Derivative Financial Instruments (Cash Flow Hedges) (Details) (Power [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 OCI	$ (5)	[1],[2]	$ 5	[1],[2]	$ (12)	[1],[2]	$ 15	[1],[2]
Operating Revenues-Electric [Member]
Derivative [Line Items]
Amount of (Gain) Loss Reclassified from OCI into Income	(1)	[2]	(4)	[2]	1	[2]	(18)	[2]
Amount of Gain (Loss) Recognized in Income on Derivatives	$ (8)	[2]	$ 7	[2]	$ (6)	[2]	$ (6)	[2]

[1] Effective portion of gain (loss).

[2] Includes amounts from Ameren registrant and nonregistrant subsidiaries.