Derivative Financial Instruments (Derivative Instruments Carrying Value) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets	$ 39	[1]	$ 24	[1]
Derivative liabilities			1	[1]
Designated as Hedging Instrument [Member] | Power [Member] | MTM derivative assets [Member]
Derivative [Line Items]
Derivative assets	25	[1]	8	[1]
Designated as Hedging Instrument [Member] | Power [Member] | Other assets [Member]
Derivative [Line Items]
Derivative assets	14	[1]	16	[1]
Designated as Hedging Instrument [Member] | Power [Member] | Other deferred credits and liabilities [Member]
Derivative [Line Items]
Derivative liabilities			1	[1]
Not Designated As Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets	123	[1],[2]	214	[1],[2]
Derivative liabilities	315	[1],[2]	280	[1],[2]
Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | MTM derivative assets [Member]
Derivative [Line Items]
Derivative assets	10	[1],[2]	29	[1],[2]
Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other current assets [Member]
Derivative [Line Items]
Derivative assets		[1],[2]		[1],[2]
Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other assets [Member]
Derivative [Line Items]
Derivative assets	5	[1],[2]	8	[1],[2]
Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | MTM derivative liabilities [Member]
Derivative [Line Items]
Derivative liabilities	3	[1],[2]	2	[1],[2]
Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other current liabilities [Member]
Derivative [Line Items]
Derivative liabilities		[1],[2]		[1],[2]
Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other deferred credits and liabilities [Member]
Derivative [Line Items]
Derivative liabilities	3	[1],[2]
Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | MTM derivative assets [Member]
Derivative [Line Items]
Derivative assets	5	[1],[2]	6	[1],[2]
Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other current assets [Member]
Derivative [Line Items]
Derivative assets		[1],[2]		[1],[2]
Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other assets [Member]
Derivative [Line Items]
Derivative assets	1	[1],[2]		[1],[2]
Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | MTM derivative liabilities [Member]
Derivative [Line Items]
Derivative liabilities	68	[1],[2]	106	[1],[2]
Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other current liabilities [Member]
Derivative [Line Items]
Derivative liabilities		[1],[2]		[1],[2]
Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other deferred credits and liabilities [Member]
Derivative [Line Items]
Derivative liabilities	45	[1],[2]	92	[1],[2]
Not Designated As Hedging Instrument [Member] | Power [Member] | MTM derivative assets [Member]
Derivative [Line Items]
Derivative assets	85	[1],[2]	72	[1],[2]
Not Designated As Hedging Instrument [Member] | Power [Member] | Other current assets [Member]
Derivative [Line Items]
Derivative assets		[1],[2]		[1],[2]
Not Designated As Hedging Instrument [Member] | Power [Member] | Other assets [Member]
Derivative [Line Items]
Derivative assets	16	[1],[2]	99	[1],[2]
Not Designated As Hedging Instrument [Member] | Power [Member] | MTM derivative liabilities [Member]
Derivative [Line Items]
Derivative liabilities	74	[1],[2]	53	[1],[2]
Not Designated As Hedging Instrument [Member] | Power [Member] | Other current liabilities [Member]
Derivative [Line Items]
Derivative liabilities		[1],[2]		[1],[2]
Not Designated As Hedging Instrument [Member] | Power [Member] | Other deferred credits and liabilities [Member]
Derivative [Line Items]
Derivative liabilities	107	[1],[2]	26	[1],[2]
Not Designated As Hedging Instrument [Member] | Coal [Member] | Other assets [Member]
Derivative [Line Items]
Derivative assets	1	[1],[2]
Not Designated As Hedging Instrument [Member] | Coal [Member] | MTM derivative liabilities [Member]
Derivative [Line Items]
Derivative liabilities	9	[1],[2]
Not Designated As Hedging Instrument [Member] | Coal [Member] | Other deferred credits and liabilities [Member]
Derivative [Line Items]
Derivative liabilities	4	[1],[2]
Not Designated As Hedging Instrument [Member] | Uranium [Member] | MTM derivative liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	[1],[2]
Not Designated As Hedging Instrument [Member] | Uranium [Member] | Other deferred credits and liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	[1],[2]	1	[1],[2]
Ameren Missouri [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets
Derivative liabilities
Ameren Missouri [Member] | Designated as Hedging Instrument [Member] | Power [Member] | Other assets [Member]
Derivative [Line Items]
Derivative assets
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets	28	[2]	55	[2]
Derivative liabilities	25	[2]	37	[2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other current assets [Member]
Derivative [Line Items]
Derivative assets	8	[2]	17	[2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other assets [Member]
Derivative [Line Items]
Derivative assets	4	[2]	6	[2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other current liabilities [Member]
Derivative [Line Items]
Derivative liabilities	2	[2]	1	[2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other deferred credits and liabilities [Member]
