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Environmental Matters - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2013
New Hampshire Division | Environmental Restoration Costs |
Site Contingency [Line Items] |
Amortization period for environmental costs | 7 years
Maine Division | Environmental Restoration Costs |
Site Contingency [Line Items] |
Amortization period for environmental costs | 5 years
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 | $ 12.0
Northern Utilities Inc |
Site Contingency [Line Items] |
Environmental Obligations | 1.8
Northern Utilities Inc | Other Current Liabilities |
Site Contingency [Line Items] |
Environmental Obligations | 1.0
Environmental Matters |
Site Contingency [Line Items] |
Income (loss) incurred in excess of recorded amount | $ 0

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- Definition Carrying value as of the balance sheet date of obligations incurred and payable, pertaining to costs that are statutory in nature, are incurred on contractual obligations, or accumulate over time and for which invoices have not yet been received or will not be rendered. Examples include taxes, interest, rent and utilities. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.20) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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