Summary Of Significant Accounting Policies (Schedule Of Material And Supplies) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Summary Of Significant Accounting Policies [Abstract]
Fuel	$ 251	[1],[2]	$ 255	[1],[2]
Gas stored underground	171	[1]	175	[1]
Other materials and supplies	290	[1]	277	[1]
Total materials and supplies	$ 712	[1]	$ 707	[1]

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

[2] Consists of coal, oil, paint, propane, and tire chips.