Commitments and Contingencies - Additional Information (Detail)																				3 Months Ended	12 Months Ended
	Nov. 27, 2023 USD ($)	Nov. 01, 2023 USD ($)	Sep. 20, 2023 USD ($)	Aug. 17, 2023 USD ($)	Jul. 27, 2023 USD ($)	May 31, 2023 USD ($)	Jan. 19, 2023	Nov. 02, 2022 USD ($)	Aug. 19, 2022 USD ($)	Jul. 28, 2022 USD ($)	Jul. 20, 2022 USD ($)	Jun. 08, 2022 USD ($)	May 12, 2022 USD ($)	May 03, 2022 USD ($)	Aug. 24, 2021 USD ($)	Dec. 30, 2020 USD ($)	Nov. 30, 2020 USD ($)	Jun. 25, 2019	Feb. 28, 2018 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($) MW	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2017	Oct. 31, 2023 USD ($)	Aug. 30, 2023 MW	Oct. 31, 2022 USD ($)	Oct. 07, 2022 USD ($)	May 27, 2022 USD ($)	May 25, 2022 MW	May 07, 2021 MW	Feb. 10, 2020 MW	May 31, 2019 MW	Jul. 31, 2018 MW MWh	Jun. 30, 2017 MW MWh
Commitments And Contingencies [Line Items]
Statutory Federal Income Tax Rate																					21.00%	21.00%	21.00%
Spending cap																					$ 491,300,000
Remuneration Percentage							2.25%											2.75%
Termination agreements																					$ 1,100,000
Approved annual increase in rates			$ 7,600,000
Requested annual increase in rates																			$ 2,100,000
Approved commitments for purchasing assets																$ 1,000,000
Environmental Remediation Expense, before Recovery, Statement of Income or Comprehensive Income [Extensible Enumeration]																					Costs and Expenses
Remediation costs																					$ 5,600,000
Percentage of Weighted Average Cost Inventory			7.22%
Stock Split Conversion Ratio			0.5201
Environmental Restoration Costs [Member]
Commitments And Contingencies [Line Items]
Estimated Costs Accrued For Remediation																					$ 2,500,000
Public Infrastructure Offering [Member]
Commitments And Contingencies [Line Items]
Approved commitments for purchasing assets																500,000
Electric Vehicle Supply Equipment [Member]
Commitments And Contingencies [Line Items]
Approved commitments for purchasing assets																300,000
Storm Cost Deferral Petition [Member]
Commitments And Contingencies [Line Items]
Recover costs of repairing damage																				$ 4,800,000
Carrying costs																				$ 1,400,000
Marketing And Outreach [Member]
Commitments And Contingencies [Line Items]
Approved commitments for purchasing assets																$ 200,000
Tax Year 2018
Commitments And Contingencies [Line Items]
Statutory Federal Income Tax Rate																								21.00%
Northern Utilities Inc
Commitments And Contingencies [Line Items]
Percentage of approved return on equity, reflecting on equity													52.00%
Percentage of approved return on equity, reflecting on debt													48.00%
Percentage of approved return on equity													9.20%
Approved annual increase in rates														$ 6,300,000
Public utilities approved increase amount of annual revenue to recover eligible capital investments						$ 1,200,000				$ 1,300,000
Northern Utilities Inc | Arrearage Management Program
Commitments And Contingencies [Line Items]
Expenses associated with the program excluded from the revenue requirement as per order and adjusted increase amount will result in reasonable rates													$ 5,900,000
Northern Utilities Inc | New Hampshire
Commitments And Contingencies [Line Items]
Percentage of approved return on equity, reflecting on equity											52.00%
Percentage of approved return on equity, reflecting on debt											48.00%
Percentage of approved return on equity											9.30%
Threshold amount that will be allowed to adjust distribution rates upward or downward during the term of stay out period											$ 200,000
Requested annual increase in rates												$ 1,600,000
Northern Utilities Inc | Maine
Commitments And Contingencies [Line Items]
Percentage of approved return on equity			9.35%
Northern Utilities Inc | Settlement Agreement [Member]
Commitments And Contingencies [Line Items]
Approved annual increase in rates											$ 6,100,000
Fitchburg Gas and Electric Light Company
Commitments And Contingencies [Line Items]
Increase (decrease) in annual revenue	$ 3,500,000	$ 3,600,000						$ 3,100,000
Power generation capacity | MWh																																			9,450,000
Approved annual increase in rates				$ 4,100,000
Transfer revenue amount				2,700,000
Fitchburg Gas and Electric Light Company | July 1, 2024 [Member]
Commitments And Contingencies [Line Items]
Approved annual increase in rates				6,800,000
Fitchburg Gas and Electric Light Company | Offshore Wind Energy
Commitments And Contingencies [Line Items]
Power generation facility | MW																																400		400	1,600
Remuneration Percentage																		2.75%
Facility power capacity to be procured in the future | MW																					5,600					400
Fitchburg Gas and Electric Light Company | Offshore Wind Energy | Second Solicitation [Member]
Commitments And Contingencies [Line Items]
Power generation facility | MW																															1,600		800
Fitchburg Gas and Electric Light Company | Qualified Clean Energy
Commitments And Contingencies [Line Items]
Power generation capacity | MWh																																		9,554,940
Fitchburg Gas and Electric Light Company | Mayflower Wind energy [Member] | Third Solicitation [Member]
Commitments And Contingencies [Line Items]
Power generation facility | MW																														400
Fitchburg Gas and Electric Light Company | Commonwealth Wind [Member] | Third Solicitation [Member]
Commitments And Contingencies [Line Items]
Power generation facility | MW																														1,200
Fitchburg Gas Company
Commitments And Contingencies [Line Items]
Regulatory assets approved increase in revenue due to be recovered																									$ 6,400,000		$ 4,500,000
Approved annual increase in rates				6,700,000
Transfer revenue amount				4,200,000
Fitchburg Gas Company | July 1, 2024 [Member]
Commitments And Contingencies [Line Items]
Public utilities interim increase decrease amount				$ 10,900,000
Granite State
Commitments And Contingencies [Line Items]
Increase (decrease) in annual revenue					$ 1,000,000				$ 300,000						$ 100,000		$ 1,300,000
Spending cap																	14,600,000
Fitchburg Grid Modernization
Commitments And Contingencies [Line Items]
Regulatory assets approved increase in revenue due to be recovered						$ 1,000,000																							$ 400,000
Amount of replacement investments approved by regulatory authority																	11,200,000
Purchase commitments for data sharing platform investments																	2,300,000
Fitchburg Grid Modernization | Track One [Member]
Commitments And Contingencies [Line Items]
Amount of capital expenditure approved by regulatory authority																												$ 9,300,000
Fitchburg Grid Modernization | Track Two [Member]
Commitments And Contingencies [Line Items]
Amount of capital expenditure approved by regulatory authority																	$ 1,500,000
Fitchburg Manufactured Gas Plant Site [Member]
Commitments And Contingencies [Line Items]
Environmental Remediation Expense, before Recovery, Statement of Income or Comprehensive Income [Extensible Enumeration]																					Costs and Expenses
Remediation costs																					$ 3,500,000
Fitchburg Manufactured Gas Plant Site [Member] | Environmental Restoration Costs [Member]
Commitments And Contingencies [Line Items]
Estimated Costs Accrued For Remediation																					$ 40,000