Derivative Financial Instruments (Cash Flow Hedges) (Details) (Power [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 OCI	$ 6	[1],[2]	$ (2)	[1],[2]
Operating Revenues-Electric [Member]
Derivative [Line Items]
Amount of (Gain) Loss Reclassified from OCI into Income	5	[1],[3]	(14)	[1],[3]
Amount of Gain (Loss) Recognized in Income on Derivatives	$ (10)	[1],[4]	$ (3)	[1],[4]

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

[2] Effective portion of gain (loss).

[3] Effective portion of (gain) loss on settlements.

[4] Ineffective portion of gain (loss) and amount excluded from effectiveness testing.