Derivative Financial Instruments (Potential Loss On Counterparty Exposures) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$ 5	$ 6
Union Electric Company
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	3	6
Ameren Illinois Company
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$ 2