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, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	$ 2	$ 2	$ 14	$ 9
Union Electric Company
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	1		7	1	[1]
Ameren Illinois Company
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	$ 1	$ 2	$ 7	$ 9

[1] Less than $1 million.