Derivative Financial Instruments (Cash Flow Hedges) (Details) (USD $) In Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010
Power [Member]
Amount of Gain (Loss) Recognized in OCI	$ (3)	[1],[2]	$ (16)	[1],[2]	$ (7)	[1],[2]	$ 10	[1],[2]
Operating Revenues-Electric [Member] | Power [Member]
Amount of (Gain) Loss Reclassified from OCI into Income	1	[2],[3]	(10)	[2],[3]	2	[2],[3]	(14)	[2],[3]
Amount of Gain (Loss) Recognized in Income on Derivatives	3	[2],[4]	(13)	[2],[4]	2	[2],[4]	(13)	[2],[4]
Interest Rate Contract [Member]
Amount of Gain (Loss) Recognized in OCI		[1],[2],[5]		[1],[2],[5]		[1],[2],[5]		[1],[2],[5]
Interest Charges [Member] | Interest Rate Contract [Member]
Amount of Gain (Loss) Recognized in Income on Derivatives				[2],[4],[5]		[2],[4],[5]		[2],[4],[5]
Cash Flow Hedge Gain (Loss) Reclassified to Interest Expense, Net	$ 1				$ 1

[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.

[5] Represents interest rate swaps settled in prior periods. The cumulative gain and loss on the interest rate swaps is being amortized into income over a 10-year period.