Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011		Dec. 31, 2011
Segment Reporting Information [Line Items]
External revenues	$ 1,658		$ 1,904
Net Income(Loss)	(403)	[1]	71	[1]
Total assets	22,899				23,645
Ameren Missouri [Member]
Segment Reporting Information [Line Items]
External revenues	686		767
Intersegment revenues	5		5
Net Income(Loss)	21	[1]	21	[1]
Total assets	12,546				12,757
Ameren Illinois Company [Member]
Segment Reporting Information [Line Items]
External revenues	721		805
Intersegment revenues	3		3
Net Income(Loss)	27	[1]	33	[1]
Total assets	7,256				7,213
Merchant Generation [Member]
Segment Reporting Information [Line Items]
External revenues	249		332
Intersegment revenues	87		47
Net Income(Loss)	(363)	[1]	20	[1]
Total assets	3,266				3,833
Other Segment [Member]
Segment Reporting Information [Line Items]
External revenues	1
Intersegment revenues	1		1
Net Income(Loss)	(88)	[1]	(3)	[1]
Total assets	1,261				1,211
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
External revenues	1
Intersegment revenues	(96)		(56)
Total assets	$ (1,430)				$ (1,369)

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