Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) (Not Designated As Hedging Instrument [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Derivative [Line Items]
Derivative assets	$ 16	$ 34
Derivative liabilities	167	178
Fuel Oils | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	3	6
Fuel Oils | Other Assets [Member]
Derivative [Line Items]
Derivative assets		3
Fuel Oils | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	5	2
Fuel Oils | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	1
Natural Gas | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	1	2
Natural Gas | Other Assets [Member]
Derivative [Line Items]
Derivative assets	1
Natural Gas | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	19	32
Natural Gas | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	9	25
Power | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	10	23
Power | Other Assets [Member]
Derivative [Line Items]
Derivative assets	1
Power | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	14	13
Power | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	116	99
Uranium | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	2	5
Uranium | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	1
Union Electric Company
Derivative [Line Items]
Derivative assets	14	33
Derivative liabilities	21	24
Union Electric Company | Fuel Oils | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	3	6
Union Electric Company | Fuel Oils | Other Assets [Member]
Derivative [Line Items]
Derivative assets		3
Union Electric Company | Fuel Oils | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	5	2
Union Electric Company | Fuel Oils | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	1
Union Electric Company | Natural Gas | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets		1
Union Electric Company | Natural Gas | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	3	5
Union Electric Company | Natural Gas | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	3	6
Union Electric Company | Power | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	10	23
Union Electric Company | Power | Other Assets [Member]
Derivative [Line Items]
Derivative assets	1
Union Electric Company | Power | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	6	4
Union Electric Company | Uranium | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	2	5
Union Electric Company | Uranium | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	1
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	2	1
Derivative liabilities	146	154
Ameren Illinois Company | Natural Gas | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	1	1
Ameren Illinois Company | Natural Gas | Other Assets [Member]
Derivative [Line Items]
Derivative assets	1
Ameren Illinois Company | Natural Gas | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	16	27
Ameren Illinois Company | Natural Gas | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	6	19
Ameren Illinois Company | Power | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	8	9
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	$ 116	$ 99