Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Accounting Policies [Line Items]
Excise tax expense	$ 193	$ 194	$ 189
Union Electric Company [Member]
Accounting Policies [Line Items]
Excise tax expense	139	137	130
Ameren Illinois Company [Member]
Accounting Policies [Line Items]
Excise tax expense	$ 54	$ 57	$ 59