Income Taxes - Additional Information (Detail) - USD ($) $ in Millions		12 Months Ended
	Dec. 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%	21.00%
Regulatory Liability	$ 49.8	$ 49.8	$ 54.0		$ 48.9
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
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
Gas Ratepayers | Massachusetts And Maine [Member]
Income Taxes [Line Items]
Regulatory liability, expected flow back to customers	$ 1.9
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%