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Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($)
$ in Millions
Sep. 30, 2025
Dec. 31, 2024
Derivative [Line Items]    
Derivative Assets $ 32 $ 16
Derivative Liability 100 103
Natural gas | Other current assets    
Derivative [Line Items]    
Derivative Assets 4 4
Natural gas | Other assets    
Derivative [Line Items]    
Derivative Assets 6 6
Natural gas | Other current liabilities    
Derivative [Line Items]    
Derivative Liability 21 27
Natural gas | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative Liability 13 19
Fuel oils | Other current liabilities    
Derivative [Line Items]    
Derivative Liability 2 2
Fuel oils | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative Liability 1 2
Power | Other current assets    
Derivative [Line Items]    
Derivative Assets 22 6
Power | Other current liabilities    
Derivative [Line Items]    
Derivative Liability 11 10
Power | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative Liability 52 43
Union Electric Company    
Derivative [Line Items]    
Derivative Assets 24 10
Derivative Liability 12 15
Union Electric Company | Natural gas | Other current assets    
Derivative [Line Items]    
Derivative Assets 1 2
Union Electric Company | Natural gas | Other assets    
Derivative [Line Items]    
Derivative Assets 1 2
Union Electric Company | Natural gas | Other current liabilities    
Derivative [Line Items]    
Derivative Liability 3 5
Union Electric Company | Natural gas | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative Liability 5 6
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 1 2
Union Electric Company | Power | Other current assets    
Derivative [Line Items]    
Derivative Assets 22 6
Union Electric Company | Power | Other current liabilities    
Derivative [Line Items]    
Derivative Liability 1 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 8 6
Derivative Liability 88 88
Ameren Illinois Company | Natural gas | Other current assets    
Derivative [Line Items]    
Derivative Assets 3 2
Ameren Illinois Company | Natural gas | Other assets    
Derivative [Line Items]    
Derivative Assets 5 4
Ameren Illinois Company | Natural gas | Other current liabilities    
Derivative [Line Items]    
Derivative Liability 18 22
Ameren Illinois Company | Natural gas | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative Liability 8 13
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 | 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 $ 52 $ 43