Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Derivative [Line Items]
Derivative assets	$ 28	$ 22
Derivative liabilities	202	221
Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	2	2
Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	3	0
Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	3	7
Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	2
Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	12	9
Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	6	4
Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	2
Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Power | Other Current Assets
Derivative [Line Items]
Derivative assets	4	7
Power | Other Assets
Derivative [Line Items]
Derivative assets	1	0
Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	25	20
Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	174	189
Union Electric Company
Derivative [Line Items]
Derivative assets	14	12
Derivative liabilities	17	21
Union Electric Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	2	2
Union Electric Company | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	3	0
Union Electric Company | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	3	7
Union Electric Company | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	2
Union Electric Company | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	2	1
Union Electric Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	2	2
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	0
Union Electric Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	4	7
Union Electric Company | Power | Other Assets
Derivative [Line Items]
Derivative assets	1	0
Union Electric Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	9	3
Union Electric Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	5	8
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	14	10
Derivative liabilities	185	200
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	8
Ameren Illinois Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	4	2
Ameren Illinois Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Ameren Illinois Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	1
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	$ 169	$ 181