Summary Of Significant Accounting Policies (Schedule Of Amortization Expense Based On Usage Of Renewable Energy Credits And Emission Allowances) (Detail) (REC and Emission Allowances [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Finite-Lived Intangible Assets [Line Items]
Amortization of Intangible Assets	$ 4		$ 1	[1]
Union Electric Company [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization of Intangible Assets	1	[1]	1	[1]
Ameren Illinois Company [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization of Intangible Assets	$ 4		$ 1	[1]

[1] Less than $1 million.