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Company's Liability for Environmental Obligations (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2015
Dec. 31, 2014
Environmental Exit Cost [Line Items]        
Total Balance at Beginning of period $ 5.5 $ 14.8    
Additions 2.9 1.3    
Less: Payments / Reductions 5.6 10.6    
Total Balance at End of period 2.8 5.5    
Less: Current Portion     $ 1.3 $ 3.5
Noncurrent Balance     1.5 2.0
Total Balance at End of period 5.5 14.8 2.8 5.5
Fitchburg Gas and Electric Light Company        
Environmental Exit Cost [Line Items]        
Total Balance at Beginning of period 1.9 12.0    
Less: Payments / Reductions 0.7 10.1    
Total Balance at End of period 1.2 1.9    
Less: Current Portion     0.2 1.9
Noncurrent Balance     1.0  
Total Balance at End of period 1.9 12.0 1.2 1.9
Northern Utilities Inc        
Environmental Exit Cost [Line Items]        
Total Balance at Beginning of period 3.6 2.8    
Additions 2.9 1.3    
Less: Payments / Reductions 4.9 0.5    
Total Balance at End of period 1.6 3.6    
Less: Current Portion     1.1 1.6
Noncurrent Balance     0.5 2.0
Total Balance at End of period $ 3.6 $ 2.8 $ 1.6 $ 3.6