Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes [Line Items]
Corporate federal income tax		21.00%	21.00%	21.00%
Regulatory Liability	$ 49.8	$ 52.1	$ 49.8
Regulatory liability, expected flow back to customers		47.1				$ 48.9
Regulatory liability, expected pass back to ratepayers		$ 0.7	0.3	$ 1.7		1.8
Net Operating Loss Carryforwards Utilized For Income Taxes			7.7
Percentage of employment retention credit		50.00%
Employment retention duties capacity		100.00%
Employment tax expense		$ 0.4	0.6
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	$ 44.3	$ 45.5			$ 48.9
Gas Ratepayers \| Massachusetts And Maine [Member]
Income Taxes [Line Items]
Regulatory liability, expected flow back to customers		3.1
Net Operating Loss Carryforward Assets
Income Taxes [Line Items]
Regulatory liability, expected pass back to ratepayers		$ 3.6			$ 2.0
Tax Year 2018
Income Taxes [Line Items]
Corporate federal income tax					21.00%	21.00%