Summary Of Significant Accounting Policies (Schedule Of Amortization Expense Based On Usage Of Renewable Energy Credits And Emission Allowances) (Detail) (REC and Emission Allowances, USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013		Sep. 30, 2012
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	$ 2	$ 2	$ 9		$ 2
Union Electric Company
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets		1	1	[1]	1
Ameren Illinois Company
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	$ 2	$ 1	$ 9		$ 1

[1] Less than $1 million.