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Revenue Recognition Revenue Recognition (Allowance for Credit Losses) (Details) - USD ($)
$ in Millions
 9 Months Ended 12 Months Ended
Sep. 30, 2021
Dec. 31, 2020
Dec. 31, 2019
Accounts Receivable Allowance For Credit Loss [Line Items]      
Accounts Receivable, Allowance for Credit Loss $ 28.3 $ 52.3 $ 13.0
Current period provisions 7.1 54.6  
Write-offs charged against allowance 40.9 29.7  
Recoveries of amounts previously written off 9.8 14.4  
Gas Distribution Operations      
Accounts Receivable Allowance For Credit Loss [Line Items]      
Accounts Receivable, Allowance for Credit Loss 24.2 41.8 9.1
Current period provisions 7.7 45.3  
Write-offs charged against allowance 34.7 26.7  
Recoveries of amounts previously written off 9.4 14.1  
Electric Operations      
Accounts Receivable Allowance For Credit Loss [Line Items]      
Accounts Receivable, Allowance for Credit Loss 3.3 9.7 3.1
Current period provisions (0.6) 9.3  
Write-offs charged against allowance 6.2 3.0  
Recoveries of amounts previously written off 0.4 0.3  
Corporate and Other      
Accounts Receivable Allowance For Credit Loss [Line Items]      
Accounts Receivable, 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