Derivative Financial Instruments (Cash Flow Hedges) (Detail) (Power [Member], Operating Revenues-Electric [Member], USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Power [Member] | Operating Revenues-Electric [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in OCI	$ 5	[1],[2]	$ (3)	[1],[2]	$ 23	[1],[2]	$ (7)	[1],[2]
Amount of (Gain) Loss Reclassified from OCI into Income	2	[2],[3]	1	[2],[3]	6	[2],[3]	2	[2],[3]
Amount of Gain (Loss) Recognized in Income on Derivatives	$ (1)	[2],[4]	$ 3	[2],[4]	$ 1	[2],[4]	$ 2	[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.