Fair Value Measurements (Schedule Of Transfers Between Fair Value Hierarchy Levels) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Derivative [Line Items]
Net fair value of Level 3 transfers	$ (3)	[1]	$ (4)	[1]	$ 185	[1]	$ (1)	[1]
Fuel Oils [Member]
Derivative [Line Items]
Assets Transfers into Level 3					2
Assets Transfers out of Level 3	1
Fuel Oils [Member] | Transfer Into/Out of Level 1 [Member]
Derivative [Line Items]
Assets Transfers into Level 3					2	[1]
Assets Transfers out of Level 3	1	[1]
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	[1]
Power [Member]
Derivative [Line Items]
Assets Transfers into Level 3			(2)		1		(1)
Assets Transfers out of Level 3	(4)		(2)		(3)
Power [Member] | Transfer Into/Out of Level 2 [Member]
Derivative [Line Items]
Assets Transfers into Level 3			(2)	[1]	1	[1]	(1)	[1]
Assets Transfers out of Level 3	(4)	[1]	(2)	[1]	(3)	[1]
Ameren Missouri [Member]
Derivative [Line Items]
Net fair value of Level 3 transfers	(2)				13
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							(1)
Assets Transfers out of Level 3	(2)				(4)		1
Ameren Missouri [Member] | Power [Member] | Transfer Into/Out of Level 2 [Member]
Derivative [Line Items]
Assets Transfers into Level 3							(1)
Assets Transfers out of Level 3	(2)				(4)		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
Genco [Member] | Fuel Oils [Member]
Derivative [Line Items]
Assets Transfers out of Level 3	1
Genco [Member] | Fuel Oils [Member] | Transfer Into/Out of Level 1 [Member]
Derivative [Line Items]
Assets Transfers out of Level 3	$ 1

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries.