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Regulatory Matters - Additional Information (Detail) |  |  |  |  |  |  |  |  |  |  | 1 Months Ended | 9 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 ($) | Feb. 28, 2018 USD ($) | Jan. 01, 2018 USD ($) | Aug. 25, 2017 MW | Aug. 01, 2017 USD ($) | Apr. 20, 2017 USD ($) | Dec. 31, 2017 | Sep. 30, 2018 USD ($) State Customer MW | Jun. 30, 2027 MW | Dec. 31, 2025 MWh | Dec. 31, 2022 MWh | Jun. 01, 2018 USD ($) | May 10, 2018 USD ($) | Jun. 30, 2017 MW | Jun. 23, 2017 kW | Nov. 09, 2016 MW
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Spending cap |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4,400,000 |  |  |
Annual revenue adjustment |  |  |  | $ 2,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power capacity facility for small customers | kW |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100 |
Tax Year 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate income tax rate |  |  |  |  |  |  |  |  |  |  | 21.00% | 21.00% |  |  |  |  |  |  |  |
Scenario Forecast |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual revenue adjustment | $ 2,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  | $ 900,000 |  |  |  |  |  |  |  | $ (0.008) |  |  |  |  |  |  |  |
Description of percentage of sales in legislation |  |  |  |  |  |  |  |  |  |  |  | The legislation contains numerous provisions, including: a requirement that increases the pace at which the Class 1 Renewable Portfolio Standard requirement increases, from the current pace of an additional 1 percent of sales each year to an additional 2 percent of sales each year during the period from January 1, 2020 through December 31, 2029. |  |  |  |  |  |  |  |
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 | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MW |  |  |  |  |  |  |  |  |  |  |  | 1,600 |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,000 | 9,450,000 |  |  |  |  |
Fitchburg Gas and Electric Light Company | Scenario Forecast | Offshore Wind Energy |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MW |  |  |  |  |  |  |  |  |  |  |  |  | 1,600 |  |  |  |  |  |  |
Northern Utilities Inc |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  | $ 2,600,000 |  |  |  | $ 3,200,000 |  | $ 2,600,000 |  |  |  |  |  |  |  |  |  |  |
Increase in annual base rate |  | 2.40% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  | $ (1,700,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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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