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Company's Liability for Environmental Obligations (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Mar. 31, 2017
Dec. 31, 2016
Mar. 31, 2016
Environmental Exit Cost [Line Items]          
Total Balance at Beginning of period $ 2.0 $ 2.8      
Additions 0.4 1.0      
Less: Payments / Reductions 0.1 1.1      
Total Balance at End of period 2.3 2.7      
Less: Current Portion     $ 0.5 $ 0.4 $ 0.7
Noncurrent Balance     1.8 1.5 2.0
Total Balance at End of period 2.3 2.7 2.3 2.0 2.7
Fitchburg Gas and Electric Light Company          
Environmental Exit Cost [Line Items]          
Total Balance at Beginning of period 0.1 1.2      
Total Balance at End of period 0.1 1.2      
Less: Current Portion     0.1   0.2
Noncurrent Balance         1.0
Total Balance at End of period 0.1 1.2 0.1 0.1 1.2
Northern Utilities Inc          
Environmental Exit Cost [Line Items]          
Total Balance at Beginning of period 1.9 1.6      
Additions 0.4 1.0      
Less: Payments / Reductions 0.1 1.1      
Total Balance at End of period 2.2 1.5      
Less: Current Portion     0.4   0.5
Noncurrent Balance     1.8   1.0
Total Balance at End of period $ 2.2 $ 1.5 $ 2.2 $ 1.9 $ 1.5