Summary Of Significant Accounting Policies (Schedule Of Material And Supplies) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Accounting Policies [Line Items]
Fuel	$ 144	[1]	$ 198	[1]
Gas stored underground	127		131
Other materials and supplies	255		241
Total materials and supplies	526		570
Union Electric Company
Accounting Policies [Line Items]
Fuel	144	[1]	198	[1]
Gas stored underground	17		18
Other materials and supplies	191		181
Total materials and supplies	352		397
Ameren Illinois Company
Accounting Policies [Line Items]
Fuel		[1]		[1]
Gas stored underground	110		113
Other materials and supplies	64		60
Total materials and supplies	$ 174		$ 173

[1] (a)Consists of coal, oil, and propane.