Fair Value Measurements (Schedule Of Transfers Between Fair Value Hierarchy Levels) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Derivative [Line Items]
Net fair value of Level 3 transfers	$ 1	$ (1)
Power
Derivative [Line Items]
Assets Transfers into Level 3		2
Assets Transfers out of Level 3	1	1
Power | Transfer Into/Out of Level 2
Derivative [Line Items]
Assets Transfers into Level 3		(2)
Assets Transfers out of Level 3	1	1
Union Electric Company
Derivative [Line Items]
Net fair value of Level 3 transfers	1	(1)
Union Electric Company | Power
Derivative [Line Items]
Assets Transfers into Level 3		2
Assets Transfers out of Level 3	1	1
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	1
Ameren Illinois Company | Power
Derivative [Line Items]
Assets Transfers into Level 3
Assets Transfers out of Level 3