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Regulatory Matters - Additional Information (Detail) - USD ($) $ in Millions |  |  |  |  |  |  |  |  |  |  | 1 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 16, 2015 | Apr. 15, 2015 | Feb. 27, 2015 | Feb. 19, 2015 | Nov. 26, 2014 | Jun. 27, 2014 | Apr. 30, 2014 | Apr. 21, 2014 | Dec. 27, 2013 | May. 12, 2010 | Jun. 30, 2015 | Jun. 30, 2015 | Dec. 31, 2014 | Jun. 30, 2014
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Current Portion of Regulatory Liabilities |  |  |  |  |  |  |  |  |  |  | $ 31.2 | $ 31.2 | $ 8.7 | $ 13.0
Regulatory Liabilities - noncurrent |  |  |  |  |  |  |  |  |  |  | 11.0 | 11.0 | 0.1 |
Fitchburg Gas and Electric Light Company |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Storm expenditures |  |  |  |  | $ 0.2 |  |  |  |  |  |  |  |  |
Construction expenditures |  |  |  |  | 0.1 |  |  |  |  |  |  |  |  |
Recovery amount |  |  |  |  |  |  |  |  |  |  | 0.3 | $ 0.3 |  |
Fitchburg Gas and Electric Light Company | Electric base |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue | $ 3.8 |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity | 5.60% |  |  |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Gas base |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue | $ 3.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity | 8.30% |  |  |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Storm Costs |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Storm expenditures |  |  |  |  | 0.3 |  |  |  |  |  |  |  |  |
Unitil Energy Systems Inc |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  |  |  |  |  |  | May 01, 2014 |  |
Adjustment to recover the increased spending for vegetation management program and reliability enhancement program and a proposed storm resiliency program |  |  |  |  |  |  | $ 1.5 |  |  |  |  |  |  |
Settlement agreement date |  |  |  |  |  |  |  |  |  |  |  | May 01, 2016 |  |
Construction expenditures |  |  |  |  | 0.4 |  |  |  |  |  |  |  |  |
Recovery of damage of electrical system |  |  |  |  | 1.7 |  |  |  |  |  |  |  |  |
Unitil Energy Systems Inc | Storm Costs |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Storm expenditures |  |  |  |  | $ 2.1 |  |  |  |  |  |  |  |  |
Northern Utilities Inc |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
TIRA initial term |  |  |  |  |  |  |  |  |  |  |  | Four years |  |
Proposed percentage increase in pipeline rates |  |  |  |  |  |  |  |  |  | 55.00% |  |  |  |
Pipeline refund received |  | $ 22.0 |  |  |  |  |  |  |  |  |  |  |  |
Current Portion of Regulatory Liabilities |  |  |  |  |  |  |  |  |  |  | 11.1 | $ 11.1 |  |
Regulatory Liabilities - noncurrent |  |  |  |  |  |  |  |  |  |  | 10.9 | $ 10.9 |  |
Northern Utilities Inc | First Annual Adjustment |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual TIRA Adjustment |  |  |  |  |  |  |  |  |  |  |  |  | $ 1.3 |
Annual TIRA Adjustment, effective date |  |  |  |  |  |  |  |  |  |  |  | May 01, 2014 |  |
Northern Utilities Inc | Second Annual Adjustment |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual TIRA Adjustment |  |  | $ 1.2 |  |  |  |  |  |  |  |  |  |  |
Annual TIRA Adjustment, effective date |  |  |  |  |  |  |  |  |  |  |  | May 01, 2015 |  |
Northern Utilities Inc | 2014 Step Adjustments |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  |  |  |  | $ 1.4 |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  |  |  |  |  |  | May 01, 2014 |  |
Northern Utilities Inc | Maine |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  |  |  |  |  | $ 3.8 |  |  |  |  |
Amendment effective date |  |  |  |  |  |  |  |  |  |  |  | Jan. 01, 2014 |  |
Northern Utilities Inc | New Hampshire |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  | $ 1.8 |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  |  |  |  |  |  | May 01, 2015 |  |
Percent of increase in annual revenue |  |  |  |  |  |  |  | 9.50% |  |  |  |  |  |
Northern Utilities Inc | New Hampshire | Natural Gas Distribution |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  |  |  |  | $ 4.6 |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  |  |  |  |  |  | May 01, 2014 |  |
Northern Utilities Inc | Maximum | Portland Natural Gas Transmission System |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Grace period to issue refund |  |  |  | 60 days |  |  |  |  |  |  |  |  |  |
Northern Utilities Inc | Maximum | Northern Utilities' customers |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Grace period to issue refund |  |  |  |  |  |  |  |  |  |  |  | 3 years |  |
Granite State Gas Transmission Inc | Gas Transportation and Storage |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  |  |  |  |  |  |  | $ 0.4 |  |  |
Amendment effective date |  |  |  |  |  |  |  |  |  |  |  | Aug. 01, 2014 |  |
Annual revenue increase in rate adjustments |  |  |  |  |  | $ 0.6 |  |  |  |  |  |  |  |

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- Definition Carrying amount as of the balance sheet date of the portion of the reserved funds established by a utility from customer payments to cover the repair costs of damages from major storms, which repairs are expected to be incurred beyond one year or the normal operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499739&loc=d3e43603-110378 + Details
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- Definition Future cash outflow to pay for construction in progress expenditures that have occurred. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4332-108586 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4313-108586 + Details
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- Definition The increase (decrease) during the reporting period in unrecovered fuel costs for a regulated utility company. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585 + Details
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- Definition The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 405 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + Details
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