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Revenue Recognition Revenue Recognition (Allowance for Credit Losses) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2020
Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]    
Allowance for Credit Loss $ 20.3 $ 13.0
Current period provisions 10.6  
Write-offs charged against allowance (7.9)  
Recoveries of amounts previously written off 4.6  
Gas Distribution Operations    
Financing Receivable, Allowance for Credit Loss [Line Items]    
Allowance for Credit Loss 15.9 9.1
Current period provisions 9.1  
Write-offs charged against allowance 6.9  
Recoveries of amounts previously written off 4.6  
Corporate and Other    
Financing Receivable, Allowance for Credit Loss [Line Items]    
Allowance for Credit Loss 0.8 0.8
Current period provisions 0.0  
Write-offs charged against allowance 0.0  
Recoveries of amounts previously written off 0.0  
Electric Operations    
Financing Receivable, Allowance for Credit Loss [Line Items]    
Allowance for Credit Loss 3.6 $ 3.1
Current period provisions 1.5  
Write-offs charged against allowance 1.0  
Recoveries of amounts previously written off $ 0.0