Derivative Financial Instruments (Other Derivatives) (Details) (Not Designated As Hedging Instrument [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	$ (1)		$ 18
Fuel Oils [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	(1)	[1]	9	[1]
Natural Gas (Generation) [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	2
Power [Member] | Operating Revenues-Electric [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	$ (2)		$ 9

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