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Company's Liability for Environmental Obligations (Detail) - USD ($)
$ in Millions
 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Dec. 31, 2015
Jun. 30, 2015
Environmental Exit Cost [Line Items]          
Total Balance at Beginning of period $ 2.8 $ 5.5      
Additions 0.9 0.3      
Less: Payments / Reductions 0.7 0.4      
Total Balance at End of period 3.0 5.4      
Less: Current Portion     $ 0.3 $ 1.3 $ 3.4
Noncurrent Balance     2.7 1.5 2.0
Total Balance at End of period 3.0 5.4 3.0 2.8 5.4
Fitchburg Gas and Electric Light Company          
Environmental Exit Cost [Line Items]          
Total Balance at Beginning of period 1.2 1.9      
Additions   0.1      
Less: Payments / Reductions 0.1 0.2      
Total Balance at End of period 1.1 1.8      
Less: Current Portion     0.1   1.8
Noncurrent Balance     1.0    
Total Balance at End of period 1.1 1.8 1.1 1.2 1.8
Northern Utilities Inc          
Environmental Exit Cost [Line Items]          
Total Balance at Beginning of period 1.6 3.6      
Additions 0.9 0.2      
Less: Payments / Reductions 0.6 0.2      
Total Balance at End of period 1.9 3.6      
Less: Current Portion     0.2   1.6
Noncurrent Balance     1.7   2.0
Total Balance at End of period $ 1.9 $ 3.6 $ 1.9 $ 1.6 $ 3.6