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, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Derivative [Line Items]
Net fair value of Level 3 transfers	$ 1	[1]	$ 3	[1]	$ 188	[1]	$ 3	[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	1		1		1		1
Assets Transfers out of Level 3			2		1		2
Power [Member] | Transfer Into/Out of Level 2 [Member]
Derivative [Line Items]
Assets Transfers into Level 3	1	[1]	1	[1]	1	[1]	1	[1]
Assets Transfers out of Level 3			2	[1]	1	[1]	2	[1]
Ameren Missouri [Member]
Derivative [Line Items]
Net fair value of Level 3 transfers	(1)		1		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							(1)
Assets Transfers out of Level 3	(1)		1		(2)		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	(1)		1		(2)		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.