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Energy Supply - Additional Information (Detail) Bcfe in Billions	1 Months Ended	12 Months Ended
	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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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Regulation S-K (SK)
-Number 229
-Section 1208
-Paragraph b
-Publisher SEC

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Amount of daily production of the applicable energy source.

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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Natural gas pipeline, capacity per day.

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Percentage Of Purchase Electric Supply

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Power supply agreement.

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- Definition

Power supply cost benefit variance percentage.

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