Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative liabilities	$ 206
Not Designated As Hedging Instrument
Derivative [Line Items]
Derivative assets	17	$ 15
Derivative liabilities		219
Not Designated As Hedging Instrument | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	5	3
Not Designated As Hedging Instrument | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	4	5
Not Designated As Hedging Instrument | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	3	4
Not Designated As Hedging Instrument | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	4	9
Not Designated As Hedging Instrument | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	3	1
Not Designated As Hedging Instrument | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	3	2
Not Designated As Hedging Instrument | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	8	12
Not Designated As Hedging Instrument | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	5	7
Not Designated As Hedging Instrument | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	1	4
Not Designated As Hedging Instrument | Power | Other Assets
Derivative [Line Items]
Derivative assets	1
Not Designated As Hedging Instrument | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	16	18
Not Designated As Hedging Instrument | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	170	169
Union Electric Company | Not Designated As Hedging Instrument
Derivative [Line Items]
Derivative assets	11	12
Derivative liabilities	12	22
Union Electric Company | Not Designated As Hedging Instrument | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	5	3
Union Electric Company | Not Designated As Hedging Instrument | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	4	5
Union Electric Company | Not Designated As Hedging Instrument | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	3	4
Union Electric Company | Not Designated As Hedging Instrument | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	4	9
Union Electric Company | Not Designated As Hedging Instrument | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Union Electric Company | Not Designated As Hedging Instrument | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	0	0
Union Electric Company | Not Designated As Hedging Instrument | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	2	4
Union Electric Company | Not Designated As Hedging Instrument | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	1	1
Union Electric Company | Not Designated As Hedging Instrument | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	1	4
Union Electric Company | Not Designated As Hedging Instrument | Power | Other Assets
Derivative [Line Items]
Derivative assets	1
Union Electric Company | Not Designated As Hedging Instrument | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	2	4
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	6	3
Derivative liabilities	194	197
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 Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Not Designated As Hedging Instrument | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Not Designated As Hedging Instrument | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	3	1
Ameren Illinois Company | Not Designated As Hedging Instrument | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	3	2
Ameren Illinois Company | Not Designated As Hedging Instrument | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	6	8
Ameren Illinois Company | Not Designated As Hedging Instrument | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	4	6
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 Assets
Derivative [Line Items]
Derivative assets	0
Ameren Illinois Company | Not Designated As Hedging Instrument | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	14	14
Ameren Illinois Company | Not Designated As Hedging Instrument | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	$ 170	$ 169