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Retirement Benefits (Summary Of Benefit Plan Costs Incurred) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Net periodic benefit cost $ (21) $ (84) $ (148)
Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Net periodic benefit cost (81) (79) (83)
Ameren Missouri | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Net periodic benefit cost (14) (44) (76)
Ameren Missouri | Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Net periodic benefit cost (28) (27) (30)
Ameren Illinois | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Net periodic benefit cost (4) (34) (62)
Ameren Illinois | Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Net periodic benefit cost (53) (52) (54)
Other | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Net periodic benefit cost (3) (6) (10)
Other | Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Net periodic benefit cost $ 0 $ 0 $ 1