Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Derivative [Line Items]
Derivative assets	$ 74	$ 22
Derivative liabilities	212	221
Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	7	2
Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	5	0
Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	1	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	33	9
Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	13	4
Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	1	2
Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Power | Other Current Assets
Derivative [Line Items]
Derivative assets	15	7
Power | Other Assets
Derivative [Line Items]
Derivative assets	1	0
Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	39	20
Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	171	189
Union Electric Company
Derivative [Line Items]
Derivative assets	38	12
Derivative liabilities	45	21
Union Electric Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	7	2
Union Electric Company | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	5	0
Union Electric Company | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	1	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	7	1
Union Electric Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	3	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	15	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	27	3
Union Electric Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	17	8
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	36	10
Derivative liabilities	167	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	26	8
Ameren Illinois Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	10	2
Ameren Illinois Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	1	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	12	17
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	$ 154	$ 181