Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Accounting Policies [Line Items]
Excise tax expense	$ 50	$ 49	$ 110	$ 104
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	39	38	73	71
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 11	$ 11	$ 37	$ 33