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Company's Liability for Environmental Obligations (Detail) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Dec. 31, 2015
Sep. 30, 2015
Environmental Exit Cost [Line Items]          
Total Balance at Beginning of period $ 2.8 $ 5.5      
Additions 0.9 1.9      
Less: Payments / Reductions 0.7 1.9      
Total Balance at End of period 3.0 5.5      
Less: Current Portion     $ 0.3 $ 1.3 $ 5.0
Noncurrent Balance     2.7 1.5 0.5
Total Balance at End of period 3.0 5.5 3.0 2.8 5.5
Fitchburg Gas and Electric Light Company          
Environmental Exit Cost [Line Items]          
Total Balance at Beginning of period 1.2 1.9      
Less: Payments / Reductions 0.1 0.3      
Total Balance at End of period 1.1 1.6      
Less: Current Portion     0.1   1.6
Noncurrent Balance     1.0    
Total Balance at End of period 1.1 1.6 1.1 1.2 1.6
Northern Utilities Inc          
Environmental Exit Cost [Line Items]          
Total Balance at Beginning of period 1.6 3.6      
Additions 0.9 1.9      
Less: Payments / Reductions 0.6 1.6      
Total Balance at End of period 1.9 3.9      
Less: Current Portion     0.2   3.4
Noncurrent Balance     1.7   0.5
Total Balance at End of period $ 1.9 $ 3.9 $ 1.9 $ 1.6 $ 3.9