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Company's Liability for Environmental Obligations (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Mar. 31, 2019
Mar. 31, 2018
Environmental Exit Cost [Line Items]        
Total Balance at Beginning of Period $ 2.0 $ 2.1    
Additions 0.1 0.2    
Less: Payments / Reductions 0.1 0.2    
Total Balance at End of Period 2.0 2.1    
Less: Current Portion     $ 0.6 $ 0.6
Noncurrent Balance     1.4 1.5
Total Balance at End of period 2.0 2.1 2.0 2.1
Fitchburg Gas and Electric Light Company        
Environmental Exit Cost [Line Items]        
Total Balance at Beginning of Period   0.1    
Additions   0.1    
Less: Payments / Reductions   0.1    
Total Balance at End of Period   0.1    
Less: Current Portion       0.1
Total Balance at End of period   0.1   0.1
Northern Utilities Inc        
Environmental Exit Cost [Line Items]        
Total Balance at Beginning of Period 2.0 2.0    
Additions 0.1 0.1    
Less: Payments / Reductions 0.1 0.1    
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 $ 2.0 $ 2.0 $ 2.0