Fair Value Measurements (Schedule Of Transfers Between Fair Value Hierarchy Levels) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2013
Derivative [Line Items]
Net fair value of Level 3 transfers			$ 2
Power
Derivative [Line Items]
Assets Transfers out of Level 3	(1)	3	4
Power | Transfer Into/Out of Level 2
Derivative [Line Items]
Assets Transfers into Level 3			(2)
Assets Transfers out of Level 3	(1)	3	4
Union Electric Company
Derivative [Line Items]
Net fair value of Level 3 transfers			2
Union Electric Company | Power
Derivative [Line Items]
Assets Transfers out of Level 3	(1)	3	4
Union Electric Company | Power | Transfer Into/Out of Level 2
Derivative [Line Items]
Assets Transfers into Level 3			(2)
Assets Transfers out of Level 3	(1)	3	4
Ameren Illinois Company
Derivative [Line Items]
Net fair value of Level 3 transfers
Ameren Illinois Company | Power
Derivative [Line Items]
Assets Transfers out of Level 3
Ameren Illinois Company | Power | Transfer Into/Out of Level 2
Derivative [Line Items]
Assets Transfers into Level 3
Assets Transfers out of Level 3