Summary Of Significant Accounting Policies (Schedule Of Material And Supplies) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Accounting Policies [Line Items]
Fuel	$ 276	[1],[2]	$ 251	[1],[2]
Gas stored underground	131	[2]	171	[2]
Other materials and supplies	297	[2]	290	[2]
Total materials and supplies	704	[2]	712	[2]
Union Electric Company [Member]
Accounting Policies [Line Items]
Fuel	198	[1]	150	[1]
Gas stored underground	18		22
Other materials and supplies	181		176
Total materials and supplies	397		348
Ameren Illinois Company [Member]
Accounting Policies [Line Items]
Fuel		[1]		[1]
Gas stored underground	113		149
Other materials and supplies	60		50
Total materials and supplies	$ 173		$ 199

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

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