Selected Quarterly Information (Summary Of Selected Quarterly Information) (Details) (USD $) In Millions, except Per Share data, 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
Selected Quarterly Information [Abstract]
Operating revenue	$ 1,578	$ 2,268	$ 1,781	$ 1,904	$ 1,706	$ 2,267	$ 1,725	$ 1,940	$ 7,531		$ 7,638		$ 7,135
Operating Income	148	550	316	227	198	89	331	298	1,241		916		1,416
Net income (loss) attributable to Ameren Corporation	$ 25	$ 285	$ 138	$ 71	$ 52	$ (167)	$ 152	$ 102	$ 519	[1]	$ 139	[1]	$ 612	[1]
Earnings per Common Share - Basic and Diluted	$ 0.10	$ 1.18	$ 0.57	$ 0.29	$ 0.21	$ (0.70)	$ 0.64	$ 0.43	$ 2.15		$ 0.58		$ 2.78

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