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Company's Liability for Environmental Obligations (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Dec. 31, 2019
Sep. 30, 2018
Environmental Exit Cost [Line Items]        
Total Balance at Beginning of Period $ 2.7 $ 2.0    
Additions   0.1    
Less: Payments / Reductions 0.5 0.1    
Total Balance at End of Period 2.2 2.0    
Less: Current Portion 0.3 0.6 $ 0.6 $ 0.6
Noncurrent Balance at End of Period 1.9 1.4 $ 2.1 1.4
Fitchburg Gas and Electric Light Company        
Environmental Exit Cost [Line Items]        
Total Balance at Beginning of Period      
Additions      
Less: Payments / Reductions      
Total Balance at End of Period      
Less: Current Portion      
Noncurrent Balance at End of Period      
Northern Utilities Inc        
Environmental Exit Cost [Line Items]        
Total Balance at Beginning of Period 2.7 2.0    
Additions   0.1    
Less: Payments / Reductions 0.5 0.1    
Total Balance at End of Period 2.2 $ 2.0    
Less: Current Portion 0.3     0.6
Noncurrent Balance at End of Period $ 1.9     $ 1.4