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Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended			12 Months Ended
	Dec. 31, 2020	Mar. 31, 2021	Sep. 30, 2020	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes [Line Items]
Corporate federal income tax		21.00%		21.00%
Regulatory Liability	$ 49,800	$ 58,200		$ 55,600	$ 49,800
Regulatory liability, expected flow back to customers					47,100			$ 48,900
Regulatory liability, expected pass back to ratepayers					300	$ 1,700		1,800
Net Operating Loss Carryforwards Utilized For Income Taxes						$ 8,200
Deferred tax assets, operating loss carryforwards, federal	2,700				2,700
Alternative minimum tax credit carryforwards	$ 900				900
Percentage of employment retention credit		50.00%
Employment retention duties capacity		100.00%
Employment tax expense			$ 600
Consolidated Appropriations Act 2021 [Member]
Income Taxes [Line Items]
Percentage of employment retention credit	70.00%
Employment retention duties capacity	100.00%
Employment tax expense		$ 350
Minimum
Income Taxes [Line Items]
Average Rate Assumption Method estimated flow back period		15 months
Maximum
Income Taxes [Line Items]
Average Rate Assumption Method estimated flow back period		20 years
Income Tax Related Liabilities
Income Taxes [Line Items]
Regulatory Liability	$ 45,500	$ 45,100		$ 46,100	$ 45,500			$ 48,900
Gas Ratepayers \| Massachusetts And Maine [Member]
Income Taxes [Line Items]
Regulatory liability, expected flow back to customers		$ 2,300
Net Operating Loss Carryforward Assets
Income Taxes [Line Items]
Regulatory liability, expected pass back to ratepayers							$ 2,000
Tax Year 2018
Income Taxes [Line Items]
Corporate federal income tax		21.00%			21.00%			21.00%