Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Derivative [Line Items]
Derivative assets	$ 31	$ 19
Derivative liabilities	237	232
Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	1	2
Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	14	22
Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	5	7
Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets		2
Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	1
Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	28	37
Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	21	19
Power | Other Current Assets
Derivative [Line Items]
Derivative assets	29	15
Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	13	14
Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	154	131
Uranium | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	2	2
Union Electric Company
Derivative [Line Items]
Derivative assets	30	18
Derivative liabilities	34	46
Union Electric Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	1	2
Union Electric Company | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	14	22
Union Electric Company | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	5	7
Union Electric Company | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets		1
Union Electric Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	0
Union Electric Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	4	6
Union Electric Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	7	6
Union Electric Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	29	15
Union Electric Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	2	3
Union Electric Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Union Electric Company | Uranium | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	2	2
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	1	1
Derivative liabilities	203	186
Ameren Illinois Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets		1
Ameren Illinois Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	1
Ameren Illinois Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	14	13
Ameren Illinois Company | Natural Gas | Mark To Market Derivative Liabilities
Derivative [Line Items]
Derivative liabilities	24	31
Ameren Illinois Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	154	131
Ameren Illinois Company | Power | Mark To Market Derivative Liabilities
Derivative [Line Items]
Derivative liabilities	11	11
Ameren Illinois Company | Uranium | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	$ 0	$ 0