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Retirement Benefits (Cash Contributions Made To Benefit Plans) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Cash contributions to benefit plans $ 3 $ 8 $ 4
Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Cash contributions to benefit plans 3 4 3
Ameren Missouri | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Cash contributions to benefit plans 1 1 1
Ameren Missouri | Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Cash contributions to benefit plans 1 2 2
Ameren Illinois | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Cash contributions to benefit plans 1 1 2
Ameren Illinois | Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Cash contributions to benefit plans 1 1 1
Other | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Cash contributions to benefit plans 1 6 1
Other | Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Cash contributions to benefit plans $ 1 $ 1 $ 0