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Environmental Matters - Additional Information (Detail) - USD ($)	1 Months Ended	6 Months Ended
	May 31, 2016	Jun. 30, 2016
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
Northern Utilities Inc | Lewiston Maine
Site Contingency [Line Items]
Period of completed maintenance activities		10 years
Period of recommended on-site monitoring	10 years

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