Segment Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011		Dec. 31, 2011
Segment Reporting Information [Line Items]
External revenues	$ 1,660		$ 1,781		$ 3,318		$ 3,685
Net income (loss) attributable to Ameren Corporation	211	[1]	138	[1]	(192)	[1]	209	[1]
Total assets	22,980				22,980				23,645
Ameren Missouri [Member]
Segment Reporting Information [Line Items]
External revenues	838		814		1,524		1,581
Intersegment revenues	6		8		11		13
Net income (loss) attributable to Ameren Corporation	143	[1]	90	[1]	164	[1]	111	[1]
Total assets	12,680				12,680				12,757
Ameren Illinois Company [Member]
Segment Reporting Information [Line Items]
External revenues	561		619		1,282		1,424
Intersegment revenues	3		4		6		7
Net income (loss) attributable to Ameren Corporation	32	[1]	37	[1]	59	[1]	70	[1]
Total assets	7,127				7,127				7,213
Merchant Generation [Member]
Segment Reporting Information [Line Items]
External revenues	261		347		510		679
Intersegment revenues	72		49		159		96
Net income (loss) attributable to Ameren Corporation	(5)	[1]	15	[1]	(368)	[1]	35	[1]
Total assets	3,254				3,254				3,833
Other Segment [Member]
Segment Reporting Information [Line Items]
External revenues			1		2		1
Intersegment revenues	2				2		1
Net income (loss) attributable to Ameren Corporation	41	[1]	(4)	[1]	(47)	[1]	(7)	[1]
Total assets	1,196				1,196				1,211
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Intersegment revenues	(83)		(61)		(178)		(117)
Total assets	$ (1,277)				$ (1,277)				$ (1,369)

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