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Regulatory Matters - Additional Information (Detail) |  | 1 Months Ended | 6 Months Ended | 9 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Nov. 01, 2017 USD ($) | Jun. 28, 2019 USD ($) | May 01, 2019 USD ($) | Feb. 28, 2019 USD ($) | May 07, 2018 USD ($) | May 02, 2018 USD ($) | Dec. 31, 2017 | Apr. 20, 2017 USD ($) | Jun. 30, 2017 MW | Sep. 30, 2019 | Jun. 30, 2027 MW | Dec. 31, 2022 MWh | Jun. 25, 2019 | Apr. 30, 2019 USD ($) | Jul. 31, 2018 MWh | Jul. 31, 2018 | Jul. 31, 2018 MW
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual revenue adjustment |  |  | $ 1,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MW |  |  |  |  |  |  |  |  |  |  | 1,600 |  |  |  |  |  |
Pro rata percentage share of contracts |  |  |  |  |  |  |  |  | 1 |  |  |  |  |  |  |  |
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] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Permanent rate increase |  |  |  |  |  |  |  | $ 4,100,000 |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  |  | $ 340,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue | $ 900,000 |  | 1,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recovery amount |  |  | $ 600,000 |  |  |  |  |  |  |  |  |  |  | $ 1,600,000 |  |  |
Annual revenue adjustment | $ 400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date | Nov. 01, 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Offshore Wind Energy |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MWh |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 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 | MWh |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 800 |  |
Fitchburg Gas and Electric Light Company | Offshore Wind Energy | Second Solicitation [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 800 |  | 400
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] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  | $ 7,000,000.0 |  |  |  | $ 3,200,000 |  |  |  |  |  |  |  |  |  |  |
Increase In Annual Base Rate |  |  |  | 2.1 | 2.4 |  |  |  |  |  |  |  |  |  |  |  |
Annual TIRA Adjustment |  |  |  | $ 1,000,000.0 | $ 1,100,000 |  |  |  |  |  |  |  |  |  |  |  |
Percentage Of Increase In Test Year Operating Revenue |  | 7.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Northern Utilities Inc | Maine |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
TIRA extended term |  |  |  |  |  |  |  |  |  | 8 years |  |  |  |  |  |  |

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