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Commitments and Contingencies - Additional Information (Detail) |  |  |  |  |  |  |  |  |  |  |  |  |  | 12 Months Ended |  |  |  |
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Nov. 30, 2020 USD ($) | Nov. 02, 2020 USD ($) | Nov. 01, 2020 USD ($) | Apr. 17, 2020 USD ($) | Mar. 26, 2020 USD ($) | Feb. 28, 2020 USD ($) | Oct. 29, 2019 USD ($) | Jun. 25, 2019 | May 01, 2019 USD ($) | Apr. 30, 2019 USD ($) | Nov. 01, 2018 USD ($) | May 02, 2018 USD ($) | Apr. 20, 2017 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 | Dec. 31, 2017 | Jan. 10, 2020 MWh | May 23, 2019 MWh | Jul. 31, 2018 MW MWh | Jun. 30, 2017 MWh MW
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accrued Revenue |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 50,900,000 | $ 50,000,000.0 |  |  |  |  |  |
Regulatory Assets |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 127,400,000 | $ 112,000,000.0 |  |  |  |  |  |
Corporate income tax rate |  |  |  |  |  |  |  |  |  |  |  |  |  | 21.00% | 21.00% | 21.00% |  |  |  |  |
Spending cap |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 571,800,000 |  |  |  |  |  |  |
Accrued Estimated Costs For Remediation |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 800,000 |  |  |  |  |  |  |
Tax Year 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate income tax rate |  |  |  |  |  |  |  |  |  |  |  |  |  | 21.00% |  | 21.00% | 21.00% |  |  |  |
Other Restructuring |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accrued Revenue |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 300,000 |  |  |  |  |  |  |
Regulatory Assets |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 300,000 |  |  |  |  |  |  |
Cost recovery period, years |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 years |  |  |  |  |  |  |
Regulatory assets approved revenue increase decrease regulatory period |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 years |  |  |  |  |  |  |
New Hampshire |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  |  |  |  |  |  |  |  |  |  | $ 3,200,000 |  |  |  |  |  |  |  |  |
Northern Utilities Inc |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  |  |  |  |  |  |  |  |  |  | $ 1,400,000 |  |  | $ 1,400,000 |  |  |  |  |  |
Northern Utilities Inc | Maine |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  |  |  | $ 3,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of increase in test year operating revenue |  |  |  |  | 3.60% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity, reflecting on equity |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity, reflecting on debt |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity |  |  |  |  | 9.48% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  | $ 1,400,000 | $ 1,100,000 | $ 900,000 |  |  | $ 1,100,000 |  | $ 1,000,000.0 |  | $ 900,000 |  |  |  |  |  |  |  |  |  |
Original Settlement Amount |  |  |  | $ 200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recovery amount |  |  |  |  |  |  |  |  | 600,000 |  |  |  |  |  |  |  |  |  |  |  |
Power generation capacity | MWh |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9,450,000
Percentage of approved return on equity, reflecting on equity |  |  |  | 52.45% |  | 52.45% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity, reflecting on debt |  |  |  | 47.55% |  | 47.55% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity |  |  |  | 9.70% |  | 9.70% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue Impact Threshold |  |  |  | $ 100,000 |  | $ 40,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Offshore Wind Energy |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MW |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 400 | 1,600
Remuneration Percentage |  |  |  |  |  |  |  | 2.75% |  |  |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Offshore Wind Energy | First Solicitation [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MWh |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 800 |  |  |
Fitchburg Gas and Electric Light Company | Offshore Wind Energy | Second Solicitation [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MWh |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 800 |  |
Fitchburg Gas and Electric Light Company | Qualified Clean Energy |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation capacity | MWh |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9,554,940 |
Remuneration Percentage |  |  |  |  |  |  |  | 2.75% |  |  |  |  |  |  |  |  |  |  |  |  |
Unitil Energy Systems Inc | New Hampshire |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Permanent rate increase |  |  |  |  |  |  |  |  |  |  |  |  | $ 4,100,000 |  |  |  |  |  |  |  |
Unitil Energy Systems Inc | New Hampshire | Second Rate Step Adjustments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  |  |  |  |  |  |  | $ 340,000 |  |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas Company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cost recovery period, years |  |  |  |  |  |  |  |  |  | 24 months |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  |  |  |  | 4,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets approved increase in revenue due to be recovered |  |  |  |  |  |  |  |  |  | $ 2,200,000 |  |  |  |  |  |  |  |  |  |  |
Regulatory assets approved revenue increase decrease regulatory period |  |  |  |  |  |  |  |  |  | 24 months |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas Company | First Solicitation [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  |  |  |  | $ 3,700,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Granite State |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue | $ 1,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Spending cap |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 14,600,000 |  |  |  |  |  |  |

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