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Regulatory Matters - Additional Information (Detail) |  |  |  |  |  |  |  |  |  |  |  |  | 1 Months Ended | 3 Months Ended |  |  |  |
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Mar. 01, 2021 USD ($) | May 01, 2020 USD ($) | Mar. 26, 2020 USD ($) | Feb. 28, 2020 USD ($) | Jan. 31, 2020 USD ($) | Dec. 17, 2019 USD ($) | Oct. 29, 2019 USD ($) | May 01, 2019 USD ($) | Apr. 30, 2019 USD ($) | Apr. 22, 2019 USD ($) | Apr. 17, 2019 USD ($) | Nov. 01, 2018 USD ($) | Mar. 31, 2020 USD ($) | Jun. 28, 2019 USD ($) | May 01, 2019 USD ($) | May 02, 2018 USD ($) | Dec. 31, 2017 | Apr. 20, 2017 USD ($) | Mar. 31, 2020 | Jun. 30, 2027 MW | Dec. 31, 2022 MWh | Jul. 31, 2018 MWh MW | Jun. 30, 2017 MW
Tax Year 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate income tax rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 21.00% |  | 21.00% |  |  |  |
Offshore Wind Energy |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MW |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,600 |  |  |
Unitil Energy Systems Inc | New Hampshire |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved annual increase in rates |  |  |  |  |  |  |  |  |  | $ 340,000 |  |  |  |  |  |  |  | $ 4,100,000 |  |  |  |  |
Fitchburg Gas and Electric Light Company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested annual increase in rates |  |  |  |  |  | $ 2,700,000 | $ 1,100,000 |  |  |  |  | $ 900,000 |  |  |  |  |  |  |  |  |  |  |
Increase in annual base rate |  |  |  |  |  | 4.10% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity |  |  |  |  | 9.70% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity, reflecting on equity |  |  |  |  | 52.45% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity, reflecting on debt |  |  |  |  | 47.55% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue impact threshold |  |  |  |  | $ 100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Offshore Wind Energy |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MW |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 400 |
Remuneration Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.75 | 2.75
Fitchburg Gas and Electric Light Company | Offshore Wind Energy | First Solicitation [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MW |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 800 |
Fitchburg Gas and Electric Light Company | Offshore Wind Energy | Second Solicitation [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MW |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 800 |
Fitchburg Gas and Electric Light Company | Qualified Clean Energy |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power Generation Capacity | MWh |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9,554,940 |
Fitchburg Gas and Electric Light Company | Minimum [Member] | Offshore Wind Energy |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MW |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 400
Fitchburg Gas and Electric Light Company | Scenario Forecast |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power Generation Capacity | MWh |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9,450,000 |  |
Northern Utilities Inc |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved annual increase in rates |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,400,000 |  |  |  |  |  |  |  |
Northern Utilities Inc | Maine |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested annual increase in rates |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 7,000,000.0 |  |  |  |  |  |  |  |  |
Increase in annual base rate |  |  | 3.60% |  |  |  |  |  |  |  | 2.10% |  |  |  |  |  |  |  |  |  |  |  |
Approved annual increase in rates |  |  | $ 3,600,000 |  |  |  |  |  |  |  | $ 1,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity |  |  | 9.48% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity, reflecting on equity |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity, reflecting on debt |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Northern Utilities Inc | New Hampshire |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved annual increase in rates |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 3,200,000 |  |  |  |  |  |  |
Northern Utilities Inc | New Hampshire | Scenario Forecast |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested annual increase in rates |  | $ 1,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas Company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested annual increase in rates |  |  |  |  |  | $ 7,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of change in revenue over previous year |  |  |  |  |  | 20.80% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved annual increase in rates |  |  |  | $ 4,600,000 | $ 1,100,000 |  |  | $ 600,000 | $ 1,600,000 |  |  |  | $ 3,700,000 |  | $ 1,000,000.0 |  |  |  |  |  |  |  |
Percentage of approved return on equity |  |  |  | 9.70% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity, reflecting on equity |  |  |  | 52.45% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity, reflecting on debt |  |  |  | 47.55% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue impact threshold |  |  |  | $ 400,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas Company | Scenario Forecast |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved annual increase in rates | $ 900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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