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Revenue Recognition Revenue Recognition (Allowance for Credit Losses) (Details) - USD ($)
 9 Months Ended 12 Months Ended
Sep. 30, 2025
Dec. 31, 2024
Dec. 31, 2023
Accounts Receivable Allowance For Credit Loss [Line Items]      
Accounts Receivable, Allowance for Credit Loss $ 18,600,000 $ 23,700,000 $ 22,900,000
Current period provisions 36,400,000 38,800,000  
Write-offs charged against allowance 48,900,000 55,700,000  
Recoveries of amounts previously written off 7,400,000 17,700,000  
Corporate and Other      
Accounts Receivable Allowance For Credit Loss [Line Items]      
Accounts Receivable, Allowance for Credit Loss 0 0 800,000
Current period provisions 0 0  
Write-offs charged against allowance 0 (800,000)  
Recoveries of amounts previously written off 0 0  
Columbia Operations      
Accounts Receivable Allowance For Credit Loss [Line Items]      
Accounts Receivable, Allowance for Credit Loss 3,000,000.0 9,800,000 10,200,000
Current period provisions 26,200,000 26,700,000  
Write-offs charged against allowance 39,700,000 43,900,000  
Recoveries of amounts previously written off 6,700,000 16,800,000  
NIPSCO Operations      
Accounts Receivable Allowance For Credit Loss [Line Items]      
Accounts Receivable, Allowance for Credit Loss 15,600,000 13,900,000 $ 11,900,000
Current period provisions 10,200,000 12,100,000  
Write-offs charged against allowance 9,200,000 11,000,000.0  
Recoveries of amounts previously written off $ 700,000 $ 900,000