Derivative Financial Instruments (Other Derivatives) (Detail) (Not Designated As Hedging Instrument [Member], 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]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	$ (13)	[1]	$ (14)	[1]	$ (12)	[1]	$ 3	[1]
Coal [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	(6)	[1]			(10)	[1]
Fuel Oils [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	(18)	[1]	(9)	[1]	(13)	[1]	10	[1]
Natural Gas (Generation) [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	4	[1]			5	[1]
Power [Member] | Operating Revenues-Electric [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	7	[1]	(5)	[1]	6	[1]	(7)	[1]
Ameren Missouri [Member] | Natural Gas (Generation) [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives							(1)
Genco [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	(15)		(9)		(14)		6
Genco [Member] | Coal [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	(5)				(8)
Genco [Member] | Fuel Oils [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	(14)		(8)		(10)		7
Genco [Member] | Natural Gas (Generation) [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	4				4
Genco [Member] | Power [Member] | Operating Revenues [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives			$ (1)				$ (1)

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