Regulatory Matters - Additional Information (Detail 1) $ in Millions											3 Months Ended		12 Months Ended
	Nov. 27, 2023 USD ($)	Nov. 01, 2023 USD ($)	Aug. 17, 2023 USD ($)	Jul. 27, 2023 USD ($)	Jul. 26, 2023 USD ($)	Dec. 30, 2022 USD ($)	Aug. 19, 2022 USD ($)	Aug. 24, 2021 USD ($)	Nov. 30, 2020 USD ($)	Jun. 25, 2019	Mar. 31, 2024 USD ($) MW	Mar. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Apr. 15, 2024 USD ($)	Dec. 31, 2023 USD ($)	Oct. 31, 2023 USD ($)	Aug. 30, 2023 MW	May 31, 2023 USD ($)	Nov. 30, 2022 USD ($)	Oct. 31, 2022 USD ($)	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 MW MWh
Regulatory Assets [Line Items]
Termination agreements											$ 1.1
Remuneration Percentage										2.75%	2.25%
Approved commitments for purchasing assets						$ 1.0
Fitchburg Gas And Electric Light Company [Member]
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	$ 3.5	$ 3.6			$ 3.1
Fitchburg Gas And Electric Light Company [Member] | Offshore Energy [Member]
Regulatory Assets [Line Items]
Facility power capacity to be procured in the future | MW											5,600						5,600
Remuneration Percentage										2.75%
Power generation facility | MW																								400		400	1,600
Fitchburg Gas And Electric Light Company [Member] | Qualified Clean Energy [Member]
Regulatory Assets [Line Items]
Power generation capacity | MWh																										9,554,940
Fitchburg Gas And Electric Light Company [Member] | Minimum [Member]
Regulatory Assets [Line Items]
Facility power capacity to be procured in the future | MW											200
Fitchburg Gas And Electric Light Company [Member] | Maximum [Member]
Regulatory Assets [Line Items]
Facility power capacity to be procured in the future | MW											2,400
Fitchburg Gas Company [Member]
Regulatory Assets [Line Items]
Regulatory assets approved increase in revenue due to be recovered			$ 6.7													$ 6.4				$ 4.5
Increase (decrease) in annual revenue			4.2
Public utilities interim increase decrease amount			$ 10.9
Granite State [Member]
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue				$ 1.0			$ 0.3	$ 0.1	$ 1.3
Spending cap									14.6
Fitchburg Grid Modernization [Member]
Regulatory Assets [Line Items]
Regulatory assets approved increase in revenue due to be recovered														$ 1.3				$ 1.0
Purchase commitments for data sharing platform investments									2.3
Amount of replacement investments approved by regulatory authority																			$ 11.2
Fitchburg Grid Modernization [Member] | Track One [Member]
Regulatory Assets [Line Items]
Amount of capital expenditure approved by regulatory authority																					$ 9.3
Fitchburg Grid Modernization [Member] | Track Two [Member]
Regulatory Assets [Line Items]
Amount of capital expenditure approved by regulatory authority									$ 1.5
Second Solicitation [Member] | Fitchburg Gas And Electric Light Company [Member] | Offshore Energy [Member]
Regulatory Assets [Line Items]
Power generation facility | MW																							1,600		800
Third Solicitation [Member] | Fitchburg Gas And Electric Light Company [Member] | Commonwealth Wind [Member]
Regulatory Assets [Line Items]
Power generation facility | MW																						1,200
Third Solicitation [Member] | Fitchburg Gas And Electric Light Company [Member] | Mayflower Wind energy [Member]
Regulatory Assets [Line Items]
Power generation facility | MW																						400
Major Storm Cost Reserve Recovery [Member]
Regulatory Assets [Line Items]
Carrying costs													$ 0.2
Recover costs of repairing damage															$ 3.7
Storm Cost Deferral Petition [Member]
Regulatory Assets [Line Items]
Carrying costs												$ 1.4
Recover costs of repairing damage												$ 4.8
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