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 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, 2010		$ 2	$ 5,520	$ 2,225	$ 0	$ (17)		$ 154				$ 1,555		$ 2,007			$ 1,952	$ 62	$ 542	$ 20
Beginning of year (shares) at Dec. 31, 2010	240.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	526									290				290	196				196
Net income (loss) attributable to Ameren Corporation	519			519						290					196
Shares issued (value)			65
Shares issued (number of shares)	2.2
Stock-based compensation activity			13
Capital contribution from parent															19		13
Common stock dividends				(375)										(403)					(327)
Preferred stock dividends														(3)					(3)
Change in derivative financial instruments					7
Divestiture of derivative financial instruments					0
Change in deferred retirement benefit costs	19					(40)									3						(3)
Divestiture of deferred retirement benefit costs						0
Net income attributable to noncontrolling interest holder								7
Dividends paid to noncontrolling interest holders								(6)
Divestiture of noncontrolling interest								0
Other								(6)
Total stockholders' equity									7,919	4,037					2,452
Stockholders' equity, end of year at Dec. 31, 2011									7,919	4,037					2,452
End of year at Dec. 31, 2011	8,068	2	5,598	2,369	7	(57)	(50)	149			511	1,555	80	1,891			1,965	62	408	17	17
End of year (shares) at Dec. 31, 2011	242.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(403)
Net income (loss) attributable to Ameren Corporation	(403)									22					28
Stockholders' equity, end of year at Mar. 31, 2012
Beginning of year at Dec. 31, 2011	8,068	2	5,598	2,369	7	(57)	(50)	149			511	1,555	80	1,891			1,965	62	408	17	17
Beginning of year (shares) at Dec. 31, 2011	242.6
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
Shares issued (value)			0
Shares issued (number of shares)	0
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
Total stockholders' equity	6,616								6,616	4,054					2,401
Stockholders' equity, end of year at Dec. 31, 2012	6,616								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									102.1					25.5
Beginning of year at Sep. 30, 2012
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(1,155)
Net income (loss) attributable to Ameren Corporation	(1,156)									16					12
Total stockholders' equity	6,616								6,616	4,054					2,401
Stockholders' equity, end of year at Dec. 31, 2012	6,616								6,616	4,054					2,401
End of year at Dec. 31, 2012	6,767	2					(8)				511		80					62		14
End of year (shares) at Dec. 31, 2012	242.6									102.1					25.5
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									102.1					25.5
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
Shares issued (value)
Shares issued (number of shares)
Stock-based compensation activity			16
Capital contribution from parent										4		4
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
Total stockholders' equity	6,544								6,544	3,993					2,448
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		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
Total stockholders' equity	6,544								6,544	3,993					2,448
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							$ 11
End of year (shares) at Dec. 31, 2013	242.6									102.1					25.5