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Revenue Recognition Revenue Recognition (Allowance for Credit Losses) (Details) - USD ($)
 3 Months Ended 9 Months Ended
Mar. 31, 2025
Sep. 30, 2024
Dec. 31, 2024
Dec. 31, 2023
Accounts Receivable Allowance For Credit Loss [Line Items]        
Accounts Receivable, Allowance for Credit Loss $ 29,600,000   $ 23,700,000 $ 22,900,000
Current period provisions 13,200,000 $ 38,800,000    
Write-offs charged against allowance 10,400,000 55,700,000    
Recoveries of amounts previously written off 3,100,000 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 14,000,000.0   9,800,000 10,200,000
Current period provisions 9,100,000 26,700,000    
Write-offs charged against allowance 7,800,000 43,900,000    
Recoveries of amounts previously written off 2,900,000 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 4,100,000 12,100,000    
Write-offs charged against allowance 2,600,000 11,000,000.0    
Recoveries of amounts previously written off $ 200,000 $ 900,000