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Regulatory Matters - Additional Information (Detail) - USD ($) $ in Millions |  |  |  |  |  |  | 3 Months Ended | 6 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 30, 2016 | Jun. 16, 2015 | May. 01, 2015 | Apr. 15, 2015 | May. 30, 2014 | Apr. 25, 2013 | Mar. 31, 2016 | Jun. 30, 2015 | Dec. 31, 2014 | Dec. 31, 2015 | Mar. 31, 2015
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Current Portion of Regulatory Liabilities |  |  |  |  |  |  | $ 18.8 |  |  | $ 15.6 | $ 39.4
Regulatory Liabilities - non current |  |  |  |  |  |  | $ 3.3 |  |  | $ 8.1 |
Fitchburg Gas and Electric Light Company |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  | May 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] |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  | $ 3.8 |  |  |  |  |  |  |  |  |
Percentage of approved return on equity |  | 5.60% |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Gas base |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [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 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 | $ 6.0 |  |  |  |  |  |  |  |  |  |
Percent of increase in total revenue | 4.00% |  |  |  |  |  |  |  |  |  |
Unitil Energy Systems Inc | Storm Costs |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Cost recovery period, years |  |  |  |  |  | 5 years |  |  |  |  |
Recovery amount |  |  |  |  |  | $ 2.3 |  |  |  |  |
Northern Utilities Inc |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
TIRA initial term |  |  |  |  |  |  | Four years |  |  |  |
Pipeline refund received |  |  |  | $ 22.0 |  |  |  |  |  |  |
Current Portion of Regulatory Liabilities |  |  |  |  |  |  | $ 6.8 |  |  |  |
Regulatory Liabilities - non current |  |  |  |  |  |  | $ 3.3 |  |  |  |
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 | 2014 Step Adjustments |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  |  |  | $ 1.4 |  |  |  |
Amendment effective date |  |  |  |  |  |  | May 01, 2014 |  |  |  |
Northern Utilities Inc | New Hampshire |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  | $ 1.8 |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  | May 01, 2015 |  |  |  |
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] |  |  |  |  |  |  |  |  |  |  |
Grace period to receive refund |  |  |  |  |  |  | 3 years |  |  |  |
Date to receive a refund as one-time lump sum payment |  |  |  |  |  |  | Oct. 05, 2015 |  |  |  |
Granite State Gas Transmission Inc | Gas Transportation and Storage |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  |  |  |  | $ 0.4 |  |  |
Amendment effective date |  |  |  |  |  |  |  | Aug. 01, 2015 |  |  |

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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=56944662&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 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 A noncash adjustment to the provision established for storm damage by an oil, gas, or 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 -URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585 + Details
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