Derivative Financial Instruments (Cash Flow Hedges) (Details) (Power [Member], Operating Revenues-Electric [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011
Power [Member] | Operating Revenues-Electric [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in OCI	$ 18	[1],[2]	$ (4)	[1],[2]
Amount of (Gain) Loss Reclassified from OCI into Income	4	[2],[3]	1	[2],[3]
Amount of Gain (Loss) Recognized in Income on Derivatives	$ 2	[2],[4]	$ (1)	[2],[4]

[1] Effective portion of gain (loss).

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

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

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