Derivative Financial Instruments (Other Derivatives) (Details) (Not Designated As Hedging Instrument [Member], USD $) In Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	$ (12)	[1]	$ 20	[1]	$ (9)	[1]	$ 33	[1]
Operating Revenues-Electric [Member] | Power [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	2	[1]	13	[1]	(5)	[1]	33	[1]
Heating Oil [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	(14)	[1]	7	[1]	(4)	[1]	1	[1]
Natural Gas [Member] | Operating Expenses-Fuel [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.