Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative assets	$ 1
Derivative liabilities	218
Not Designated As Hedging Instrument
Derivative [Line Items]
Derivative assets	21	$ 17
Derivative liabilities		226
Not Designated As Hedging Instrument | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	10	5
Not Designated As Hedging Instrument | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	6	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	11	13
Not Designated As Hedging Instrument | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	3	9
Not Designated As Hedging Instrument | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	16	14
Not Designated As Hedging Instrument | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	174	182
Union Electric Company | Not Designated As Hedging Instrument
Derivative [Line Items]
Derivative assets	19	17
Derivative liabilities	10	9
Union Electric Company | Not Designated As Hedging Instrument | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	10	5
Union Electric Company | Not Designated As Hedging Instrument | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	6	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	2	3
Union Electric Company | Not Designated As Hedging Instrument | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	3	9
Union Electric Company | Not Designated As Hedging Instrument | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	3	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	208	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	9	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	$ 174	$ 182