Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) (Not Designated As Hedging Instrument [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Derivative [Line Items]
Derivative assets	$ 37	[1]	$ 29	[1]
Derivative liabilities	183	[1]	230	[1]
Fuel Oils [Member] | Mark To Market Derivative Assets [Member]
Derivative [Line Items]
Derivative assets	6	[1]	8	[1]
Fuel Oils [Member] | Other Assets [Member]
Derivative [Line Items]
Derivative assets	4	[1]	4	[1]
Fuel Oils [Member] | Mark To Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	[1]	2	[1]
Fuel Oils [Member] | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	2	[1]	2	[1]
Natural Gas [Member] | Mark To Market Derivative Assets [Member]
Derivative [Line Items]
Derivative assets	6	[1]	1	[1]
Natural Gas [Member] | Other Assets [Member]
Derivative [Line Items]
Derivative assets	2	[1]	1	[1]
Natural Gas [Member] | Mark To Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	45	[1]	64	[1]
Natural Gas [Member] | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	34	[1]	45	[1]
Power [Member] | Mark To Market Derivative Assets [Member]
Derivative [Line Items]
Derivative assets	15	[1]	14	[1]
Power [Member] | Other Assets [Member]
Derivative [Line Items]
Derivative assets	4	[1]	1	[1]
Power [Member] | Mark To Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	25	[1]	25	[1]
Power [Member] | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	74	[1]	90	[1]
Uranium [Member] | Mark To Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	2	[1]	1	[1]
Uranium [Member] | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities			1	[1]
Union Electric Company [Member]
Derivative [Line Items]
Derivative assets	28	[1]	28	[1]
Derivative liabilities	32	[1]	25	[1]
Union Electric Company [Member] | Fuel Oils [Member] | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	6	[1]	8	[1]
Union Electric Company [Member] | Fuel Oils [Member] | Other Assets [Member]
Derivative [Line Items]
Derivative assets	4	[1]	4	[1]
Union Electric Company [Member] | Fuel Oils [Member] | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	[1]	2	[1]
Union Electric Company [Member] | Fuel Oils [Member] | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	2	[1]	2	[1]
Union Electric Company [Member] | Natural Gas [Member] | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	1	[1]
Union Electric Company [Member] | Natural Gas [Member] | Other Assets [Member]
Derivative [Line Items]
Derivative assets	1	[1]	1	[1]
Union Electric Company [Member] | Natural Gas [Member] | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	7	[1]	8	[1]
Union Electric Company [Member] | Natural Gas [Member] | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	5	[1]	7	[1]
Union Electric Company [Member] | Power [Member] | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	15	[1]	14	[1]
Union Electric Company [Member] | Power [Member] | Other Assets [Member]
Derivative [Line Items]
Derivative assets	1	[1]	1	[1]
Union Electric Company [Member] | Power [Member] | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	14	[1]	4	[1]
Union Electric Company [Member] | Power [Member] | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	[1]
Union Electric Company [Member] | Uranium [Member] | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	2	[1]	1	[1]
Union Electric Company [Member] | Uranium [Member] | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities			1	[1]
Ameren Illinois Company [Member]
Derivative [Line Items]
Derivative assets	9	[1]	1	[1]
Derivative liabilities	151	[1]	205	[1]
Ameren Illinois Company [Member] | Natural Gas [Member] | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	5	[1]	1	[1]
Ameren Illinois Company [Member] | Natural Gas [Member] | Other Assets [Member]
Derivative [Line Items]
Derivative assets	1	[1]
Ameren Illinois Company [Member] | Natural Gas [Member] | Mark To Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	38	[1]	56	[1]
Ameren Illinois Company [Member] | Natural Gas [Member] | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	29	[1]	38	[1]
Ameren Illinois Company [Member] | Power [Member] | Other Assets [Member]
Derivative [Line Items]
Derivative assets	3	[1]
Ameren Illinois Company [Member] | Power [Member] | Mark To Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	11	[1]	21	[1]
Ameren Illinois Company [Member] | Power [Member] | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	$ 73	[1]	$ 90	[1]

[1] Includes derivatives subject to regulatory deferral.