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Supplemental Information (Allowance for Doubtful Accounts) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Dec. 31, 2024
Allowance for Doubtful Accounts Receivable [Roll Forward]          
Beginning of period $ 39 $ 37 $ 30 $ 30  
Bad debt expense 18 12 37 28  
Charged to other accounts 2 2 5 6  
Net write-offs (21) (20) (34) (33)  
End of period 38 31 38 31  
Payables for purchased receivables 55   55   $ 43
Union Electric Company          
Allowance for Doubtful Accounts Receivable [Roll Forward]          
Beginning of period 12 12 12 12  
Bad debt expense 7 3 11 8  
Net write-offs (4) (3) (8) (8)  
End of period 15 12 15 12  
Ameren Illinois Company          
Allowance for Doubtful Accounts Receivable [Roll Forward]          
Beginning of period 27 25 18 18  
Bad debt expense 11 9 26 20  
Charged to other accounts 2 2 5 6  
Net write-offs (17) (17) (26) (25)  
End of period 23 $ 19 23 $ 19  
Payables for purchased receivables $ 55   $ 55   $ 43