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Environmental Matters - Additional Information (Detail) - USD ($) | 6 Months Ended |  |
--- | --- | --- | ---
Jun. 30, 2015 | Jun. 30, 2014 | Dec. 31, 2014 | Dec. 31, 2013
Site Contingency [Line Items] |  |  |  |
Environmental obligations | $ 5,400,000 | $ 8,400,000 | $ 5,500,000 | $ 14,800,000
Environmental obligations, spent during the year | $ 400,000 | 12,500,000 |  |
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 |  |  |  |
Site Contingency [Line Items] |  |  |  |
Environmental obligations | 1,800,000 | 5,200,000 | 1,900,000 | 12,000,000
Environmental obligations, spent during the year | $ 200,000 | 12,300,000 |  |
Fitchburg Gas and Electric Light Company | Environmental Restoration Costs |  |  |  |
Site Contingency [Line Items] |  |  |  |
Amortization period for environmental costs | 7 years |  |  |
Fitchburg Gas and Electric Light Company | Environmental Clean Up Costs |  |  |  |
Site Contingency [Line Items] |  |  |  |
Environmental obligations | $ 5,500,000 |  |  |
Environmental obligations, spent during the year | 3,800,000 |  |  |
Northern Utilities Inc |  |  |  |
Site Contingency [Line Items] |  |  |  |
Accrued remedial costs | 1,700,000 |  |  |
Environmental obligations | 3,600,000 | 3,200,000 | $ 3,600,000 | $ 2,800,000
Environmental obligations, spent during the year | $ 200,000 | $ 200,000 |  |

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