Derivative Financial Instruments (Derivative Instruments Carrying Value) (Details) (USD $) In Millions	Sep. 30, 2011		Dec. 31, 2010
Designated As Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets hedging instruments	$ 10	[1]	$ 5	[1]
Derivative liabilities as hedging instruments	11	[1]	1	[1]
Designated As Hedging Instrument [Member] | Power [Member] | Mark to Market Derivative Assets [Member]
Derivative [Line Items]
Derivative assets hedging instruments	7	[1]	3	[1]
Designated As Hedging Instrument [Member] | Power [Member] | Other Assets [Member]
Derivative [Line Items]
Derivative assets hedging instruments	3	[1]	2	[1]
Designated As Hedging Instrument [Member] | Power [Member] | Mark to Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities as hedging instruments	3	[1]	1	[1]
Designated As Hedging Instrument [Member] | Power [Member] | Other Deferred Credits and Liabilities [Member]
Derivative [Line Items]
Derivative liabilities as hedging instruments	8	[1]
Not Designated As Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets hedging instruments	200	[1],[2]	169	[1],[2]
Derivative liabilities as hedging instruments	198	[1],[2]	252	[1],[2]
Not Designated As Hedging Instrument [Member] | Heating Oil [Member] | Mark to Market Derivative Assets [Member]
Derivative [Line Items]
Derivative assets hedging instruments	28	[1],[2]	42	[1],[2]
Not Designated As Hedging Instrument [Member] | Heating Oil [Member] | Other Assets [Member]
Derivative [Line Items]
Derivative assets hedging instruments	9	[1],[2]	22	[1],[2]
Not Designated As Hedging Instrument [Member] | Heating Oil [Member] | Mark to Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities as hedging instruments	4	[1],[2]	12	[1],[2]
Not Designated As Hedging Instrument [Member] | Heating Oil [Member] | Other Deferred Credits and Liabilities [Member]
Derivative [Line Items]
Derivative liabilities as hedging instruments	3	[1],[2]	1	[1],[2]
Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Mark to Market Derivative Assets [Member]
Derivative [Line Items]
Derivative assets hedging instruments	6	[1],[2]	4	[1],[2]
Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Assets [Member]
Derivative [Line Items]
Derivative assets hedging instruments			1	[1],[2]
Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Mark to Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities as hedging instruments	84	[1],[2]	87	[1],[2]
Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Deferred Credits and Liabilities [Member]
Derivative [Line Items]
Derivative liabilities as hedging instruments	66	[1],[2]	84	[1],[2]
Not Designated As Hedging Instrument [Member] | Power [Member] | Mark to Market Derivative Assets [Member]
Derivative [Line Items]
Derivative assets hedging instruments	53	[1],[2]	78	[1],[2]
Not Designated As Hedging Instrument [Member] | Power [Member] | Other Assets [Member]
Derivative [Line Items]
Derivative assets hedging instruments	104	[1],[2]	20	[1],[2]
Not Designated As Hedging Instrument [Member] | Power [Member] | Mark to Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities as hedging instruments	27	[1],[2]	61	[1],[2]
Not Designated As Hedging Instrument [Member] | Power [Member] | Other Deferred Credits and Liabilities [Member]
Derivative [Line Items]
Derivative liabilities as hedging instruments	13	[1],[2]	7	[1],[2]
Not Designated As Hedging Instrument [Member] | Uranium [Member] | Mark to Market Derivative Assets [Member]
Derivative [Line Items]
Derivative assets hedging instruments			2	[1],[2]
Not Designated As Hedging Instrument [Member] | Uranium [Member] | Other Deferred Credits and Liabilities [Member]
Derivative [Line Items]
Derivative liabilities as hedging instruments	$ 1	[1],[2]

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

[2] Includes derivatives subject to regulatory deferral.