Schedule I - Condensed Financial Information Of Parent (Statement of Income) (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
Operating revenue	$ 1,578	$ 2,268	$ 1,781	$ 1,904	$ 1,706	$ 2,267	$ 1,725	$ 1,940	$ 7,531		$ 7,638		$ 7,135
Goodwill and other impairment charges									125	[1]	589	[1]	7	[1]
Operating expenses									6,290		6,722		5,719
Operating income (loss)	148	550	316	227	198	89	331	298	1,241		916		1,416
Interest income affiliates									4	[2]	5	[2]	2	[2]
Miscellaneous income									69	[2]	90	[2]	71	[2]
Interest charges									451		497		508
Income tax (benefit)									310	[3]	325	[3]	332	[3]
Net Income	25	285	138	71	52	(167)	152	102	519	[4]	139	[4]	612	[4]
Parent Company [Member]
Goodwill and other impairment charges											372
Operating expenses									15		24		20
Operating income (loss)									(15)		(396)		(20)
Equity in earnings of subsidiaries									527		535		625
Interest income affiliates									44		28		36
Miscellaneous income									4		3		4
Interest charges									41		56		37
Income tax (benefit)									(8)		(31)		(12)
Net Income									$ 519		$ 139		$ 612

[1] Includes amounts for registrant and nonregistrant subsidiaries.

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

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

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