Fair Value Measurements (Schedule Of Transfers Between Fair Value Hierarchy Levels) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Fuel Oils
Derivative [Line Items]
Assets Transfers out of Level 3	$ 2	$ 0	$ 2	$ 0
Power
Derivative [Line Items]
Assets Transfers out of Level 3	0	(1)
Union Electric Company | Fuel Oils
Derivative [Line Items]
Assets Transfers out of Level 3	2	0	2	0
Union Electric Company | Power
Derivative [Line Items]
Assets Transfers out of Level 3	0	(1)
Ameren Illinois Company | Fuel Oils
Derivative [Line Items]
Assets Transfers out of Level 3	0	0	$ 0	$ 0
Ameren Illinois Company | Power
Derivative [Line Items]
Assets Transfers out of Level 3	$ 0	$ 0