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Regulatory Matters - Additional Information (Detail) |  |  |  |  |  |  |  |  |  |  |  |  | 1 Months Ended | 6 Months Ended |  |  |  |  |  |  |
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May 01, 2019 USD ($) | May 07, 2018 USD ($) | May 02, 2018 USD ($) | May 01, 2018 USD ($) | Apr. 30, 2018 USD ($) | Feb. 28, 2018 USD ($) | Dec. 20, 2017 USD ($) | Oct. 31, 2017 USD ($) | Aug. 25, 2017 MW | Aug. 01, 2017 USD ($) | Apr. 20, 2017 USD ($) | Apr. 29, 2016 USD ($) | Dec. 31, 2017 | Jun. 30, 2018 USD ($) State Customer | Jun. 30, 2027 MW | Dec. 31, 2022 MWh | Jun. 01, 2018 USD ($) | May 10, 2018 USD ($) | Jun. 30, 2017 MW | Jun. 23, 2017 kW | Aug. 19, 2016 MW
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual revenue adjustment |  |  |  | $ 2.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Spending cap |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4,400,000 |  |  |
Power capacity facility for small customers | kW |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100 |
Scenario Forecast |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual revenue adjustment | $ 2.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Tax Year 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate income tax rate |  |  |  |  |  |  |  |  |  |  |  |  | 21.00% | 21.00% |  |  |  |  |  |  |
Unitil Energy Systems Inc | New Hampshire |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  |  |  | $ 300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Permanent rate increase |  |  |  |  |  |  |  |  |  |  | $ 4,100,000 |  |  |  |  |  |  |  |  |  |
Reduction of federal tax decrease |  |  |  |  | 2,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reconciliation adjustment of recouped difference between temporary rates and final rates |  |  |  |  | 1,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unitil Energy Systems Inc | New Hampshire | First Rate Step Adjustments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Permanent rate increase |  |  |  |  |  |  |  |  |  |  | $ 900,000 |  |  |  |  |  |  |  |  |  |
Unitil Energy Systems Inc | New Hampshire | Second rate Step Adjustments [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  |  |  | $ 3,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  |  |  |  |  |  | $ 400,000 |  |  |  |  |  | $ (800,000) |  |  |  |  |  |  |
Recovery amount |  |  |  |  |  |  |  | $ 900,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of waiver approved on annual changes in the revenue requirement eligible for recovery |  |  |  |  | 1.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  |  | May 01, 2018 |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues sought approval to collect, deferred for recovery in future periods |  |  |  |  |  |  |  | $ 900,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of waiver sought on annual changes in the revenue requirement eligible for recovery |  |  |  |  | 1.50% |  |  | 3.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Number of states with electric distribution companies | State |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Solar Generation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MW |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.3
Recovery amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 300,000 |  |  |  |
Fitchburg Gas and Electric Light Company | Minimum | Offshore Wind Energy |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MW |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 400 |  |
Fitchburg Gas and Electric Light Company | Solar Massachusetts Renewable Target Program | Solar Generation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation capacity | MW |  |  |  |  |  |  |  |  | 1,600 |  |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Scenario Forecast |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation capacity | MWh |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9,450,000 |  |  |  |  |
Fitchburg Gas and Electric Light Company | Scenario Forecast | Offshore Wind Energy |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MW |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,600 |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Gas base |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  |  |  |  |  |  |  |  |  |  | $ 1,600,000 |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  |  |  |  |  |  | May 01, 2016 |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Electric base |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  |  |  |  |  |  |  |  |  |  | $ 2,100,000 |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  | Jan. 01, 2018 |  |  |  |  | May 01, 2016 |  | Jan. 01, 2017 |  |  |  |  |  |  |
Annual revenue adjustment |  |  |  |  |  |  | $ 400,000 |  |  |  |  |  |  | $ 500,000 |  |  |  |  |  |  |
Northern Utilities Inc |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  | $ 2.6 |  |  |  |  |  |  | $ 2.6 |  |  |  |  |  |  |  |  |  |  |
Increase in annual base rate |  | $ 0.024 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual TIRA Adjustment |  | $ 1,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
TIRA initial term |  |  |  |  |  |  |  |  |  |  |  |  |  | Four years |  |  |  |  |  |  |
Northern Utilities Inc | Revenue Offset [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  | $ (1.7) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Northern Utilities Inc | Maine |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  |  |  |  | $ 2,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity |  |  |  |  |  | 9.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity, reflecting on equity |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity, reflecting on debt |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reduction of annual operating costs |  |  |  |  |  | $ 500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
TIRA extended term |  |  |  |  |  |  |  |  |  |  |  |  |  | 8 years |  |  |  |  |  |  |
Construction completion period |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 years |  |  |  |  |  |  |
Number of new customers | Customer |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,000 |  |  |  |  |  |  |
Potential regulated operating revenue |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,000,000 |  |  |  |  |  |  |
Northern Utilities Inc | Maine | Second TAB program |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of new customers | Customer |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,000 |  |  |  |  |  |  |
Potential regulated operating revenue |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,000,000 |  |  |  |  |  |  |
Northern Utilities Inc | Maine | Revenue Offset [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  |  |  |  | $ 2,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=77907324&loc=d3e43603-110378 + Details
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