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Revenue Recognition (Allowance for Credit Losses) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Financing Receivable, Allowance for Credit Loss [Line Items]      
Allowance for Credit Loss $ 23.7 $ 22.9 $ 23.9
Current period provisions 38.8 39.8  
Write-offs charged against allowance (55.7) (61.6)  
Recoveries of amounts previously written off 17.7 20.8  
Electric Operations      
Financing Receivable, Allowance for Credit Loss [Line Items]      
Allowance for Credit Loss 13.9 11.9 12.0
Current period provisions 12.1 11.5  
Write-offs charged against allowance (11.0) (12.4)  
Recoveries of amounts previously written off 0.9 0.8  
Corporate Segment and Other Operating Segment      
Financing Receivable, Allowance for Credit Loss [Line Items]      
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      
Financing Receivable, Allowance for Credit Loss [Line Items]      
Allowance for Credit Loss 9.8 10.2 $ 11.1
Current period provisions 26.7 28.3  
Write-offs charged against allowance (43.9) (49.2)  
Recoveries of amounts previously written off $ 16.8 $ 20.0