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Environmental Matters - Additional Information (Detail) | 9 Months Ended
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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

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