Consolidated Statement Of Stockholders' Equity (USD $) In Millions, except Share data		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]	Union Electric Company [Member] Accumulated Other Comprehensive Income (Loss) [Member]	Union Electric Company [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]		Ameren Illinois Company [Member]		Ameren Energy Generating Company [Member] Other Paid-In Capital [Member]		Ameren Energy Generating Company [Member] Retained Earnings [Member]		Ameren Energy Generating Company [Member] Derivative Financial Instruments [Member]		Ameren Energy Generating Company [Member] Deferred Retirement Benefit Costs [Member]		Ameren Energy Generating Company [Member] Accumulated Other Comprehensive Income (Loss) [Member]		Ameren Energy Generating Company [Member] Noncontrolling Interest [Member]		Ameren Energy Generating Company [Member] Total Ameren Corporation Stockholders' Equity [Member]		Ameren Energy Generating Company [Member]		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]	Total
Beginning of year at Dec. 31, 2008			$ 1,119	$ 113	$ 1,794	$ 25		$ 1,951	[1]	$ 115	[1]	$ 566	[1]	$ 23	[1]					$ 620	[2]	$ 315	[2]	$ (6)	[2]	$ (61)	[2]			$ 16	[2]					$ 2	$ 4,780	$ 2,181	$ 48	$ (43)		$ 211
Beginning of year (shares) at Dec. 31, 2008																																												212,300,000
Net income (loss)					265		265					247	[1]					247	[1],[3]															162	[2]									624
Net income (loss) attributable to Ameren Company																						160	[2]											160	[2]			612						612	[4]
Shares issued (value)																																					617
Shares issued (number of shares)																																												25,100,000
Stock-based compensation activity																																					15
Capital contribution from parent			436				436	272	[1]									272	[3]
Common stock dividends					(175)							(98)	[1]									(43)	[2]															(338)
Preferred stock dividends					(6)							(6)	[1]
Change in derivative financial instruments						(25)																																	(38)
Change in deferred retirement benefit costs														(4)	[1]			(4)	[1]							19	[2]							21	[2]					20				22
Change in accumulated other comprehensive income from discontinued operations	[1]													6				6
Net income attributable to noncontrolling interest holder																														2	[2]			2	[2]							12		12
Dividends paid to noncontrolling interest holders																														(11)	[2]											(21)
Other																														2	[2]											2
Total stockholders' equity							4,057											3,072	[1]
Stockholders' equity, end of year at Dec. 31, 2009							4,057											3,072	[1]
End of year at Dec. 31, 2009		511	1,555	113	1,878			2,223	[1]	115	[1]	709	[1]	25	[1]	25	[1]			620	[2]	432	[2]	(6)	[2]	(42)	[2]	(48)	[2]	9	[2]	1,004	[2]	1,013	[2]	2	5,412	2,455	10	(23)	(13)	204	7,856	8,060
End of year (shares) at Dec. 31, 2009																																												237,400,000
Net income (loss)					369		369					252						252																(36)										151
Net income (loss) attributable to Ameren Company																						(39)												(39)				139						139	[4]
Shares issued (value)																																					80
Shares issued (number of shares)																																												3,000,000
Stock-based compensation activity																																					14
Regulatory recovery of prior-period common stock issuance costs																																					14
Capital contribution from parent								6												29														5
Contribution of Ameren owned preferred stock without consideration								33		(33)
Transfer of AERG to parent (Notes 1 and 16)								(310)				(281)
Common stock dividends					(235)							(133)																										(368)
Preferred stock dividends					(5)							(4)
Other										1		1																										(1)
Change in derivative financial instruments																																							(10)
Change in deferred retirement benefit costs														(4)				(4)								4								3						6				4
Change in accumulated other comprehensive income from discontinued operations														(1)				(1)
Net income attributable to noncontrolling interest holder																														3				3								12		12
Dividends paid to noncontrolling interest holders																																										(8)
Redemptions of preferred stock				(33)						(19)																																(52)
Other																														(1)												(2)
Total stockholders' equity							4,153											2,576																998										7,730
Stockholders' equity, end of year at Dec. 31, 2010							4,153											2,576																998										7,730
End of year at Dec. 31, 2010		511	1,555	80	2,007			1,952		62		542		20		20				649		393		(6)		(38)		(44)		11		998		1,009		2	5,520	2,225		(17)	(17)	154	7,730	7,884
End of year (shares) at Dec. 31, 2010							102,100,000											25,500,000																2,000										240,400,000
Net income (loss)					290		290					196						196																45										526
Net income (loss) attributable to Ameren Company																						44												44				519						519	[4]
Shares issued (value)																																					65
Shares issued (number of shares)																																												2,200,000
Stock-based compensation activity																																					13
Capital contribution from parent								13										19		4														28
Common stock dividends					(403)							(327)																										(375)
Preferred stock dividends					(3)							(3)
Change in derivative financial instruments																								1															7
Change in deferred retirement benefit costs														(3)				(3)								(29)								(34)						(40)				(46)
Net income attributable to noncontrolling interest holder																														1				1								7		7
Dividends paid to noncontrolling interest holders																																										(6)
Other																														(5)												(6)
Total stockholders' equity							4,037											2,452																1,018										7,919
Stockholders' equity, end of year at Dec. 31, 2011							4,037											2,452																1,018										7,919
End of year at Dec. 31, 2011		$ 511	$ 1,555	$ 80	$ 1,891			$ 1,965		$ 62		$ 408		$ 17		$ 17				$ 653		$ 437		$ (5)		$ (67)		$ (72)		$ 7		$ 1,018		$ 1,025		$ 2	$ 5,598	$ 2,369	$ 7	$ (57)	$ (50)	$ 149	$ 7,919	$ 8,068
End of year (shares) at Dec. 31, 2011							102,100,000											25,500,000																2,000										242,600,000

[1] Prior period has been adjusted to reflect the AIC Merger as discussed in Note 1 - Summary of Significant Accounting Policies.

[2] Prior period has been adjusted to include EEI as discussed in Note 1 - Summary of Significant Accounting Policies.

[3] Prior period has been adjusted to reflect the Ameren Illinois Merger as discussed in Note 1 - Summary of Significant Accounting Policies.

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