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Commitments and Contingencies - Additional Information (Detail) $ in Millions																								1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 09, 2024 USD ($)	Nov. 26, 2024 USD ($)	Nov. 01, 2024 USD ($)	Oct. 04, 2024 USD ($)	Sep. 11, 2024 USD ($)	Jul. 18, 2024 USD ($)	Jul. 16, 2024 USD ($)	Jun. 28, 2024 USD ($)	Sep. 20, 2023 USD ($)	Aug. 17, 2023 USD ($)	Jul. 27, 2023 USD ($)	Jul. 26, 2023 USD ($)	May 31, 2023 USD ($)	Dec. 30, 2022 USD ($)	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 ($)	Jun. 25, 2019	Feb. 28, 2018 USD ($)	Dec. 31, 2024 USD ($) MW Megawatts	Mar. 31, 2023 USD ($)	Dec. 31, 2024 USD ($) MW Megawatts	Dec. 31, 2023 USD ($)	Oct. 31, 2024 USD ($)	Sep. 06, 2024 MW	Jul. 15, 2024 USD ($)	May 31, 2024 USD ($)	Oct. 31, 2023 USD ($)	Aug. 30, 2023 MW	Dec. 31, 2022 MW	Nov. 30, 2022 USD ($)	Oct. 07, 2022 USD ($)	May 25, 2022 MW	May 07, 2021 MW	Feb. 10, 2020 MW	May 31, 2019 MW	Jul. 31, 2018 MWh MW	Jun. 30, 2017 MWh MW
Commitments And Contingencies [Line Items]
Spending cap																								$ 753.9		$ 753.9
Remuneration Percentage														2.25%								2.75%
Termination agreements																										$ 1.1
Approved annual increase in rates									$ 7.6
Requested annual increase in rates																							$ 2.4
Public utilities approved increase amount of annual revenue to recover eligible capital investments													$ 1.2
Approved commitments for purchasing assets														$ 1.0
Environmental Remediation Expense, before Recovery, Statement of Income or Comprehensive Income [Extensible Enumeration]																										Costs and Expenses
Energy storage systems \| Megawatts																								5,000
Mid-Duration Storage \| Megawatts																								1,500
Percentage of Weighted Average Cost Inventory									7.22%
Stock Split Conversion Ratio									0.5201
Public Infrastructure Offering [Member]
Commitments And Contingencies [Line Items]
Approved commitments for purchasing assets														0.5
Electric Vehicle Supply Equipment [Member]
Commitments And Contingencies [Line Items]
Approved commitments for purchasing assets														0.3
Storm Cost Deferral Petition [Member]
Commitments And Contingencies [Line Items]
Recover costs of repairing damage																									$ 4.8
Carrying costs																									$ 1.4
Major Storm Cost Reserve Recovery [Member]
Commitments And Contingencies [Line Items]
Recover costs of repairing damage																											$ 3.7
Carrying costs																											$ 0.2
Marketing And Outreach [Member]
Commitments And Contingencies [Line Items]
Approved commitments for purchasing assets														$ 0.2
Gas Division [Member]
Commitments And Contingencies [Line Items]
Accumulated deferred income taxes	$ 0.6
Electric Division [Member]
Commitments And Contingencies [Line Items]
Accumulated deferred income taxes	0.8
Environmental Restoration Costs [Member]
Commitments And Contingencies [Line Items]
Estimated Costs Accrued For Remediation																										$ 5.6
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																	$ 6.1			$ 6.3
Public utilities approved increase amount of annual revenue to recover eligible capital investments																$ 1.3
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																			$ 5.9
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%
Requested annual increase in rates																		$ 1.6
Northern Utilities Inc \| Maine
Commitments And Contingencies [Line Items]
Percentage of approved return on equity									9.35%
Fitchburg Gas and Electric Light Company
Commitments And Contingencies [Line Items]
Increase (decrease) in annual revenue			$ 0.5		$ 3.5							$ 3.1
Power generation capacity \| MWh																																										9,450,000
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								9.40%
Approved annual increase in rates							$ 0.8
Accumulated deferred income taxes	1.4	$ 0.1				$ 1.4
Transfer revenue amount								$ 2.2
Facility power capacity to be procured in the future \| MW																																	400
Energy storage systems \| Megawatts																										5,000
Mid-Duration Storage \| Megawatts																										1,500
Fitchburg Gas and Electric Light Company \| Gas Division [Member]
Commitments And Contingencies [Line Items]
Accumulated deferred income taxes	(0.6)
Fitchburg Gas and Electric Light Company \| Electric Division [Member]
Commitments And Contingencies [Line Items]
Accumulated deferred income taxes	(0.8)
Fitchburg Gas and Electric Light Company \| Maximum [Member]
Commitments And Contingencies [Line Items]
Extended long-term renewable contracts																								30 years		30 years
Facility power capacity to be procured in the future \| MW																								2,400		2,400
Fitchburg Gas and Electric Light Company \| Minimum [Member]
Commitments And Contingencies [Line Items]
Facility power capacity to be procured in the future \| MW																								200		200
Fitchburg Gas and Electric Light Company \| July 1, 2024 [Member]
Commitments And Contingencies [Line Items]
Approved annual increase in rates										$ 6.8
Approved annual increase in rates								4.7
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		5,600			2,678				5,600
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 \| Offshore Wind Energy \| Third Solicitation [Member]
Commitments And Contingencies [Line Items]
Power generation facility \| MW																																					1,200
Fitchburg Gas and Electric Light Company \| Offshore Wind Energy \| Maximum [Member]
Commitments And Contingencies [Line Items]
Power generation capacity \| MW																																		1,600
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 Company
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										9.40%
Regulatory assets approved increase in revenue due to be recovered																												$ 3.5				$ 6.4
Approved annual increase in rates										$ 6.7
Accumulated deferred income taxes	$ 1.4					$ 1.4
Transfer revenue amount										4.2
Increase in cost							$ 0.9			4.9
Fitchburg Gas Company \| Maximum [Member]
Commitments And Contingencies [Line Items]
Regulatory assets approved increase in revenue due to be recovered																														$ 6.4
Fitchburg Gas Company \| Minimum [Member]
Commitments And Contingencies [Line Items]
Regulatory assets approved increase in revenue due to be recovered																														$ 4.0
Fitchburg Gas Company \| July 1, 2024 [Member]
Commitments And Contingencies [Line Items]
Public utilities interim increase decrease amount								10.1		$ 10.9
Granite State
Commitments And Contingencies [Line Items]
Increase (decrease) in annual revenue				$ 3.0							$ 1.0				$ 0.3						$ 0.1
Spending cap				$ 29.9
Fitchburg Grid Modernization
Commitments And Contingencies [Line Items]
Order providing cost								$ 1.6
Regulatory assets approved increase in revenue due to be recovered													$ 1.0																		$ 1.3
Amount of replacement investments approved by regulatory authority																																			$ 11.2
Purchase commitments for data sharing platform investments																																			2.3
Fitchburg Grid Modernization \| Track One [Member]
Commitments And Contingencies [Line Items]
Amount of capital expenditure approved by regulatory authority																																				$ 9.3
Fitchburg Grid Modernization \| Track Two [Member]
Commitments And Contingencies [Line Items]
Amount of capital expenditure approved by regulatory authority																																			$ 1.5
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																								$ 3.5		$ 3.5
Fitchburg Manufactured Gas Plant Site [Member] \| Environmental Restoration Costs [Member]
Commitments And Contingencies [Line Items]
Estimated Costs Accrued For Remediation																										$ 40,000.0