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Goodwill and Other Intangible Assets (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Goodwill And Other Intangible Assets [Line Items]      
Goodwill $ 1,690.7 $ 1,690.7  
Intangible assets excluding goodwill 220.7 231.7  
Amortization 221.5 210.5  
Amortization of Intangible Assets $ 11.0 $ 11.0 $ 11.0
Finite-Lived Intangible Asset, Useful Life 40 years    
Finite-Lived Intangible Assets, Amortization End Date 2039    
Gas Distribution Operations      
Goodwill And Other Intangible Assets [Line Items]      
Goodwill $ 1,690.7    
Electric Operations      
Goodwill And Other Intangible Assets [Line Items]      
Goodwill 0.0    
Corporate and Other      
Goodwill And Other Intangible Assets [Line Items]      
Goodwill 0.0    
Columbia Of Massachusetts      
Goodwill And Other Intangible Assets [Line Items]      
Goodwill $ 204.8