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Derivative Financial Instruments (Derivative Instruments Carrying Value) (Details) - Not Designated As Hedging Instrument - USD ($)
$ in Millions
Dec. 31, 2025
Dec. 31, 2024
Derivative [Line Items]    
Derivative assets $ 16 $ 16
Derivative liabilities 126 103
Natural gas | Other current assets    
Derivative [Line Items]    
Derivative assets 2 4
Natural gas | Other assets    
Derivative [Line Items]    
Derivative assets 3 6
Natural gas | Other current liabilities    
Derivative [Line Items]    
Derivative liabilities 26 27
Natural gas | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative liabilities 16 19
Power | Other current assets    
Derivative [Line Items]    
Derivative assets 11 6
Power | Other current liabilities    
Derivative [Line Items]    
Derivative liabilities 24 10
Power | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative liabilities 55 43
Fuel oils | Other current liabilities    
Derivative [Line Items]    
Derivative liabilities 3 2
Fuel oils | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative liabilities 2 2
Ameren Missouri    
Derivative [Line Items]    
Derivative assets 12 10
Derivative liabilities 15 15
Ameren Missouri | Natural gas | Other current assets    
Derivative [Line Items]    
Derivative assets 1 2
Ameren Missouri | Natural gas | Other assets    
Derivative [Line Items]    
Derivative assets 0 2
Ameren Missouri | Natural gas | Other current liabilities    
Derivative [Line Items]    
Derivative liabilities 5 5
Ameren Missouri | Natural gas | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative liabilities 5 6
Ameren Missouri | Power | Other current assets    
Derivative [Line Items]    
Derivative assets 11 6
Ameren Missouri | Power | Other current liabilities    
Derivative [Line Items]    
Derivative liabilities 0 0
Ameren Missouri | Power | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative liabilities 0 0
Ameren Missouri | Fuel oils | Other current liabilities    
Derivative [Line Items]    
Derivative liabilities 3 2
Ameren Missouri | Fuel oils | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative liabilities 2 2
Ameren Illinois    
Derivative [Line Items]    
Derivative assets 4 6
Derivative liabilities 111 88
Ameren Illinois | Natural gas | Other current assets    
Derivative [Line Items]    
Derivative assets 1 2
Ameren Illinois | Natural gas | Other assets    
Derivative [Line Items]    
Derivative assets 3 4
Ameren Illinois | Natural gas | Other current liabilities    
Derivative [Line Items]    
Derivative liabilities 21 22
Ameren Illinois | Natural gas | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative liabilities 11 13
Ameren Illinois | Power | Other current assets    
Derivative [Line Items]    
Derivative assets 0 0
Ameren Illinois | Power | Other current liabilities    
Derivative [Line Items]    
Derivative liabilities 24 10
Ameren Illinois | Power | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative liabilities 55 43
Ameren Illinois | Fuel oils | Other current liabilities    
Derivative [Line Items]    
Derivative liabilities 0 0
Ameren Illinois | Fuel oils | Other deferred credits and liabilities    
Derivative [Line Items]    
Derivative liabilities $ 0 $ 0