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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Aug. 31, 2022	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2017
Income Taxes [Line Items]
Statutory Federal Income Tax Rate		21.00%	21.00%	21.00%
Regulatory Liability		$ 64.0	$ 47.9
Regulatory liability, expected flow back to customers					$ 47.1
Offset future years income			7.3
Regulatory liability, expected pass back to ratepayers		1.6
Net Operating Loss Carryforwards Utilized For Income Taxes			2.9
Deferred tax assets, operating loss carryforwards, federal		6.6
Income Tax Expense (Benefit)		14.0	13.2	$ 11.2
Percentage of alternate minimum tax	15.00%
Number of years used for calculating adjusted financial statement income	3 years
Adjustment financial statement income	$ 1,000.0
FederalTaxCreditsToOffsetCurrentTaxes		1.3
Income Tax Related Liabilities
Income Taxes [Line Items]
Regulatory Liability		50.4	$ 38.6		$ 48.9
Gas Ratepayers | Massachusetts And Maine [Member]
Income Taxes [Line Items]
Regulatory liability, expected flow back to customers		$ 11.2
Tax Year 2018
Income Taxes [Line Items]
Statutory Federal Income Tax Rate					21.00%

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible domestic operating loss carryforwards. Excludes state and local operating loss carryforwards.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

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