Commitments and Contingencies - Additional Information (Detail)														12 Months Ended
	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