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Revenue Recognition Revenue Recognition (Allowance for Credit Losses) (Details) - USD ($)
$ in Millions
 9 Months Ended 12 Months Ended
Sep. 30, 2024
Dec. 31, 2023
Dec. 31, 2022
Accounts Receivable Allowance For Credit Loss [Line Items]      
Accounts Receivable, Allowance for Credit Loss $ 17.7 $ 22.9 $ 23.9
Current period provisions (28.2) (39.8)  
Write-offs charged against allowance 43.0 61.6  
Recoveries of amounts previously written off 9.6 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 3.7 10.2 11.1
Current period provisions (18.9) (28.3)  
Write-offs charged against allowance 34.3 49.2  
Recoveries of amounts previously written off 8.9 20.0  
NIPSCO Operations      
Accounts Receivable Allowance For Credit Loss [Line Items]      
Accounts Receivable, Allowance for Credit Loss 14.0 11.9 $ 12.0
Current period provisions (9.3) (11.5)  
Write-offs charged against allowance 7.9 12.4  
Recoveries of amounts previously written off $ 0.7 $ 0.8