Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Derivative assets	$ 30	$ 26
Derivative liabilities	301	255
Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	3	4
Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	1	2
Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	19	4
Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	12	3
Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	2	3
Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	2	1
Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	20	13
Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	5	7
Power | Other Current Assets
Derivative [Line Items]
Derivative assets	18	14
Power | Other Assets
Derivative [Line Items]
Derivative assets	4	2
Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	20	19
Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	224	208
Uranium [Member] | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	1	1
Union Electric Company
Derivative [Line Items]
Derivative assets	26	22
Derivative liabilities	38	13
Union Electric Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	3	4
Union Electric Company | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	1	2
Union Electric Company | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	19	4
Union Electric Company | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	12	3
Union Electric Company | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Union Electric Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	0	0
Union Electric Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	3	1
Union Electric Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Union Electric Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	18	14
Union Electric Company | Power | Other Assets
Derivative [Line Items]
Derivative assets	4	2
Union Electric Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	2	2
Union Electric Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	1	1
Union Electric Company | Uranium [Member] | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	1	1
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	4	4
Derivative liabilities	263	242
Ameren Illinois Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Fuel Oils | Other 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	2	3
Ameren Illinois Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	2	1
Ameren Illinois Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	17	12
Ameren Illinois Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	5	6
Ameren Illinois Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Power | Other Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	18	17
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	223	207
Ameren Illinois Company | Uranium [Member] | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	$ 0	$ 0