Financial Instruments (Derivative Instruments Carrying Value) (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative assets	$ 1
Derivative liabilities	225
Not Designated As Hedging Instrument
Derivative [Line Items]
Derivative assets	22	$ 17
Derivative liabilities		226
Not Designated As Hedging Instrument | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	8	5
Not Designated As Hedging Instrument | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	5	2
Not Designated As Hedging Instrument | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	1
Not Designated As Hedging Instrument | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	1
Not Designated As Hedging Instrument | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	1	1
Not Designated As Hedging Instrument | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	16	17
Not Designated As Hedging Instrument | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	16	13
Not Designated As Hedging Instrument | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	7	9
Not Designated As Hedging Instrument | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	15	14
Not Designated As Hedging Instrument | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	177	182
Union Electric Company | Not Designated As Hedging Instrument
Derivative [Line Items]
Derivative assets	20	17
Derivative liabilities	10	9
Union Electric Company | Not Designated As Hedging Instrument | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	8	5
Union Electric Company | Not Designated As Hedging Instrument | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	5	2
Union Electric Company | Not Designated As Hedging Instrument | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	1
Union Electric Company | Not Designated As Hedging Instrument | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	0
Union Electric Company | Not Designated As Hedging Instrument | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	0	1
Union Electric Company | Not Designated As Hedging Instrument | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	4	5
Union Electric Company | Not Designated As Hedging Instrument | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	3	3
Union Electric Company | Not Designated As Hedging Instrument | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	7	9
Union Electric Company | Not Designated As Hedging Instrument | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	2	1
Union Electric Company | Not Designated As Hedging Instrument | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Not Designated As Hedging Instrument
Derivative [Line Items]
Derivative assets	2	0
Derivative liabilities	215	217
Ameren Illinois Company | Not Designated As Hedging Instrument | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Not Designated As Hedging Instrument | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Not Designated As Hedging Instrument | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0
Ameren Illinois Company | Not Designated As Hedging Instrument | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	1
Ameren Illinois Company | Not Designated As Hedging Instrument | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	1	0
Ameren Illinois Company | Not Designated As Hedging Instrument | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	12	12
Ameren Illinois Company | Not Designated As Hedging Instrument | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	13	10
Ameren Illinois Company | Not Designated As Hedging Instrument | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Not Designated As Hedging Instrument | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	13	13
Ameren Illinois Company | Not Designated As Hedging Instrument | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	$ 177	$ 182