Fair Value Measurements (Schedule Of Transfers Between Fair Value Hierarchy Levels) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Derivative [Line Items]
Net fair value of Level 3 transfers	$ (1)	$ 186
Fuel Oils [Member]
Derivative [Line Items]
Assets Transfers into Level 3		2
Fuel Oils [Member] | Transfer Into/Out of Level 1 [Member]
Derivative [Line Items]
Assets Transfers into Level 3		2
Natural Gas [Member]
Derivative [Line Items]
Assets Transfers out of Level 3		185
Natural Gas [Member] | Transfer Into/Out of Level 2 [Member]
Derivative [Line Items]
Assets Transfers out of Level 3		185
Power [Member]
Derivative [Line Items]
Assets Transfers into Level 3	(2)
Assets Transfers out of Level 3	1	(1)	[1]
Power [Member] | Transfer Into/Out of Level 2 [Member]
Derivative [Line Items]
Assets Transfers into Level 3	(2)
Assets Transfers out of Level 3	1	(1)
Union Electric Company [Member]
Derivative [Line Items]
Net fair value of Level 3 transfers	(1)	16
Union Electric Company [Member] | Fuel Oils [Member]
Derivative [Line Items]
Assets Transfers into Level 3		2
Union Electric Company [Member] | Fuel Oils [Member] | Transfer Into/Out of Level 1 [Member]
Derivative [Line Items]
Assets Transfers into Level 3		2
Union Electric Company [Member] | Natural Gas [Member]
Derivative [Line Items]
Assets Transfers out of Level 3		15
Union Electric Company [Member] | Natural Gas [Member] | Transfer Into/Out of Level 2 [Member]
Derivative [Line Items]
Assets Transfers out of Level 3		15
Union Electric Company [Member] | Power [Member]
Derivative [Line Items]
Assets Transfers into Level 3	(2)
Assets Transfers out of Level 3	1	(1)	[1]
Union Electric Company [Member] | Power [Member] | Transfer Into/Out of Level 2 [Member]
Derivative [Line Items]
Assets Transfers into Level 3	(2)
Assets Transfers out of Level 3	1	(1)
Ameren Illinois Company [Member] | Natural Gas [Member]
Derivative [Line Items]
Assets Transfers out of Level 3		170
Ameren Illinois Company [Member] | Natural Gas [Member] | Transfer Into/Out of Level 2 [Member]
Derivative [Line Items]
Assets Transfers out of Level 3		170
Ameren Illinois Company [Member] | Power [Member]
Derivative [Line Items]
Assets Transfers into Level 3
Assets Transfers out of Level 3			[1]

[1] Ameren amounts include the elimination of financial power contracts between Ameren Illinois and Marketing Company.