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Environmental Matters - Additional Information (Detail)
 9 Months Ended
Sep. 30, 2016
USD ($)
New Hampshire | Environmental Restoration Costs  
Site Contingency [Line Items]  
Amortization period for environmental costs 7 years
Maine | Environmental Restoration Costs  
Site Contingency [Line Items]  
Amortization period for environmental costs 5 years
Environmental Matters  
Site Contingency [Line Items]  
Income (loss) incurred in excess of recorded amount $ 0
Fitchburg Gas and Electric Light Company | Environmental Restoration Costs  
Site Contingency [Line Items]  
Amortization period for environmental costs 7 years