Segment Information (Details) (USD $) In Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010		Dec. 31, 2010
External revenues	$ 1,781		$ 1,725		$ 3,685		$ 3,665
Net income (loss) attributable to Ameren Corporation	138	[1]	152	[1]	209	[1]	254	[1]
Total assets	23,391				23,391				23,515
Ameren Missouri [Member]
External revenues	814		756		1,581		1,433
Intersegment revenues	8		5		13		10
Net income (loss) attributable to Ameren Corporation	90	[1]	113	[1]	111	[1]	140	[1]
Total assets	12,527				12,527				12,504
Ameren Illinois [Member]
External revenues	619		644		1,424		1,553
Intersegment revenues	4		3		7		5
Net income (loss) attributable to Ameren Corporation	37	[1]	46	[1]	70	[1]	81	[1]
Total assets	7,154				7,154				7,406
Merchant Generation [Member]
External revenues	347		325		679		679
Intersegment revenues	49		60		96		134
Net income (loss) attributable to Ameren Corporation	15	[1]	(2)	[1]	35	[1]	42	[1]
Total assets	3,861				3,861				3,934
Other [Member]
External revenues	1				1
Intersegment revenues			3		1		6
Net income (loss) attributable to Ameren Corporation	(4)	[1]	(5)	[1]	(7)	[1]	(9)	[1]
Total assets	1,324				1,324				1,354
Intersegment Eliminations [Member]
Intersegment revenues	(61)		(71)		(117)		(155)
Total assets	$ (1,475)				$ (1,475)				$ (1,683)

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