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Company's Liability for Environmental Obligations (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Environmental Exit Cost [Line Items]        
Total Balance at Beginning of Period $ 2.1 $ 1.9    
Additions 0.3 0.4    
Less: Payments / Reductions 0.4 0.2    
Total Balance at End of Period 2.0 2.1    
Less: Current Portion     $ 0.6 $ 0.5
Noncurrent Balance     1.4 1.6
Total Balance at End of Period 2.1 1.9 2.0 2.1
Fitchburg Gas and Electric Light Company        
Environmental Exit Cost [Line Items]        
Total Balance at Beginning of Period 0.1 0.1    
Less: Payments / Reductions 0.1      
Total Balance at End of Period   0.1    
Noncurrent Balance       0.1
Total Balance at End of Period 0.1 0.1   0.1
Northern Utilities Inc        
Environmental Exit Cost [Line Items]        
Total Balance at Beginning of Period 2.0 1.8    
Additions 0.3 0.4    
Less: Payments / Reductions 0.3 0.2    
Total Balance at End of Period 2.0 2.0    
Less: Current Portion     0.6 0.5
Noncurrent Balance     1.4 1.5
Total Balance at End of Period $ 2.0 $ 1.8 $ 2.0 $ 2.0