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, 2013	Jun. 30, 2012		Mar. 31, 2012	Jun. 30, 2013	Jun. 30, 2012
Derivative [Line Items]
Net fair value of Level 3 transfers	$ 3	$ (1)			$ 2	$ 185
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	3	(1)	[1]		4	(2)	[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	3	(1)			4	(2)
Ameren Missouri [Member]
Derivative [Line Items]
Net fair value of Level 3 transfers	3	(1)			2	15
Ameren Missouri [Member] | Fuel Oils [Member]
Derivative [Line Items]
Assets Transfers into Level 3						(2)
Ameren Missouri [Member] | Fuel Oils [Member] | Transfer Into/Out of Level 1 [Member]
Derivative [Line Items]
Assets Transfers into Level 3						2
Ameren Missouri [Member] | Natural Gas [Member]
Derivative [Line Items]
Assets Transfers out of Level 3						15
Ameren Missouri [Member] | Natural Gas [Member] | Transfer Into/Out of Level 2 [Member]
Derivative [Line Items]
Assets Transfers out of Level 3						15
Ameren Missouri [Member] | Power [Member]
Derivative [Line Items]
Assets Transfers into Level 3					2
Assets Transfers out of Level 3	3	(1)			4	(2)
Ameren Missouri [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	3	(1)			4	(2)
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 out of Level 3

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