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		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	$ 3	$ 3	$ 12	$ 7
Union Electric Company
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets			6	1	[1]
Ameren Illinois Company
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	$ 3	$ 3	$ 6	$ 7

[1] Less than $1 million.