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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	6 Months Ended		12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2021	Jun. 30, 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	$ 49.8		$ 60.3	$ 57.4	$ 49.8
Regulatory liability, expected flow back to customers					47.1			$ 48.9
Regulatory liability, expected pass back to ratepayers					0.3	$ 1.7		1.8
Net Operating Loss Carryforwards Utilized For Income Taxes						$ 8.2
Deferred tax assets, operating loss carryforwards, federal	2.7				2.7
Alternative minimum tax credit carryforwards	$ 0.9				0.9
Percentage of employment retention credit			50.00%
Employment retention duties capacity			100.00%
Employment tax expense		$ 0.6	$ 0.4
Consolidated Appropriations Act 2021 [Member]
Income Taxes [Line Items]
Percentage of employment retention credit	70.00%
Employment retention duties capacity	100.00%
Minimum
Income Taxes [Line Items]
Average Rate Assumption Method estimated flow back period			15 years
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	$ 45.5		$ 44.8	$ 45.9	$ 45.5			$ 48.9
Gas Ratepayers \| Massachusetts And Maine [Member]
Income Taxes [Line Items]
Regulatory liability, expected flow back to customers			$ 2.5
Net Operating Loss Carryforward Assets
Income Taxes [Line Items]
Regulatory liability, expected pass back to ratepayers							$ 2.0
Tax Year 2018
Income Taxes [Line Items]
Corporate federal income tax			21.00%		21.00%			21.00%