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Regulatory Matters - Additional Information (Detail)													1 Months Ended						3 Months Ended
	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

X

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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