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Retirement Benefits (Summary of Benefit Plan Costs Incurred) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Pension Plan        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Net periodic benefit cost $ (6) $ (21) $ (16) $ (63)
Pension Plan | Union Electric Company        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Net periodic benefit cost (4) (11) (11) (33)
Pension Plan | Ameren Illinois Company        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Net periodic benefit cost (1) (8) (3) (25)
Pension Plan | Other Affiliated Entities And Intercompany Eliminations        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Net periodic benefit cost (1) (2) (2) (5)
Other Postretirement Benefit Plan, Defined Benefit        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Net periodic benefit cost (20) (19) (60) (59)
Other Postretirement Benefit Plan, Defined Benefit | Union Electric Company        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Net periodic benefit cost (7) (6) (21) (20)
Other Postretirement Benefit Plan, Defined Benefit | Ameren Illinois Company        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Net periodic benefit cost (13) (13) (39) (39)
Other Postretirement Benefit Plan, Defined Benefit | Other Affiliated Entities And Intercompany Eliminations        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Net periodic benefit cost $ 0 $ 0 $ 0 $ 0