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Revenue Recognition Revenue Recognition (Allowance for Credit Losses) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Mar. 31, 2024
Jun. 30, 2023
Dec. 31, 2023
Dec. 31, 2022
Accounts Receivable Allowance For Credit Loss [Line Items]        
Accounts Receivable, Allowance for Credit Loss $ 31.6   $ 22.9 $ 23.9
Current period provisions 14.6   39.8  
Write-offs charged against allowance 10.2   61.6  
Recoveries of amounts previously written off 4.3   20.8  
Corporate and Other        
Accounts Receivable Allowance For Credit Loss [Line Items]        
Accounts Receivable, Allowance for Credit Loss 0.0   0.8 0.8
Current period provisions 0.0 $ 0.0    
Write-offs charged against allowance (0.8) 0.0    
Recoveries of amounts previously written off 0.0 0.0    
Columbia Operations        
Accounts Receivable Allowance For Credit Loss [Line Items]        
Accounts Receivable, Allowance for Credit Loss 18.0   10.2 11.1
Current period provisions 11.0 28.3    
Write-offs charged against allowance 7.3 49.2    
Recoveries of amounts previously written off 4.1 20.0    
NIPSCO Operations        
Accounts Receivable Allowance For Credit Loss [Line Items]        
Accounts Receivable, Allowance for Credit Loss 13.6   $ 11.9 $ 12.0
Current period provisions 3.6 11.5    
Write-offs charged against allowance 2.1 12.4    
Recoveries of amounts previously written off $ 0.2 $ 0.8