Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Accounting Policies [Line Items]
Excise tax expense	$ 60	$ 55
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	34	33
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 26	$ 22