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Environmental Obligations Recognized by Company (Detail) - USD ($) $ in Millions | 6 Months Ended |
--- | --- | ---
Jun. 30, 2015 | Jun. 30, 2014 | Dec. 31, 2014
Site Contingency [Line Items] |  |  |
Beginning Balance | $ 5.5 | $ 14.8 |
Additions | 0.3 | 6.1 |
Less: Payments / Reductions | 0.4 | 12.5 |
Ending Balance | 5.4 | 8.4 |
Less: Current Portion | 3.4 | 6.4 | $ 3.5
Ending Balance | 2.0 | 2.0 | $ 2.0
Fitchburg Gas and Electric Light Company |  |  |
Site Contingency [Line Items] |  |  |
Beginning Balance | 1.9 | 12.0 |
Additions | 0.1 | 5.5 |
Less: Payments / Reductions | 0.2 | 12.3 |
Ending Balance | 1.8 | 5.2 |
Less: Current Portion | 1.8 | 5.2 |
Northern Utilities Inc |  |  |
Site Contingency [Line Items] |  |  |
Beginning Balance | 3.6 | 2.8 |
Additions | 0.2 | 0.6 |
Less: Payments / Reductions | 0.2 | 0.2 |
Ending Balance | 3.6 | 3.2 |
Less: Current Portion | 1.6 | 1.2 |
Ending Balance | $ 2.0 | $ 2.0 |

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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 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349 Reference 2: 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 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 The aggregate amount disbursed during the reporting period to settle environmental loss contingencies. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 5.Y.Q2) -URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751 + Details
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- Definition The amount added to the accrual for newly incurred environmental loss contingencies. These additions include remediation expense and interest accretion. These additions do not relate to changes in the environmental accrual related to a business acquisition or divestiture, to currency movements, or revisions in previously existing estimates. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 5.Y.Q2) -URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751 + Details
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- Definition Carrying value of the obligation (known or estimated) arising from requirements to perform activities to remediate one or more sites, payable in twelve months or in the next operating cycle if longer. + 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 210 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6911-107765 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 10 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13237-110859 Reference 5: 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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