Summary Of Significant Accounting Policies (Schedule Of Amortization Expense) (Details) (Emission Allowances, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ 13		$ 4		$ 3
Union Electric Company
Finite-Lived Intangible Assets [Line Items]
Amortization expense	1	[1]	1	[1]	1	[1]
Ameren Illinois Company
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ 13		$ 4		$ 3

[1] (a)Less than $1 million.