Derivative Financial Instruments (Derivative Instruments Carrying Value) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Designated As Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets used hedging instruments	$ 24	[1]	$ 5	[1]
Derivative liabilities used as hedging instruments	1	[1]	1	[1]
Designated As Hedging Instrument [Member] | Power [Member] | Mark to Market Derivative Assets [Member]
Derivative [Line Items]
Derivative assets used hedging instruments	8	[1]	3	[1]
Designated As Hedging Instrument [Member] | Power [Member] | Other Assets [Member]
Derivative [Line Items]
Derivative assets used hedging instruments	16	[1]	2	[1]
Designated As Hedging Instrument [Member] | Power [Member] | Mark to Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities used as hedging instruments			1	[1]
Designated As Hedging Instrument [Member] | Power [Member] | Other Deferred Credits and Liabilities [Member]
Derivative [Line Items]
Derivative liabilities used as hedging instruments	1	[1]
Not Designated As Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets used hedging instruments	214	[1],[2]	169	[1],[2]
Derivative liabilities used as hedging instruments	280	[1],[2]	252	[1],[2]
Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Mark to Market Derivative Assets [Member]
Derivative [Line Items]
Derivative assets used hedging instruments	29	[1],[2]	42	[1],[2]
Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Assets [Member]
Derivative [Line Items]
Derivative assets used hedging instruments	8	[1],[2]	22	[1],[2]
Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Mark to Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities used as hedging instruments	2	[1],[2]	12	[1],[2]
Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Deferred Credits and Liabilities [Member]
Derivative [Line Items]
Derivative liabilities used as hedging instruments			1	[1],[2]
Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Mark to Market Derivative Assets [Member]
Derivative [Line Items]
Derivative assets used 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 used 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 used as hedging instruments	106	[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 used as hedging instruments	92	[1],[2]	84	[1],[2]
Not Designated As Hedging Instrument [Member] | Power [Member] | Mark to Market Derivative Assets [Member]
Derivative [Line Items]
Derivative assets used hedging instruments	72	[1],[2]	78	[1],[2]
Not Designated As Hedging Instrument [Member] | Power [Member] | Other Assets [Member]
Derivative [Line Items]
Derivative assets used hedging instruments	99	[1],[2]	20	[1],[2]
Not Designated As Hedging Instrument [Member] | Power [Member] | Mark to Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities used as hedging instruments	53	[1],[2]	61	[1],[2]
Not Designated As Hedging Instrument [Member] | Power [Member] | Other Deferred Credits and Liabilities [Member]
Derivative [Line Items]
Derivative liabilities used as hedging instruments	26	[1],[2]	7	[1],[2]
Not Designated As Hedging Instrument [Member] | Uranium [Member] | Mark to Market Derivative Assets [Member]
Derivative [Line Items]
Derivative assets used hedging instruments			2	[1],[2]
Not Designated As Hedging Instrument [Member] | Uranium [Member] | Other Deferred Credits and Liabilities [Member]
Derivative [Line Items]
Derivative liabilities used 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.