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Environmental Matters - Additional Information (Detail) (USD $) In Millions, unless otherwise specified |  |  |  | 9 Months Ended |
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Sep. 30, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Sep. 30, 2014 Fitchburg Gas and Electric Light Company Environmental Clean Up Costs | Sep. 30, 2014 Fitchburg Gas and Electric Light Company Environmental Restoration Costs | Sep. 30, 2014 New Hampshire Environmental Restoration Costs | Sep. 30, 2014 Maine Environmental Restoration Costs | Sep. 30, 2014 Environmental Matters | Sep. 30, 2014 Revised Estimate Fitchburg Gas and Electric Light Company Environmental Clean Up Costs
Site Contingency [Line Items] |  |  |  |  |  |  |  |  |
Income (loss) incurred in excess of recorded amount |  |  |  |  |  |  |  | $ 0 |
Amortization period for environmental costs |  |  |  |  | 7 years | 7 years | 5 years |  |
Environmental obligations | 7.3 | 14.8 | 14.9 | 12.0 |  |  |  |  | 5.5
Environmental obligations, spent during the year |  |  |  | $ 1.4 |  |  |  |  |

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- Definition Total costs accrued as of the balance sheet date for environmental loss contingencies. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346 + Details
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- Definition Disclosure that no losses are anticipated exceeding those amounts recorded. + References No definition available. + Details
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