Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Accounting Policies [Line Items]
Excise tax expense	$ 51	$ 50	$ 108	$ 110
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	41	39	75	73
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 10	$ 11	$ 33	$ 37