Consolidated Statement Of Stockholders' Equity (USD $) In Millions, unless otherwise specified	Total		Common Stock [Member]	Other Paid-In Capital [Member]	Retained Earnings [Member]	Derivative Financial Instruments [Member]	Deferred Retirement Benefit Costs [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Noncontrolling Interest [Member]	Total Ameren Corporation Stockholders' Equity [Member]	Union Electric Company [Member]	Union Electric Company [Member] Common Stock [Member]	Union Electric Company [Member] Other Paid-In Capital [Member]	Union Electric Company [Member] Preferred Stock Not Subject To Mandatory Redemption [Member]	Union Electric Company [Member] Retained Earnings [Member]	Ameren Illinois Company [Member]	Ameren Illinois Company [Member] Common Stock [Member]	Ameren Illinois Company [Member] Other Paid-In Capital [Member]	Ameren Illinois Company [Member] Preferred Stock Not Subject To Mandatory Redemption [Member]	Ameren Illinois Company [Member] Retained Earnings [Member]	Ameren Illinois Company [Member] Deferred Retirement Benefit Costs [Member]	Ameren Illinois Company [Member] Accumulated Other Comprehensive Income (Loss) [Member]
Beginning of year at Dec. 31, 2009			$ 2	$ 5,412	$ 2,455	$ 10	$ (23)		$ 204				$ 1,555	$ 113	$ 1,878			$ 2,223	$ 115	$ 709	$ 25
Beginning of year (shares) at Dec. 31, 2009	237.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	151										369				369	252				252
Net income (loss) attributable to Ameren Corporation	139	[1]			139						369					252
Shares issued (value)				80
Shares issued (number of shares)	3.0
Stock-based compensation activity				14
Regulatory recovery of prior-period common stock issuance costs				14
Capital contribution from parent																		6
Contribution of Ameren owned preferred stock without consideration																		33	(33)
Transfer of AERG to parent (Notes 1 and 16)																		(310)		(281)
Common stock dividends					(368)										(235)					(133)
Preferred stock dividends															(5)					(4)
Other					(1)														1	1
Change in derivative financial instruments						(10)
Change in deferred retirement benefit costs	(4)						6															(4)
Change in accumulated other comprehensive income from discontinued operations																(1)						(1)
Net income attributable to noncontrolling interest holder	12								12
Dividends paid to noncontrolling interest holders									(8)
Redemptions of preferred stock									(52)					(33)					(19)
Other									(2)
Total stockholders' equity										7,730	4,153					2,576
Stockholders' equity, end of year at Dec. 31, 2010										7,730	4,153					2,576
End of year at Dec. 31, 2010	7,884		2	5,520	2,225		(17)	(17)	154			511	1,555	80	2,007			1,952	62	542	20	20
End of year (shares) at Dec. 31, 2010	240.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Ameren Corporation	71	[2]									22					34
Stockholders' equity, end of year at Mar. 31, 2011
Beginning of year at Dec. 31, 2010	7,884		2	5,520	2,225		(17)	(17)	154			511		80	2,007			1,952	62	542	20	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	[1]			519						290					196
Shares issued (value)				65
Shares issued (number of shares)	2.2
Stock-based compensation activity				13
Regulatory recovery of prior-period common stock issuance costs
Capital contribution from parent																19		13
Contribution of Ameren owned preferred stock without consideration
Transfer of AERG to parent (Notes 1 and 16)
Common stock dividends					(375)										(403)					(327)
Preferred stock dividends															(3)					(3)
Other
Change in derivative financial instruments						7
Change in deferred retirement benefit costs	46						(40)															(3)
Change in accumulated other comprehensive income from discontinued operations
Net income attributable to noncontrolling interest holder	7								7
Dividends paid to noncontrolling interest holders									(6)
Redemptions of preferred stock
Other									(6)
Total stockholders' equity	7,919									7,919	4,037					2,452
Stockholders' equity, end of year at Dec. 31, 2011	7,919									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		80	1,891			1,965	62	408	17	17
End of year (shares) at Dec. 31, 2011	242.6										102.1					25.5
Beginning of year at Sep. 30, 2011
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Ameren Corporation	25	[2]									(14)					26
Total stockholders' equity	7,919									7,919	4,037					2,452
Stockholders' equity, end of year at Dec. 31, 2011	7,919									7,919	4,037					2,452
End of year at Dec. 31, 2011	8,068		2					(50)				511									17
End of year (shares) at Dec. 31, 2011	242.6										102.1					25.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Ameren Corporation	(403)	[2]									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		1,891			1,965		408	17	17
Beginning of year (shares) at Dec. 31, 2011	242.6										102.1					25.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(974)										419				419	144				144
Net income (loss) attributable to Ameren Corporation	(974)	[1]			(974)						419					144
Shares issued (value)
Shares issued (number of shares)
Stock-based compensation activity				18
Capital contribution from parent											1		1
Common stock dividends					(389)										(400)					(189)
Preferred stock dividends															(3)					(3)
Other
Change in derivative financial instruments						18
Change in deferred retirement benefit costs	(32)						24															(3)
Change in accumulated other comprehensive income from discontinued operations
Net income attributable to noncontrolling interest holder
Dividends paid to noncontrolling interest holders									(6)
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) attributable to Ameren Corporation	(1,156)	[2]									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

[1] Represents net income (loss) available to common stockholders.

[2] The sum of quarterly amounts, including per share amounts, may not equal amounts reported for year-to-date periods. This is due to the effects of rounding and changes in the number of weighted-average shares outstanding each period.