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
	Mar. 31, 2014		Mar. 31, 2013
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	$ 9		$ 4
Union Electric Company
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	6	[1]	1	[2]
Ameren Illinois Company
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	$ 3		$ 4

[1] igher priced renewable energy credits.

[2] Less than $1 million.