v3.26.3
Revenue Recognition Revenue Recognition (Allowance for Credit Losses) (Details) - USD ($)
$ in Millions
 9 Months Ended 12 Months Ended
Sep. 30, 2026
Dec. 31, 2025
Dec. 31, 2024
Accounts Receivable Allowance For Credit Loss [Line Items]      
Accounts Receivable, Allowance for Credit Loss $ 33.6 $ 40.6 $ 23.7
Accounts Receivable, Credit Loss Expense (Reversal) 54.8 67.3  
Accounts Receivable, Allowance for Credit Loss, Writeoff (71.3) (65.8)  
Accounts Receivable, Allowance for Credit Loss, Recovery 9.5 15.4  
Columbia Operations      
Accounts Receivable Allowance For Credit Loss [Line Items]      
Accounts Receivable, Allowance for Credit Loss 8.8 15.6 9.8
Accounts Receivable, Credit Loss Expense (Reversal) 39.5 42.3  
Accounts Receivable, Allowance for Credit Loss, Writeoff (53.8) (50.8)  
Accounts Receivable, Allowance for Credit Loss, Recovery 7.5 14.3  
NIPSCO Operations      
Accounts Receivable Allowance For Credit Loss [Line Items]      
Accounts Receivable, Allowance for Credit Loss 24.8 25.0 $ 13.9
Accounts Receivable, Credit Loss Expense (Reversal) 15.3 25.0  
Accounts Receivable, Allowance for Credit Loss, Writeoff (17.5) (15.0)  
Accounts Receivable, Allowance for Credit Loss, Recovery $ 2.0 $ 1.1