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Company's Liability for Environmental Obligations (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Environmental Exit Cost [Line Items]          
Total Balance at Beginning of Period $ 2.1 $ 2.0      
Additions 0.2 0.4      
Less: Payments / Reductions 0.2 0.1      
Total Balance at End of Period 2.1 2.3      
Less: Current Portion     $ 0.6 $ 0.5 $ 0.5
Noncurrent Balance     1.5 1.6 1.8
Total Balance at End of period 2.1 2.3 2.1 2.1 2.3
Fitchburg Gas and Electric Light Company          
Environmental Exit Cost [Line Items]          
Total Balance at Beginning of Period 0.1 0.1      
Additions 0.1        
Less: Payments / Reductions 0.1        
Total Balance at End of Period 0.1 0.1      
Less: Current Portion     0.1   0.1
Total Balance at End of period 0.1 0.1 0.1 0.1 0.1
Northern Utilities Inc          
Environmental Exit Cost [Line Items]          
Total Balance at Beginning of Period 2.0 1.9      
Additions 0.1 0.4      
Less: Payments / Reductions 0.1 0.1      
Total Balance at End of Period 2.0 2.2      
Less: Current Portion     0.5   0.4
Noncurrent Balance     1.5   1.8
Total Balance at End of period $ 2.0 $ 2.2 $ 2.0 $ 2.0 $ 2.2