Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Accounting Policies [Line Items]
Excise tax expense	$ 61	$ 56	$ 169	$ 166
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	52	47	127	120
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 9	$ 9	$ 42	$ 46