Income Taxes - Schedule of Accumulated Deferred Income Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disclosure Income Taxes Schedule Of Accumulated Deferred Income Tax Assets And Liabilities [Abstract]
Deferred State and Local Income Tax Expense (Benefit)	$ 4,700	$ 3,500	$ 900
Deferred income tax liabilities:
Property, plant and equipment	173,466	153,423
Acquisition adjustment	6,969	8,896
Deferred Tax Liabilities Loss On Reacquired Debt	220	32
Deferred Income Tax Liability - Deferred Gas Costs	1,223	1,139
Deferred Income Tax Liability, Natural Gas Conversion Costs	4,956	3,987
Deferred Income Tax Liability, Storm Reserve	10,316	97
Other	1,456	2,544
Total deferred income tax liabilities	198,606	170,118
Deferred income tax assets:
Pension and other employee benefits	3,818	3,711
Environmental costs	1,486	1,710
Net operating loss carryforwards	5,523	2,010
Self-insurance	146	151
Storm reserve liability	96	0
Other	6,881	5,716
Total deferred income tax assets	17,950	13,298
Deferred Income Tax Liabilities, Net	$ 180,656	$ 156,820