Summary Of Significant Accounting Policies (Schedule Of Amortization Expense) (Details) (Emission Allowances [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ 7	[1]	$ 6	[1]	$ 35	[1]
Union Electric Company [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization expense	1	[2]	1	[2]	6
Ameren Illinois Company [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization expense	4		3		7
Other [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ 3	[1],[3]	$ 3	[1],[3]	$ 22	[1],[3]

[1] Includes allowances consumed that were recorded through purchase accounting.

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

[3] Consists of renewable energy credit expense for Marketing Company and emission allowance expense for Genco and AERG