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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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