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Company's Liability for Environmental Obligations (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Mar. 31, 2016
Dec. 31, 2015
Mar. 31, 2015
Environmental Exit Cost [Line Items]          
Total Balance at Beginning of period $ 2.8 $ 5.5      
Additions 1.0 0.1      
Less: Payments / Reductions 1.1 0.2      
Total Balance at End of period 2.7 5.4      
Less: Current Portion     $ 0.7 $ 1.3 $ 3.4
Noncurrent Balance     2.0 1.5 2.0
Total Balance at End of period 2.7 5.4 2.7 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      
Less: Payments / Reductions   0.1      
Total Balance at End of period 1.2 1.8      
Less: Current Portion     0.2   1.8
Noncurrent Balance     1.0    
Total Balance at End of period 1.2 1.8 1.2 1.2 1.8
Northern Utilities Inc          
Environmental Exit Cost [Line Items]          
Total Balance at Beginning of period 1.6 3.6      
Additions 1.0 0.1      
Less: Payments / Reductions 1.1 0.1      
Total Balance at End of period 1.5 3.6      
Less: Current Portion     0.5   1.6
Noncurrent Balance     1.0   2.0
Total Balance at End of period $ 1.5 $ 3.6 $ 1.5 $ 1.6 $ 3.6