Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Derivative [Line Items]
Derivative assets	$ 33	$ 26
Derivative liabilities	246	255
Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	2	4
Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	0	2
Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	14	4
Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	6	3
Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	12	3
Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	6	1
Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	2	13
Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	1	7
Power | Other Current Assets
Derivative [Line Items]
Derivative assets	12	14
Power | Other Assets
Derivative [Line Items]
Derivative assets	1	2
Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	22	19
Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	201	208
Uranium | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Union Electric Company
Derivative [Line Items]
Derivative assets	19	22
Derivative liabilities	30	13
Union Electric Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	2	4
Union Electric Company | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	0	2
Union Electric Company | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	14	4
Union Electric Company | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	6	3
Union Electric Company | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	2	0
Union Electric Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	2	0
Union Electric Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	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	12	14
Union Electric Company | Power | Other Assets
Derivative [Line Items]
Derivative assets	1	2
Union Electric Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	6	2
Union Electric Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	4	1
Union Electric Company | Uranium | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	14	4
Derivative liabilities	216	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	10	3
Ameren Illinois Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	4	1
Ameren Illinois Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	2	12
Ameren Illinois Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	1	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	16	17
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	197	207
Ameren Illinois Company | Uranium | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	$ 0	$ 0