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, 2013	Sep. 30, 2012		Sep. 30, 2013	Sep. 30, 2012
Derivative [Line Items]
Net fair value of Level 3 transfers	$ (1)	$ (2)		$ 1	$ 183
Fuel Oils
Derivative [Line Items]
Assets Transfers into Level 3					(2)
Fuel Oils | Transfer Into/Out of Level 1
Derivative [Line Items]
Assets Transfers into Level 3					2
Natural Gas
Derivative [Line Items]
Assets Transfers out of Level 3					185
Natural Gas | Transfer Into/Out of Level 2
Derivative [Line Items]
Assets Transfers out of Level 3					185
Power
Derivative [Line Items]
Assets Transfers into Level 3	(1)			3
Assets Transfers out of Level 3		(2)	[1]	4	4
Power | Transfer Into/Out of Level 2
Derivative [Line Items]
Assets Transfers into Level 3	(1)			(3)
Assets Transfers out of Level 3		(2)		4	(4)
Union Electric Company
Derivative [Line Items]
Net fair value of Level 3 transfers	(1)	(2)		1	13
Union Electric Company | Fuel Oils
Derivative [Line Items]
Assets Transfers into Level 3					(2)
Union Electric Company | Fuel Oils | Transfer Into/Out of Level 1
Derivative [Line Items]
Assets Transfers into Level 3					2
Union Electric Company | Natural Gas
Derivative [Line Items]
Assets Transfers out of Level 3					15
Union Electric Company | Natural Gas | Transfer Into/Out of Level 2
Derivative [Line Items]
Assets Transfers out of Level 3					15
Union Electric Company | Power
Derivative [Line Items]
Assets Transfers into Level 3	(1)			3
Assets Transfers out of Level 3		(2)		4	(4)
Union Electric Company | Power | Transfer Into/Out of Level 2
Derivative [Line Items]
Assets Transfers into Level 3	(1)			(3)
Assets Transfers out of Level 3		(2)		4	(4)
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Assets Transfers out of Level 3					170
Ameren Illinois Company | Natural Gas | Transfer Into/Out of Level 2
Derivative [Line Items]
Assets Transfers out of Level 3					170
Ameren Illinois Company | Power
Derivative [Line Items]
Assets Transfers into Level 3
Assets Transfers out of Level 3				$ 0

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