Segment Information (Details) (USD $) In Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010		Dec. 31, 2010
Segment Reporting Information [Line Items]
External revenues	$ 2,268		$ 2,267		$ 5,953		$ 5,932
Intersegment revenues
Net income (loss) attributable to Ameren Corporation	285	[1]	(167)	[1]	494	[1]	87	[1]
Total assets	23,356				23,356				23,515
Ameren Missouri [Member]
Segment Reporting Information [Line Items]
External revenues	1,109		1,053		2,690		2,486
Intersegment revenues	6		7		19		17
Net income (loss) attributable to Ameren Corporation	190	[1]	223	[1]	301	[1]	363	[1]
Total assets	12,638				12,638				12,504
Ameren Illinois Company [Member]
Segment Reporting Information [Line Items]
External revenues	742		743		2,166		2,296
Intersegment revenues	3		3		10		8
Net income (loss) attributable to Ameren Corporation	98	[1]	90	[1]	168	[1]	171	[1]
Total assets	7,064				7,064				7,406
Merchant Generation [Member]
Segment Reporting Information [Line Items]
External revenues	415		470		1,094		1,149
Intersegment revenues	67		44		163		178
Net income (loss) attributable to Ameren Corporation	(9)	[1]	(470)	[1]	26	[1]	(428)	[1]
Total assets	3,794				3,794				3,934
Other Segment [Member]
Segment Reporting Information [Line Items]
External revenues	2		1		3		1
Intersegment revenues	2		4		3		10
Net income (loss) attributable to Ameren Corporation	6	[1]	(10)	[1]	(1)	[1]	(19)	[1]
Total assets	1,209				1,209				1,354
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Intersegment revenues	(78)		(58)		(195)		(213)
Total assets	$ (1,349)				$ (1,349)				$ (1,683)

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