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Regulatory Matters - Additional Information (Detail) - USD ($) $ in Millions | Jul. 01, 2025 | May 01, 2025 | Feb. 28, 2025 | Dec. 09, 2024 | Nov. 26, 2024 | Nov. 01, 2024 | Oct. 04, 2024 | Sep. 11, 2024 | Jul. 18, 2024 | Jul. 16, 2024 | Jun. 28, 2024 | May 01, 2024 | Sep. 20, 2023 | Aug. 17, 2023 | Jul. 26, 2023 | Jul. 20, 2022 | Jun. 08, 2022 | Apr. 15, 2025 | Oct. 31, 2024 | May 31, 2024 | May 31, 2023
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Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested annual increase in rates |  |  |  |  |  |  |  |  |  |  |  | $ 2.1 |  |  |  |  |  |  |  |  |
Northern Utilities Inc |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved annual increase in rates |  |  |  |  |  |  |  |  |  |  |  |  | $ 7.6 |  |  |  |  |  |  |  |
Return on equity |  |  |  |  |  |  |  |  |  |  |  |  | 9.35% |  |  |  |  |  |  |  |
Equity ratio |  |  |  |  |  |  |  |  |  |  |  |  | 52.01% |  |  |  |  |  |  |  |
Weighted average cost of capital |  |  |  |  |  |  |  |  |  |  |  |  | 7.22% |  |  |  |  |  |  |  |
Northern Utilities Inc | Settlement Agreement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved annual increase in rates |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 6.1 |  |  |  |  |
Northern Utilities Inc | New Hampshire |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9.30% |  |  |  |  |
Percentage of approved return on equity, reflecting on equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 52.00% |  |  |  |  |
Percentage of approved return on equity, reflecting on debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 48.00% |  |  |  |  |
Requested annual increase in rates |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1.6 |  |  |  |
Unitil Energy Systems Inc |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
percent above present rates | 3.70% | 7.30% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved annual increase in rates |  | $ 18.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities approved increase amount of annual revenue to recover eligible capital investments | $ 7.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  |  |  |  | $ 0.5 |  | $ 3.5 |  |  |  |  |  |  | $ 3.1 |  |  |  |  |  |
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% |  |  |  |  |  |  |  |  |  |
Increase in cost |  |  |  |  |  |  |  |  |  | $ 0.8 | $ 2.2 |  |  |  |  |  |  |  |  |  |
Accumulated deferred income taxes |  |  |  | $ 1.4 | $ 0.1 |  |  |  | $ 1.4 |  |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | July 1, 2024 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved annual increase in rates |  |  |  |  |  |  |  |  |  |  | $ 4.7 |  |  | $ 6.8 |  |  |  |  |  |  |
Fitchburg Gas Company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.2 |  |  |  |  |  |  |
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% |  |  |  |  |  |  |  |  |  |
Regulatory assets approved increase in revenue due to be recovered |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 3.5 |  |
Approved annual increase in rates |  |  |  |  |  |  |  |  |  |  | $ 10.1 |  |  |  |  |  |  |  |  |  |
Increase in cost |  |  |  |  |  |  |  |  |  | $ 0.9 | $ 4.9 |  |  |  |  |  |  |  |  |  |
Accumulated deferred income taxes |  |  |  | 1.4 |  |  |  |  | $ 1.4 |  |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas Company | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 6.7 |  |  |  |  |  |  |
Granite State |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  |  |  |  |  | $ 3.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fitchburg - Grid Modernization |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets approved increase in revenue due to be recovered |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1.4 |  | $ 1.3 | $ 1.0
Fitchburg Performance Base Rate Adjustment Electric [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  | $ 1.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fitchburg Performance Base Rate Adjustment Gas [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in annual revenue |  |  | $ 0.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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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- Definition The increase (decrease) during the reporting period in the account that represents the temporary difference that results from Income or Loss that is recognized for accounting purposes but not for tax purposes and vice versa. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 45 -Paragraph 28 -Subparagraph (a) -SubTopic 10 -Topic 230 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482740/230-10-45-28 + Details
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