Derivative Financial Instruments (Cash Flow Hedges) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Interest Charges [Member]
Derivative [Line Items]
(Gain) Loss Reclassified from Accumulated OCI	$ 1	[1],[2]
Power [Member]
Derivative [Line Items]
Gain (Loss) Recognized in OC	34	[3],[4]	6	[3],[4]
Power [Member] | Operating Revenues-Electric [Member]
Derivative [Line Items]
(Gain) Loss Reclassified from Accumulated OCI	(6)	[1],[4]	5	[1],[4]
Gain (Loss) Recognizedin Income	(12)	[4],[5]	(10)	[4],[5]
Maximum [Member] | Interest Charges [Member]
Derivative [Line Items]
(Gain) Loss Reclassified from Accumulated OCI			$ 1

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

[2] 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.

[3] Effective portion of gain (loss).

[4] Includes amounts for Ameren registrant and nonregistrant subsidiaries.

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