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Environmental Obligations Recognized by Company (Detail) - USD ($)
$ in Millions
 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Dec. 31, 2014
Site Contingency [Line Items]      
Beginning Balance $ 5.5 $ 14.8  
Additions 0.3 6.1  
Less: Payments / Reductions 0.4 12.5  
Ending Balance 5.4 8.4  
Less: Current Portion 3.4 6.4 $ 3.5
Ending Balance 2.0 2.0 $ 2.0
Fitchburg Gas and Electric Light Company      
Site Contingency [Line Items]      
Beginning Balance 1.9 12.0  
Additions 0.1 5.5  
Less: Payments / Reductions 0.2 12.3  
Ending Balance 1.8 5.2  
Less: Current Portion 1.8 5.2  
Northern Utilities Inc      
Site Contingency [Line Items]      
Beginning Balance 3.6 2.8  
Additions 0.2 0.6  
Less: Payments / Reductions 0.2 0.2  
Ending Balance 3.6 3.2  
Less: Current Portion 1.6 1.2  
Ending Balance $ 2.0 $ 2.0