Consolidated Statement Of Stockholders' Equity - USD ($) shares in Millions, $ in Millions	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 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, 2012		$ 2	$ 5,616	$ 1,006	$ 25	$ (33)		$ 151				$ 1,556		$ 1,907			$ 1,965		$ 360	$ 14
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation activity			16
Net income (loss) attributable to Ameren Corporation	$ 289			289						$ 398					$ 163
Net income (loss)	295									398				398	163				163
Common stock dividends				(388)										(460)					(110)
Capital contribution from parent										4		4					0
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)
End of year (shares) at Dec. 31, 2013	242.6
End of year at Dec. 31, 2013	$ 6,686	2	5,632	907	0	3	$ 3	142			$ 511	1,560	$ 80	1,842		$ 0	1,965	$ 62	410	11	11
Stockholders' equity, end of year at Dec. 31, 2013									$ 6,544	3,993					2,448
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Return of capital to parent												0
Stock-based compensation activity			(15)
Net income (loss) attributable to Ameren Corporation	586			586						393					204
Net income (loss)	592									393				393	204				204
Common stock dividends				(390)										(340)					0
Capital contribution from parent										$ 215		224			15		15
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
End of year (shares) at Dec. 31, 2014	242.6									102.1					25.5
End of year at Dec. 31, 2014	$ 6,855	2	5,617	1,103	0	(9)	(9)	142			511	1,569	80	1,892		0	1,980	62	611	8	8
Stockholders' equity, end of year at Dec. 31, 2014	6,713								6,713	$ 4,052					$ 2,661
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Return of capital to parent										215		(215)
Stock-based compensation activity			(1)
Net income (loss) attributable to Ameren Corporation	630			630						355					217
Net income (loss)	636									355				355	217				217
Common stock dividends				(402)										(575)					0
Capital contribution from parent										$ 224		253			25		25
Preferred stock dividends														(3)					(3)
Change in derivative financial instruments					0
Divestiture of derivative financial instruments					0
Change in deferred retirement benefit costs	$ (6)					6									$ 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
End of year (shares) at Dec. 31, 2015	242.6									102.1					25.5
End of year at Dec. 31, 2015	$ 7,088	$ 2	$ 5,616	$ 1,331	$ 0	$ (3)	$ (3)	$ 142			$ 511	1,822	$ 80	$ 1,669		$ 0	$ 2,005	$ 62	$ 825	$ 5	$ 5
Stockholders' equity, end of year at Dec. 31, 2015	$ 6,946								$ 6,946	$ 4,082					$ 2,897
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Return of capital to parent										$ 0		$ 0