Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Segment Reporting Information [Line Items]
External revenues	$ 1,578	$ 2,268	$ 1,781	$ 1,904	$ 1,706	$ 2,267	$ 1,725	$ 1,940	$ 7,531		$ 7,638		$ 7,135
Depreciation and amortization									785		765		725
Interest and dividend income									32		33		30
Interest charges									451		497		508
Income taxes (benefit)									310	[1]	325	[1]	332	[1]
Net income (loss) attributable to Ameren Corporation	25	285	138	71	52	(167)	152	102	519	[2]	139	[2]	612	[2]
Capital expenditures									1,030		1,042		1,710
Total assets	23,645				23,511				23,645		23,511		23,702
Ameren Missouri [Member]
Segment Reporting Information [Line Items]
External revenues									3,358		3,176		2,847
Intersegment revenues									25		21		27
Depreciation and amortization									408		382		357
Interest and dividend income									30		31		29
Interest charges									209		213		229
Income taxes (benefit)									161		199		128
Net income (loss) attributable to Ameren Corporation									287	[2]	364	[2]	259	[2]
Capital expenditures									550		624		882
Total assets	12,757				12,504				12,757		12,504		12,219
Ameren Illinois Company [Member]
Segment Reporting Information [Line Items]
External revenues									2,774		3,002		2,957
Intersegment revenues									13		12		27
Depreciation and amortization									215		210		216
Interest and dividend income									1		1		6
Interest charges									136		143		153
Income taxes (benefit)									127		137		79
Net income (loss) attributable to Ameren Corporation									193	[2]	208	[2]	127	[2]
Capital expenditures									351		281		352
Total assets	7,213				7,406				7,213		7,406		7,181
Merchant Generation [Member]
Segment Reporting Information [Line Items]
External revenues									1,394		1,459		1,322
Intersegment revenues									235		234		390
Depreciation and amortization									143		146		126
Interest and dividend income											1
Interest charges									105		133		119
Income taxes (benefit)									32		6		151
Net income (loss) attributable to Ameren Corporation									45	[2]	(409)	[2]	247	[2]
Capital expenditures									153		101		408
Total assets	3,833				3,934				3,833		3,934		4,751
Other Segment [Member]
Segment Reporting Information [Line Items]
External revenues									5		1		9
Intersegment revenues									4		13		19
Depreciation and amortization									19		27		26
Interest and dividend income									44		25		33
Interest charges									44		35		48
Income taxes (benefit)									(10)		(17)		(26)
Net income (loss) attributable to Ameren Corporation									(6)	[2]	(24)	[2]	(21)	[2]
Capital expenditures									(24)	[2]	36		68
Total assets	1,211				1,354				1,211		1,354		1,814
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Intersegment revenues									(277)		(280)		(463)
Interest and dividend income									(43)		(25)		(38)
Interest charges									(43)		(27)		(41)
Total assets	$ (1,369)				$ (1,687)				$ (1,369)		$ (1,687)		$ (2,263)

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

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