Derivative Financial Instruments (Other Derivatives) (Details) (Not Designated As Hedging Instrument [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	$ 1	[1]	$ 17	[1]
Coal [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	(4)	[1]
Fuel Oils [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	5	[1]	19	[1]
Natural Gas (Generation) [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	1	[1]
Power [Member] | Operating Revenues-Electric [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Nondesignated Derivatives	$ (1)	[1]	$ (2)	[1]

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