v3.26.1
Revenue Recognition Revenue Recognition (Allowance for Credit Losses) (Details) - USD ($)
$ in Millions
 6 Months Ended 9 Months Ended
Jun. 30, 2026
Sep. 30, 2025
Dec. 31, 2025
Dec. 31, 2024
Accounts Receivable Allowance For Credit Loss [Line Items]        
Accounts Receivable, Allowance for Credit Loss $ 40.2   $ 40.6 $ 23.7
Current period provisions 34.6 $ 67.3    
Write-offs charged against allowance 40.4 65.8    
Recoveries of amounts previously written off 5.4 15.4    
Columbia Operations        
Accounts Receivable Allowance For Credit Loss [Line Items]        
Accounts Receivable, Allowance for Credit Loss 14.5   15.6 9.8
Current period provisions 24.4 42.3    
Write-offs charged against allowance 30.2 50.8    
Recoveries of amounts previously written off 4.7 14.3    
NIPSCO Operations        
Accounts Receivable Allowance For Credit Loss [Line Items]        
Accounts Receivable, Allowance for Credit Loss 25.7   $ 25.0 $ 13.9
Current period provisions 10.2 25.0    
Write-offs charged against allowance 10.2 15.0    
Recoveries of amounts previously written off $ 0.7 $ 1.1