Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Derivative [Line Items]
Derivative Assets	$ 16	$ 22
Derivative Liability	156	168
Fuel oils | Other current assets
Derivative [Line Items]
Derivative Assets	0	2
Fuel oils | Other current liabilities
Derivative [Line Items]
Derivative Liability	3	1
Fuel oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	2	1
Natural gas | Other current assets
Derivative [Line Items]
Derivative Assets	3	0
Natural gas | Other assets
Derivative [Line Items]
Derivative Assets	3	6
Natural gas | Other current liabilities
Derivative [Line Items]
Derivative Liability	35	57
Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	38	40
Power | Other current assets
Derivative [Line Items]
Derivative Assets	10	5
Power | Other current liabilities
Derivative [Line Items]
Derivative Liability	11	13
Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	67	56
Uranium | Other current assets
Derivative [Line Items]
Derivative Assets	0	9
Union Electric Company
Derivative [Line Items]
Derivative Assets	13	19
Derivative Liability	21	25
Union Electric Company | Fuel oils | Other current assets
Derivative [Line Items]
Derivative Assets	0	2
Union Electric Company | Fuel oils | Other current liabilities
Derivative [Line Items]
Derivative Liability	3	1
Union Electric Company | Fuel oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	2	1
Union Electric Company | Natural gas | Other current assets
Derivative [Line Items]
Derivative Assets	1	0
Union Electric Company | Natural gas | Other assets
Derivative [Line Items]
Derivative Assets	2	3
Union Electric Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative Liability	7	12
Union Electric Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	9	10
Union Electric Company | Power | Other current assets
Derivative [Line Items]
Derivative Assets	10	5
Union Electric Company | Power | Other current liabilities
Derivative [Line Items]
Derivative Liability	0	1
Union Electric Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	0	0
Union Electric Company | Uranium | Other current assets
Derivative [Line Items]
Derivative Assets	0	9
Ameren Illinois Company
Derivative [Line Items]
Derivative Assets	3	3
Derivative Liability	135	143
Ameren Illinois Company | Fuel oils | Other current assets
Derivative [Line Items]
Derivative Assets	0	0
Ameren Illinois Company | Fuel oils | Other current liabilities
Derivative [Line Items]
Derivative Liability	0	0
Ameren Illinois Company | Fuel oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	0	0
Ameren Illinois Company | Natural gas | Other current assets
Derivative [Line Items]
Derivative Assets	2	0
Ameren Illinois Company | Natural gas | Other assets
Derivative [Line Items]
Derivative Assets	1	3
Ameren Illinois Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative Liability	28	45
Ameren Illinois Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	29	30
Ameren Illinois Company | Power | Other current assets
Derivative [Line Items]
Derivative Assets	0	0
Ameren Illinois Company | Power | Other current liabilities
Derivative [Line Items]
Derivative Liability	11	12
Ameren Illinois Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	67	56
Ameren Illinois Company | Uranium | Other current assets
Derivative [Line Items]
Derivative Assets	$ 0	$ 0