Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) (Not Designated As Hedging Instrument [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Derivative [Line Items]
Derivative assets	$ 41	[1]	$ 29	[1]
Derivative liabilities	182	[1]	230	[1]
Fuel Oils | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	6	[1]	8	[1]
Fuel Oils | Other Assets [Member]
Derivative [Line Items]
Derivative assets	2	[1]	4	[1]
Fuel Oils | Mark To Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	3	[1]	2	[1]
Fuel Oils | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	[1]	2	[1]
Natural Gas | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	1	[1]	1	[1]
Natural Gas | Other Assets [Member]
Derivative [Line Items]
Derivative assets			1	[1]
Natural Gas | Mark To Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	45	[1]	64	[1]
Natural Gas | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	30	[1]	45	[1]
Power | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	30	[1]	14	[1]
Power | Other Assets [Member]
Derivative [Line Items]
Derivative assets	1	[1]	1	[1]
Power | Mark To Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	13	[1]	25	[1]
Power | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	84	[1]	90	[1]
Uranium | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	1	[1]
Uranium | Mark To Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	4	[1]	1	[1]
Uranium | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	2		1	[1]
Union Electric Company
Derivative [Line Items]
Derivative assets	41	[1]	28	[1]
Derivative liabilities	26	[1]	25	[1]
Union Electric Company | Fuel Oils | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	6	[1]	8	[1]
Union Electric Company | Fuel Oils | Other Assets [Member]
Derivative [Line Items]
Derivative assets	2	[1]	4	[1]
Union Electric Company | Fuel Oils | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	3	[1]	2	[1]
Union Electric Company | Fuel Oils | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	[1]	2	[1]
Union Electric Company | Natural Gas | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	1	[1]
Union Electric Company | Natural Gas | Other Assets [Member]
Derivative [Line Items]
Derivative assets			1	[1]
Union Electric Company | Natural Gas | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	7	[1]	8	[1]
Union Electric Company | Natural Gas | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	6	[1]	7	[1]
Union Electric Company | Power | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	30	[1]	14	[1]
Union Electric Company | Power | Other Assets [Member]
Derivative [Line Items]
Derivative assets	1	[1]	1	[1]
Union Electric Company | Power | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	3	[1]	4	[1]
Union Electric Company | Uranium | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	1	[1]
Union Electric Company | Uranium | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	4	[1]	1	[1]
Union Electric Company | Uranium | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	2		1	[1]
Ameren Illinois Company
Derivative [Line Items]
Derivative assets			1	[1]
Derivative liabilities	156	[1]	205	[1]
Ameren Illinois Company | Natural Gas | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets			1	[1]
Ameren Illinois Company | Natural Gas | Mark To Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	38	[1]	56	[1]
Ameren Illinois Company | Natural Gas | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	24	[1]	38	[1]
Ameren Illinois Company | Power | Mark To Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	10	[1]	21	[1]
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	$ 84	[1]	$ 90	[1]

[1] Includes derivatives subject to regulatory deferral.