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			6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	$ 3	$ 1	[1]	$ 7		$ 1	[1]
Ameren Missouri [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets		1	[1]	1	[1]	1	[1]
Ameren Illinois Company [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	$ 3	$ 1	[1]	$ 7		$ 1	[1]

[1] Less than $1 million.