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Regulatory Matters - Additional Information (Detail 1) $ in Millions													1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
	Apr. 15, 2025 USD ($)	Dec. 09, 2024 USD ($)	Nov. 26, 2024 USD ($)	Nov. 01, 2024 USD ($)	Oct. 04, 2024 USD ($)	Sep. 11, 2024 USD ($)	Jul. 18, 2024 USD ($)	Jun. 28, 2024 USD ($)	Aug. 17, 2023 USD ($)	Jul. 26, 2023 USD ($)	Dec. 30, 2022 USD ($)	Jun. 25, 2019	Dec. 31, 2024 Megawatts	Mar. 31, 2023 USD ($)	Sep. 30, 2025 USD ($) MW	Dec. 31, 2024 Megawatts	Oct. 31, 2025 USD ($)	Apr. 30, 2025 USD ($)	Feb. 28, 2025 USD ($)	Nov. 25, 2024 USD ($)	Oct. 31, 2024 USD ($)	Sep. 06, 2024 MW	May 31, 2024 USD ($)	Aug. 30, 2023 MW	May 31, 2023 USD ($)	Dec. 31, 2022 MW	Nov. 30, 2022 USD ($)	Oct. 07, 2022 USD ($)	May 25, 2022 MW	May 07, 2021 MW	Nov. 30, 2020 USD ($)	Feb. 10, 2020 MW	May 31, 2019 MW	Jul. 31, 2018 MWh MW	Jun. 30, 2017 MWh MW
Regulatory Assets [Line Items]
Remuneration Percentage											2.25%	2.75%
Approved commitments for purchasing assets											$ 1.0
Recover costs of repairing damage														$ 4.8
Energy storage systems \| Megawatts																5,000
Mid Duration Storage \| Megawatts																1,500
Fitchburg Gas and Electric Light Company
Regulatory Assets [Line Items]
Facility power capacity to be procured in the future \| MW																								400
Power generation capacity \| MWh																																			9,450,000
Increase (decrease) in annual revenue				$ 0.5		$ 3.5				$ 3.1
Increase in cost								$ 2.2
Percentage of approved return on equity								9.40%
Percentage of approved return on equity, reflecting on equity								52.00%
Percentage of approved return on equity, reflecting on debt								48.00%
Accumulated deferred income taxes		$ 1.4	$ 0.1				$ 1.4
Energy storage systems \| Megawatts													5,000
Mid Duration Storage \| Megawatts													1,500
Reduction in total residential sector budget															$ 500.0
Fitchburg Gas and Electric Light Company \| Offshore Energy [Member]
Regulatory Assets [Line Items]
Facility power capacity to be procured in the future \| MW															5,600							2,678		5,600
Remuneration Percentage												2.75%
Power generation facility \| MW																																400		400	1,600
Fitchburg Gas and Electric Light Company \| Qualified Clean Energy [Member]
Regulatory Assets [Line Items]
Power generation capacity \| MWh																																		9,554,940
Fitchburg Gas and Electric Light Company \| Minimum [Member]
Regulatory Assets [Line Items]
Facility power capacity to be procured in the future \| MW															200
Fitchburg Gas and Electric Light Company \| Maximum [Member]
Regulatory Assets [Line Items]
Facility power capacity to be procured in the future \| MW															2,400
Extended long-term renewable contracts													30 years			30 years
Fitchburg Gas and Electric Light Company \| Maximum [Member] \| Offshore Energy [Member]
Regulatory Assets [Line Items]
Power generation capacity \| MW																										1,600
Fitchburg Gas Company
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue									$ 4.2
Public utilities interim increase decrease amount									10.9
Approved annual increase in rates								$ 10.1
Increase in cost								$ 4.9
Percentage of approved return on equity								9.40%
Percentage of approved return on equity, reflecting on equity								52.00%
Percentage of approved return on equity, reflecting on debt								48.00%
Accumulated deferred income taxes		1.4					$ 1.4
Fitchburg Gas Company \| Maximum [Member]
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue									$ 6.7
Granite State
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue					$ 3.0
Spending cap					$ 29.9
Revenue increase approved by regulator																				$ 1.2
Fitchburg - Grid Modernization
Regulatory Assets [Line Items]
Regulatory assets approved increase in revenue due to be recovered	$ 1.4																						$ 1.3		$ 1.0
Purchase commitments for data sharing platform investments																															$ 2.3
Amount of replacement investments approved by regulatory authority																											$ 11.2
Order providing cost	$ 1.5							$ 1.6
Fitchburg - Grid Modernization \| Track One [Member]
Regulatory Assets [Line Items]
Amount of capital expenditure approved by regulatory authority																												$ 9.3
Fitchburg - Grid Modernization \| Track Two [Member]
Regulatory Assets [Line Items]
Amount of capital expenditure approved by regulatory authority																											$ 1.5
Fitchburg Gas System Enhancement Program
Regulatory Assets [Line Items]
Regulatory assets approved increase in revenue due to be recovered																	$ 5.4				$ 3.5
Regulatory assets approved recovery amount																		$ 3.5
Regulatory assets deferred cost																	1.0	1.6
Regulatory recovery amount																	$ 6.4	$ 5.1
Second Solicitation [Member] \| Fitchburg Gas and Electric Light Company \| Offshore Energy [Member]
Regulatory Assets [Line Items]
Power generation facility \| MW																														1,600			800
Third Solicitation [Member] \| Fitchburg Gas and Electric Light Company \| Offshore Energy [Member]
Regulatory Assets [Line Items]
Power generation facility \| MW																													1,200
Third Solicitation [Member] \| Fitchburg Gas and Electric Light Company \| Mayflower Wind energy [Member]
Regulatory Assets [Line Items]
Power generation facility \| MW																													400
Major Storm Cost Reserve Recovery [Member]
Regulatory Assets [Line Items]
Recover costs of repairing damage																			$ 1.8
Storm Cost Deferral Petition [Member]
Regulatory Assets [Line Items]
Carrying costs														$ 1.4
Marketing And Outreach [Member]
Regulatory Assets [Line Items]
Approved commitments for purchasing assets											0.2
Electric Vehicle Supply Equipment [Member]
Regulatory Assets [Line Items]
Approved commitments for purchasing assets											0.3
Public Infrastructure Offering [Member]
Regulatory Assets [Line Items]
Approved commitments for purchasing assets											$ 0.5
Gas Division [Member]
Regulatory Assets [Line Items]
Accumulated deferred income taxes		(0.6)
Gas Division [Member] \| Fitchburg Gas and Electric Light Company
Regulatory Assets [Line Items]
Accumulated deferred income taxes		(0.6)
Electric Division [Member]
Regulatory Assets [Line Items]
Accumulated deferred income taxes		(0.8)
Electric Division [Member] \| Fitchburg Gas and Electric Light Company
Regulatory Assets [Line Items]
Accumulated deferred income taxes		$ (0.8)
GSEP 2025 \| Fitchburg Gas System Enhancement Program \| Minimum [Member]
Regulatory Assets [Line Items]
Revenue cap																		2.50%
GSEP 2025 \| Fitchburg Gas System Enhancement Program \| Maximum [Member]
Regulatory Assets [Line Items]
Revenue cap																		3.00%
GSEP 2026 \| Fitchburg Gas System Enhancement Program
Regulatory Assets [Line Items]
Recovery cap rate																		2.00%
GSEP 2027 \| Fitchburg Gas System Enhancement Program
Regulatory Assets [Line Items]
Recovery cap rate																		1.50%