Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended		12 Months Ended
	Aug. 31, 2022	Dec. 31, 2020	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2017
Income Taxes [Line Items]
Statutory Federal Income Tax Rate			21.00%	21.00%	21.00%
Regulatory Liability			$ 51.9	$ 52.1
Regulatory liability, expected flow back to customers					$ 47.1
Regulatory liability, expected pass back to ratepayers			1.0			$ 1.8
Net Operating Loss Carryforwards Utilized For Income Taxes				2.4
Deferred tax assets, operating loss carryforwards, federal			$ 2.8
Percentage of employment retention credit			50.00%
Employment retention duties capacity			100.00%
Employment tax expense				0.4	$ 0.6
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
Consolidated Appropriations Act 2021 [Member]
Income Taxes [Line Items]
Percentage of employment retention credit		70.00%
Employment retention duties capacity		100.00%
Income Tax Related Liabilities
Income Taxes [Line Items]
Regulatory Liability			$ 41.0	$ 44.3		$ 48.9
Gas Ratepayers | Massachusetts And Maine [Member]
Income Taxes [Line Items]
Regulatory liability, expected flow back to customers			$ 6.4
Tax Year 2018
Income Taxes [Line Items]
Statutory Federal Income Tax Rate						21.00%