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Supplemental Information (Allowance for Doubtful Accounts) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Beginning balance at January 1 $ 30 $ 30
Bad debt expense 51 39
Accounts Receivable, Credit Loss charged to other accounts 7 8
Net write-offs (49) (47)
Ending balance at December 31 39 30
Ameren Missouri    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Beginning balance at January 1 12 12
Bad debt expense 17 11
Accounts Receivable, Credit Loss charged to other accounts 0 0
Net write-offs (12) (11)
Ending balance at December 31 17 12
Ameren Illinois    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Beginning balance at January 1 18 18
Bad debt expense 34 28
Accounts Receivable, Credit Loss charged to other accounts 7 8
Net write-offs (37) (36)
Ending balance at December 31 $ 22 $ 18