Goodwill, Impairment And Other Charges (Summary Of Goodwill And Other Asset Impairment Pretax Charges) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Goodwill, Impairment And Other Charges [Abstract]
Impairment charge on goodwill			$ 420	[1],[2]
Impairment charge on long-lived assets and related charges	123	[1]	101	[1]	7	[1]
Impairment charge on emission allowances	2	[1]	68	[1]
Total impairment charge	$ 125	[1]	$ 589	[1]	$ 7	[1]

[1] Includes amounts for registrant and nonregistrant subsidiaries.

[2] Includes amounts for Ameren registrants and nonregistrant subsidiaries.