Derivative Financial Instruments (Other Derivatives) (Details) (Not Designated as Hedging Instrument [Member], USD $) In Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	$ (14)	[1]	$ (18)	[1]	$ 3	[1]	$ 13	[1]
Operating Revenues-Electric [Member] | Power [Member]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	(5)	[1]	(11)	[1]	(7)	[1]	20	[1]
Heating Oil [Member] | Operating Expenses-Fuel [Member]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	(9)	[1]	(7)	[1]	10	[1]	(6)	[1]
Natural Gas [Member] | Operating Expenses-Fuel [Member]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives							$ (1)	[1]

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