Derivative Financial Instruments (Potential Loss On Counterparty Exposures) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$ 13	$ 6
Ameren Illinois Company
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	6
Union Electric Company
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$ 7	$ 6