Derivative [Line Items]
Derivative liabilities	2	[2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other current assets [Member]
Derivative [Line Items]
Derivative assets		[2]	2	[2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other assets [Member]
Derivative [Line Items]
Derivative assets	1	[2]		[2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other current liabilities [Member]
Derivative [Line Items]
Derivative liabilities	8	[2]	13	[2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other deferred credits and liabilities [Member]
Derivative [Line Items]
Derivative liabilities	7	[2]	13	[2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Other current assets [Member]
Derivative [Line Items]
Derivative assets	14	[2]	30	[2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Other assets [Member]
Derivative [Line Items]
Derivative assets	1	[2]		[2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Other current liabilities [Member]
Derivative [Line Items]
Derivative liabilities	4	[2]	9	[2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Other deferred credits and liabilities [Member]
Derivative [Line Items]
Derivative liabilities		[2]		[2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Coal [Member]
Derivative [Line Items]
Derivative liabilities		[2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Uranium [Member] | Other current liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	[2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Uranium [Member] | Other deferred credits and liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	[2]	1	[2]
Ameren Illinois [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets
Derivative liabilities
Ameren Illinois [Member] | Designated as Hedging Instrument [Member] | Power [Member] | Other assets [Member]
Derivative [Line Items]
Derivative assets
Ameren Illinois [Member] | Designated as Hedging Instrument [Member] | Power [Member] | MTM derivative liabilities [Member]
Derivative [Line Items]
Derivative liabilities
Ameren Illinois [Member] | Not Designated As Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets	1	[2]	79	[2]
Derivative liabilities	205	[2]	386	[2]
Ameren Illinois [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other current assets [Member]
Derivative [Line Items]
Derivative assets		[2]		[2]
Ameren Illinois [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other assets [Member]
Derivative [Line Items]
Derivative assets		[2]		[2]
Ameren Illinois [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | MTM derivative liabilities [Member]
Derivative [Line Items]
Derivative liabilities		[2]		[2]
Ameren Illinois [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other current liabilities [Member]
Derivative [Line Items]
Derivative liabilities		[2]		[2]
Ameren Illinois [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other current assets [Member]
Derivative [Line Items]
Derivative assets	1	[2]	1	[2]
Ameren Illinois [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other assets [Member]
Derivative [Line Items]
Derivative assets		[2]	1	[2]
Ameren Illinois [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | MTM derivative liabilities [Member]
Derivative [Line Items]
Derivative liabilities	56	[2]	90	[2]
Ameren Illinois [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other current liabilities [Member]
Derivative [Line Items]
Derivative liabilities		[2]		[2]
Ameren Illinois [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other deferred credits and liabilities [Member]
Derivative [Line Items]
Derivative liabilities	38	[2]	79	[2]
Ameren Illinois [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Other current assets [Member]
Derivative [Line Items]
Derivative assets		[2]		[2]
Ameren Illinois [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Other assets [Member]
Derivative [Line Items]
Derivative assets		[2]	77	[2]
Ameren Illinois [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | MTM derivative liabilities [Member]
Derivative [Line Items]
Derivative liabilities	21	[2]	9	[2]
Ameren Illinois [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | MTM derivative liabilities - affiliates [Member]
Derivative [Line Items]
Derivative liabilities			200	[2]
Ameren Illinois [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Other current liabilities [Member]
Derivative [Line Items]
Derivative liabilities		[2]		[2]
Ameren Illinois [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Other deferred credits and liabilities [Member]
Derivative [Line Items]
Derivative liabilities	90	[2]	8	[2]
Ameren Illinois [Member] | Not Designated As Hedging Instrument [Member] | Coal [Member]
Derivative [Line Items]
Derivative liabilities		[2]
Ameren Illinois [Member] | Not Designated As Hedging Instrument [Member] | Coal [Member] | MTM derivative liabilities [Member]
Derivative [Line Items]
Derivative liabilities		[2]
Ameren Illinois [Member] | Not Designated As Hedging Instrument [Member] | Uranium [Member] | Other deferred credits and liabilities [Member]
Derivative [Line Items]
Derivative liabilities		[2]		[2]

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

[2] Includes derivatives subject to regulatory deferral.