Environmental Matters - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2016 USD ($)
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 | Environmental Restoration Costs
Site Contingency [Line Items]
Amortization period for environmental costs	7 years