Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Counterparty default risk liability valuation adjustment related to derivative contracts	$ 1	$ 3
Union Electric Company
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
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Counterparty default risk liability valuation adjustment related to derivative contracts	$ 1	$ 3