Derivative Financial Instruments (Potential Loss On Counterparty Exposures) (Detail) $ in Millions	Sep. 30, 2015 USD ($)
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$ 11
Ameren Illinois Company
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	0
Union Electric Company
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$ 11