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Regulatory Matters - Additional Information (Detail 1) $ in Millions |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 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 ($) | Jul. 16, 2024 USD ($) | Jun. 28, 2024 USD ($) | Aug. 17, 2023 USD ($) | Jul. 26, 2023 USD ($) | Dec. 30, 2022 USD ($) | Jun. 25, 2019 | Mar. 31, 2025 USD ($) | Mar. 31, 2023 USD ($) | Dec. 31, 2024 MW Megawatts | Feb. 28, 2025 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 MW MWh | Jun. 30, 2017 MW MWh
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 |  |  |  |  |  |  |  | $ 0.8 | $ 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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Maximum [Member] | Offshore Energy [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation capacity | MW |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,600 |  |  |  |  |  |  |  |  |
Fitchburg Gas Company [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets approved increase in revenue due to be recovered |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 3.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  | $ 0.9 | $ 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 [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  |  |  |  |  |  |  |  | $ 6.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Granite State [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  |  |  | $ 3.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Spending cap |  |  |  |  | $ 29.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fitchburg Grid Modernization [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 [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 | 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) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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