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Goodwill (Schedule of Goodwill) (Details) - USD ($)
$ in Millions
Sep. 30, 2020
Dec. 31, 2019
Goodwill [Line Items]    
Goodwill $ 1,485.9 $ 1,485.9
Gas Distribution Operations    
Goodwill [Line Items]    
Goodwill 1,485.9  
Electric Operations    
Goodwill [Line Items]    
Goodwill 0.0  
Corporate and Other    
Goodwill [Line Items]    
Goodwill $ 0.0