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Environmental Matters - Additional Information (Detail) - USD ($)
 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Dec. 31, 2014
Dec. 31, 2013
Site Contingency [Line Items]        
Environmental obligations $ 5,500,000 $ 7,300,000 $ 5,500,000 $ 14,800,000
Environmental obligations, spent during the year $ 1,900,000 14,100,000    
Maine | Environmental Restoration Costs        
Site Contingency [Line Items]        
Amortization period for environmental costs 5 years      
New Hampshire | Environmental Restoration Costs        
Site Contingency [Line Items]        
Amortization period for environmental costs 7 years      
Environmental Matters        
Site Contingency [Line Items]        
Income (loss) incurred in excess of recorded amount $ 0      
Fitchburg Gas and Electric Light Company        
Site Contingency [Line Items]        
Environmental obligations 1,600,000 4,100,000 1,900,000 12,000,000
Environmental obligations, spent during the year $ 300,000 13,400,000    
Fitchburg Gas and Electric Light Company | Environmental Restoration Costs        
Site Contingency [Line Items]        
Amortization period for environmental costs 7 years      
Fitchburg Gas and Electric Light Company | Environmental Clean Up Costs        
Site Contingency [Line Items]        
Environmental obligations $ 5,500,000      
Environmental obligations, spent during the year 3,900,000      
Northern Utilities Inc        
Site Contingency [Line Items]        
Accrued remedial costs 1,600,000      
Environmental obligations 3,900,000 3,200,000 $ 3,600,000 $ 2,800,000
Environmental obligations, spent during the year $ 1,600,000 $ 700,000