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Regulatory Matters - Additional Information (Detail)															1 Months Ended					9 Months Ended
	Nov. 01, 2023 USD ($)	Nov. 01, 2022 USD ($)	Nov. 01, 2021 USD ($)	Mar. 01, 2021 USD ($)	Nov. 01, 2020 USD ($)	May 01, 2020 USD ($)	Mar. 26, 2020 USD ($)	Feb. 28, 2020 USD ($)	Jan. 31, 2020 USD ($)	Oct. 29, 2019 USD ($)	May 01, 2019 USD ($)	Apr. 30, 2019 USD ($)	Apr. 22, 2019 USD ($)	Nov. 01, 2018 USD ($)	Mar. 31, 2020 USD ($)	May 01, 2019 USD ($)	May 02, 2018 USD ($)	Dec. 31, 2017	Apr. 20, 2017 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2027 MW	Dec. 31, 2022 MWh	Jul. 31, 2018 MW MWh	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										$ 1,100,000				$ 900,000
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]
Increase in annual base rate							3.60%
Approved annual increase in rates							$ 3,600,000
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]
Requested annual increase in rates						$ 1,400,000
Approved annual increase in rates																	$ 3,200,000
Fitchburg Gas Company
Regulatory Assets [Line Items]
Approved annual increase in rates								$ 4,600,000	$ 1,100,000		$ 600,000	$ 1,600,000			$ 3,700,000	$ 1,000,000.0
Approved annual increase in rates																				$ 900,000
Apporved annual decrease in rates																				$ 200,000
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								$ 40,000
Regulatory assets approved increase in revenue due to be recovered				$ 2,100,000
Regulatory assets approved revenue increase decrease regulatory period				24 months
Fitchburg Gas Company | Scenario Forecast
Regulatory Assets [Line Items]
Approved annual increase in rates				$ 900,000
Granite State
Regulatory Assets [Line Items]
Approved annual increase in rates					$ 1,300,000
Granite State | Scenario Forecast
Regulatory Assets [Line Items]
Approved annual increase in rates	$ 14,600,000	$ 14,600,000	$ 14,600,000

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