Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain (loss) recognized related to valuation adjustments for counterparty default risk	$ (1)	$ (2)	$ 1
Counterparty default risk liability valuation adjustment related to derivative contracts	7	1
Union Electric Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Counterparty default risk liability valuation adjustment related to derivative contracts	1	1
Ameren Illinois Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Counterparty default risk liability valuation adjustment related to derivative contracts	$ 7	$ 19