v3.25.2
Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($)
$ in Millions
Jun. 30, 2025
Dec. 31, 2024
Derivative [Line Items]    
Derivative Assets $ 29 $ 16
Derivative Liability 113 103
Fuel oils | Other current liabilities    
Derivative [Line Items]    
Derivative Liability 2 2
Fuel oils | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative Liability 2 2
Natural gas | Other current assets    
Derivative [Line Items]    
Derivative Assets 6 4
Natural gas | Other assets    
Derivative [Line Items]    
Derivative Assets 10 6
Natural gas | Other current liabilities    
Derivative [Line Items]    
Derivative Liability 20 27
Natural gas | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative Liability 13 19
Power | Other current assets    
Derivative [Line Items]    
Derivative Assets 13 6
Power | Other current liabilities    
Derivative [Line Items]    
Derivative Liability 10 10
Power | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative Liability 66 43
Union Electric Company    
Derivative [Line Items]    
Derivative Assets 17 10
Derivative Liability 13 15
Union Electric Company | Fuel oils | Other current liabilities    
Derivative [Line Items]    
Derivative Liability 2 2
Union Electric Company | Fuel oils | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative Liability 2 2
Union Electric Company | Natural gas | Other current assets    
Derivative [Line Items]    
Derivative Assets 2 2
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 Liability 4 5
Union Electric Company | Natural gas | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative Liability 5 6
Union Electric Company | Power | Other current assets    
Derivative [Line Items]    
Derivative Assets 13 6
Union Electric Company | Power | Other current liabilities    
Derivative [Line Items]    
Derivative Liability 0 0
Union Electric Company | Power | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative Liability 0 0
Ameren Illinois Company    
Derivative [Line Items]    
Derivative Assets 12 6
Derivative Liability 100 88
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 4 2
Ameren Illinois Company | Natural gas | Other assets    
Derivative [Line Items]    
Derivative Assets 8 4
Ameren Illinois Company | Natural gas | Other current liabilities    
Derivative [Line Items]    
Derivative Liability 16 22
Ameren Illinois Company | Natural gas | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative Liability 8 13
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 10 10
Ameren Illinois Company | Power | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative Liability $ 66 $ 43