Consolidated Statement Of Stockholders' Equity (USD $) In Millions, unless otherwise specified	Total	Common Stock	Other Paid-In Capital	Retained Earnings	Derivative Financial Instruments	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 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, 2011		$ 2	$ 5,598	$ 2,369	$ 7	$ (57)		$ 149				$ 1,555		$ 1,891		$ 1,965	$ 62	$ 408	$ 17
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(974)									419				419	144			144
Net income (loss) attributable to Ameren Corporation	(974)			(974)						419					144
Stock-based compensation activity			18
Capital contribution from parent										1		1				0
Common stock dividends				(389)										(400)				(189)
Preferred stock dividends														(3)				(3)
Change in derivative financial instruments					18
Divestiture of derivative financial instruments					0
Change in deferred retirement benefit costs	8					24									3					(3)
Divestiture of deferred retirement benefit costs						0
Net income attributable to noncontrolling interest holder								0
Dividends paid to noncontrolling interest holders								(6)
Divestiture of noncontrolling interest								0
Other								8
Dividends	0
Stockholders' equity, end of year at Dec. 31, 2012									6,616	4,054					2,401
End of year at Dec. 31, 2012	6,767	2	5,616	1,006	25	(33)	(8)	151			511	1,556	80	1,907		1,965	62	360	14	14
End of year (shares) at Dec. 31, 2012	242.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(143)
Net income (loss) attributable to Ameren Corporation	(145)									41					32
Stockholders' equity, end of year at Mar. 31, 2013
Beginning of year at Dec. 31, 2012	6,767	2	5,616	1,006	25	(33)	(8)	151			511	1,556	80	1,907		1,965	62	360	14	14
Beginning of year (shares) at Dec. 31, 2012	242.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	295									398				398	163			163
Net income (loss) attributable to Ameren Corporation	289			289						398					163
Stock-based compensation activity			16
Capital contribution from parent										4		4				0
Common stock dividends				(388)										(460)				(110)
Preferred stock dividends														(3)				(3)
Change in derivative financial instruments					(21)
Divestiture of derivative financial instruments					(4)
Change in deferred retirement benefit costs	(30)					29									3					(3)
Divestiture of deferred retirement benefit costs						7
Net income attributable to noncontrolling interest holder								6
Dividends paid to noncontrolling interest holders								(6)
Divestiture of noncontrolling interest								(9)
Other								0
Dividends	0
Stockholders' equity, end of year at Dec. 31, 2013	6,544								6,544	3,993					2,448
End of year at Dec. 31, 2013	6,686	2	5,632	907	0	3	3	142			511	1,560	80	1,842		1,965	62	410	11	11
End of year (shares) at Dec. 31, 2013	242.6									102.1					25.5
Beginning of year at Sep. 30, 2013
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	38
Net income (loss) attributable to Ameren Corporation	37									33					22
Stockholders' equity, end of year at Dec. 31, 2013	6,544								6,544	3,993					2,448
End of year at Dec. 31, 2013	6,686	2					3				511		80				62		11
End of year (shares) at Dec. 31, 2013	242.6									102.1					25.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	98
Net income (loss) attributable to Ameren Corporation	96									48					54
Stockholders' equity, end of year at Mar. 31, 2014
Beginning of year at Dec. 31, 2013	6,686	2	5,632	907	0	3	3	142			511	1,560	80	1,842		1,965	62	410	11	11
Beginning of year (shares) at Dec. 31, 2013	242.6									102.1					25.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	592									393				393	204			204
Net income (loss) attributable to Ameren Corporation	586			586						393					204
Stock-based compensation activity			(15)
Capital contribution from parent										215		224			15	15
Common stock dividends				(390)										(340)				0
Preferred stock dividends														(3)				(3)
Change in derivative financial instruments					0
Divestiture of derivative financial instruments					0
Change in deferred retirement benefit costs	12					(12)									3					(3)
Divestiture of deferred retirement benefit costs						0
Net income attributable to noncontrolling interest holder								6
Dividends paid to noncontrolling interest holders								(6)
Divestiture of noncontrolling interest								0
Other								0
Return of capital to parent										215		(215)
Stockholders' equity, end of year at Dec. 31, 2014	6,713								6,713	4,052					2,661
End of year at Dec. 31, 2014	6,855	2	5,617	1,103	0	(9)	(9)	142			511	1,569	80	1,892		1,980		611	8	8
End of year (shares) at Dec. 31, 2014	242.6									102.1					25.5
Beginning of year at Sep. 30, 2014
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	49
Net income (loss) attributable to Ameren Corporation	48									(5)					46
Stockholders' equity, end of year at Dec. 31, 2014	6,713								6,713	4,052					2,661
End of year at Dec. 31, 2014	$ 6,855	$ 2					$ (9)				$ 511		$ 80						$ 8
End of year (shares) at Dec. 31, 2014	242.6									102.1					25.5