net Income (Loss) Available to Common Stockholders, Basic	191			191
Common stock dividends				(116)									0
Preferred stock dividends													(1)					(1)
Stockholders' Equity, Other				0
Change in deferred retirement benefit costs	(1)				1
Net income attributable to noncontrolling interest holder	$ 2						2
Dividends paid to noncontrolling interest holders							(2)
Beginning of year (shares) at Dec. 31, 2018	244.5								102.1					25.5
Stock Issued During Period, Shares, New Issues	0.3
Stock Issued During Period, Shares, Other	0.8
End of year (shares) at Mar. 31, 2019	245.6								102.1					25.5
End of year 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
Stockholders' equity, end of year at Mar. 31, 2019	$ 7,705							$ 7,705	$ 4,268					$ 3,894