Income Taxes - Schedule of Accumulated Deferred Income Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure Income Taxes Schedule Of Accumulated Deferred Income Tax Assets And Liabilities [Abstract]
Deferred State and Local Income Tax Expense (Benefit)	$ 4,900	$ 4,700	$ 3,500
Deferred income tax liabilities:
Property, plant and equipment	199,287	173,466
Acquisition adjustment	6,618	6,969
Deferred Tax Liabilities Loss On Reacquired Debt	201	220
Deferred Income Tax Liability - Deferred Gas Costs	509	1,223
Deferred Income Tax Liability, Natural Gas Conversion Costs	5,379	4,956
Deferred Income Tax Liability, Storm Reserve	7,073	10,316
Other	5,587	1,456
Total deferred income tax liabilities	224,654	198,606
Deferred income tax assets:
Pension and other employee benefits	4,636	3,818
Environmental costs	1,064	1,486
Net operating loss carryforwards	1,587	5,523
Self-insurance	0	146
Storm reserve liability	409	96
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals	6,153	2,064
Other	5,417	4,817
Total deferred income tax assets	19,266	17,950
Deferred Tax Assets, Net	$ 205,388	$ 180,656