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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

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