Derivative Financial Instruments (Other Derivatives) (Detail) (Not Designated As Hedging Instrument [Member], 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]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	$ 9	[1]	$ (12)	[1]	$ (3)	[1]	$ (9)	[1]
Coal [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives					(10)	[1]
Fuel Oils [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	6	[1]	(14)	[1]	(7)	[1]	(4)	[1]
Natural Gas (Generation) [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	(1)	[1]			4	[1]
Power [Member] | Operating Revenues-Electric [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	4	[1]	2	[1]	10	[1]	(5)	[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	4		(11)		(10)		(5)
Genco [Member] | Coal [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives					(8)
Genco [Member] | Fuel Oils [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	5		(10)		(5)		(3)
Genco [Member] | Natural Gas (Generation) [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	(1)		1		3		1
Genco [Member] | Power [Member] | Operating Revenues-Electric [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives			$ (2)				$ (3)

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