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Revenue Recognition Revenue Recognition (Allowance for Credit Losses) (Details) - USD ($)
$ in Millions
 6 Months Ended 12 Months Ended
Jun. 30, 2021
Dec. 31, 2020
Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]      
Allowance for Credit Loss $ 41.6 $ 52.3 $ 13.0
Current period provisions 5.5 54.6  
Write-offs charged against allowance (23.2) (29.7)  
Recoveries of amounts previously written off 7.0 14.4  
Gas Distribution Operations      
Financing Receivable, Allowance for Credit Loss [Line Items]      
Allowance for Credit Loss 35.4 41.8 9.1
Current period provisions 5.1 45.3  
Write-offs charged against allowance (18.3) (26.7)  
Recoveries of amounts previously written off 6.8 14.1  
Electric Operations      
Financing Receivable, Allowance for Credit Loss [Line Items]      
Allowance for Credit Loss 5.4 9.7 3.1
Current period provisions 0.4 9.3  
Write-offs charged against allowance (4.9) (3.0)  
Recoveries of amounts previously written off 0.2 0.3  
Corporate and Other      
Financing Receivable, Allowance for Credit Loss [Line Items]      
Allowance for Credit Loss 0.8 0.8 $ 0.8
Current period provisions 0.0 0.0  
Write-offs charged against allowance 0.0 0.0  
Recoveries of amounts previously written off $ 0.0 $ 0.0