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Energy Supply - Additional Information (Detail) Bcfe in Billions | 1 Months Ended | 12 Months Ended
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Nov. 30, 2024 | Dec. 31, 2024 | Dec. 31, 2024 | Dec. 31, 2024 BTU | Dec. 31, 2024 Bcfe
Northern Utilities Inc |  |  |  |  |
Gas and Oil Acreage [Line Items] |  |  |  |  |
Natural gas available under firm contract per day of year-round and seasonal transportation and underground storage capacity to distribution facilities |  |  |  | 85,500,000,000 |
Natural gas, underground storage |  |  |  | 44,000 | 6.3
Northern Utilities Inc | Maximum |  |  |  |  |
Gas and Oil Acreage [Line Items] |  |  |  |  |
Purchases of natural gas, contract duration |  | 1 year |  |  |
Fitchburg Gas and Electric Light Company |  |  |  |  |
Gas and Oil Acreage [Line Items] |  |  |  |  |
Natural gas available under firm contract per day of year-round and seasonal transportation and underground storage capacity to distribution facilities |  |  |  | 14,439 |
Natural gas, underground storage | Bcfe |  |  |  |  | 0.4
Percentage of power supply requirement | 50.00% |  | 50.00% |  |
Power supply contract duration |  | 12 months |  |  |
Percentage Of Purchase Electric Supply | 50.00% |  |  |  |
ISO New England markets |  |  |  |  |
Gas and Oil Acreage [Line Items] |  |  |  |  |
Percentage of power supply requirement |  |  | 10.00% |  |
Unitil Energy Systems Inc |  |  |  |  |
Gas and Oil Acreage [Line Items] |  |  |  |  |
Percentage of power supply requirement |  |  | 90.00% |  |
Power supply contract duration |  | 6 months |  |  |

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