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Regulatory Matters - Additional Information (Detail) $ in Millions |  |  |  |  |  |  |  |  | 6 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 28, 2016 USD ($) | Jun. 24, 2016 USD ($) | Apr. 29, 2016 USD ($) | Jun. 16, 2015 USD ($) | May 01, 2015 USD ($) | Apr. 15, 2015 USD ($) | May 30, 2014 USD ($) | Apr. 25, 2013 USD ($) | Jun. 30, 2016 USD ($) Customer | Dec. 31, 2014 USD ($) | Dec. 31, 2015 USD ($) | Jun. 30, 2015 USD ($)
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Current Portion of Regulatory Liabilities |  |  |  |  |  |  |  |  | $ 15.9 |  | $ 15.6 | $ 31.2
Regulatory Liabilities - non current |  |  |  |  |  |  |  |  | $ 3.7 |  | $ 8.1 | $ 11.0
Northern Utilities Inc |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
TIRA initial term |  |  |  |  |  |  |  |  | Four years |  |  |
Amendment approved date |  |  |  |  |  |  |  |  | Apr. 28, 2016 |  |  |
Pipeline refund received |  |  |  |  |  | $ 22.0 |  |  |  |  |  |
Current Portion of Regulatory Liabilities |  |  |  |  |  |  |  |  | $ 5.0 |  |  |
Regulatory Liabilities - non current |  |  |  |  |  |  |  |  | $ 3.6 |  |  |
Northern Utilities Inc | 2014 Step Adjustments |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  |  |  | May 01, 2014 |  |  |
Increase in annual revenue |  |  |  |  |  |  |  |  | $ 1.4 |  |  |
Northern Utilities Inc | First Annual Adjustment |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Annual TIRA Adjustment |  |  |  |  |  |  |  |  |  | $ 1.5 |  |
Annual TIRA Adjustment, effective date |  |  |  |  |  |  |  |  | May 01, 2016 |  |  |
Northern Utilities Inc | New Hampshire |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  |  |  | May 01, 2015 |  |  |
Increase in annual revenue |  |  |  |  | $ 1.8 |  |  |  |  |  |  |
Grace period to receive refund |  |  |  |  |  |  |  |  | 3 years |  |  |
Northern Utilities Inc | New Hampshire | Natural Gas Distribution |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  |  |  |  |  | $ 4.6 |  |  |
Northern Utilities Inc | Maine |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Construction completion period |  |  |  |  |  |  |  |  | 3 years |  |  |
Number of new customers | Customer |  |  |  |  |  |  |  |  | 1,000 |  |  |
Potential regulated operating revenue |  |  |  |  |  |  |  |  | $ 1.0 |  |  |
Grace period to receive refund |  |  |  |  |  |  |  |  | 3 years |  |  |
Date to receive a refund as one-time lump sum payment |  |  |  |  |  |  |  |  | Oct. 05, 2015 |  |  |
Fitchburg Gas and Electric Light Company |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  | May 01, 2016 |  |  |  |  |  |  |  |  |
Revenue requirement reconciled rates effective date |  |  | Nov. 01, 2016 |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Cumulative capital investments for 2015 and 2016 |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Recovery amount |  |  | $ 0.9 |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Electric base |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  | May 01, 2016 |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  | $ 2.1 | $ 3.8 |  |  |  |  |  |  |  |
Percentage of approved return on equity |  |  |  | 5.60% |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Gas base |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  | May 01, 2016 |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  | $ 1.6 | $ 3.0 |  |  |  |  |  |  |  |
Percentage of approved return on equity |  |  |  | 8.30% |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Storm Costs |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Storm expenditures |  |  |  |  |  |  | $ 5.0 |  |  |  |  |
Cost recovery period, years |  |  |  |  |  |  | 3 years |  |  |  |  |
Unitil Energy Systems Inc |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Interest rate allowed by regulators on the unrecovered amount of storm costs which have been approved for recovery through rate adjustments |  |  |  |  |  |  |  | 4.52% |  |  |  |
Recovery of damage of electrical system |  |  |  |  |  |  |  | $ 1.7 |  |  |  |
Unitil Energy Systems Inc | New Hampshire |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Increase in base rates, amount |  |  | $ 6.3 |  |  |  |  |  |  |  |  |
Percent of increase in total revenue |  |  | 3.60% |  |  |  |  |  |  |  |  |
Temporary rate increase | $ 2.4 |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  |  |  | Jul. 01, 2016 |  |  |
Unitil Energy Systems Inc | Storm Costs |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Recovery amount |  |  |  |  |  |  |  | $ 2.3 |  |  |  |
Cost recovery period, years |  |  |  |  |  |  |  | 5 years |  |  |  |
Granite State Gas Transmission Inc | Gas Transportation and Storage |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  | Aug. 01, 2016 |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  | $ 0.3 |  |  |  |  |  |  |  |  |  |
Amendment approved date |  | Jul. 13, 2017 |  |  |  |  |  |  |  |  |  |

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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 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=66902209&loc=d3e3602-108585 + Details
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- Definition Describes the term of the registration payment arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6449706&loc=d3e16207-108621 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + 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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- Definition Remaining recovery period of regulatory assets for which no return on investment during recovery period is provided, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 55 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6591318&loc=d3e44360-110383 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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- Definition Represents the interest rate allowed by regulators on the unrecovered amount of storm costs which have been approved for recovery through rate adjustments. + References No definition available. + Details
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