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Regulatory Matters - Additional Information (Detail) - USD ($) $ in Millions | Nov. 01, 2024 | Sep. 11, 2024 | Jul. 16, 2024 | Jun. 28, 2024 | May 01, 2024 | Sep. 20, 2023 | Aug. 17, 2023 | Jul. 26, 2023 | May 31, 2023 | Jul. 28, 2022 | Jul. 20, 2022 | Jun. 08, 2022 | May 12, 2022 | May 03, 2022
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Regulatory Asset [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested annual increase in rates |  |  |  |  | $ 2.4 |  |  |  |  |  |  |  |  |
Northern Utilities Inc |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Asset [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity |  |  |  |  |  |  |  |  |  |  |  |  | 9.20% |
Percentage of approved return on equity, reflecting on equity |  |  |  |  |  |  |  |  |  |  |  |  | 52.00% |
Percentage of approved return on equity, reflecting on debt |  |  |  |  |  |  |  |  |  |  |  |  | 48.00% |
Approved annual increase in rates |  |  |  |  |  | $ 7.6 |  |  |  |  |  |  |  | $ 6.3
Return on equity |  |  |  |  |  | 9.35% |  |  |  |  |  |  |  |
Equity ratio |  |  |  |  |  | 52.01% |  |  |  |  |  |  |  |
Weighted Average Cost Of Capital |  |  |  |  |  | 7.22% |  |  |  |  |  |  |  |
Public utilities approved increase amount of annual revenue to recover eligible capital investments |  |  |  |  |  |  |  |  | $ 1.2 | $ 1.3 |  |  |  |
Northern Utilities Inc | Settlement Agreement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Asset [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved annual increase in rates |  |  |  |  |  |  |  |  |  |  | $ 6.1 |  |  |
Northern Utilities Inc | Arrearage Management Program [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Asset [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 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Asset [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 |  |
Fitchburg Gas and Electric Light Company |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Asset [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 |  |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | July 1, 2024 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Asset [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved annual increase in rates |  |  |  | $ 4.7 |  |  | $ 6.8 |  |  |  |  |  |  |

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