Income Taxes - Schedule of Accumulated Deferred Income Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure Income Taxes Schedule Of Accumulated Deferred Income Tax Assets And Liabilities [Abstract]
Deferred State and Local Income Tax Expense (Benefit)	$ 3,500	$ 900	$ 2,100
Deferred income tax liabilities:
Property, plant and equipment	153,423	133,581
Acquisition adjustment	8,896	9,323
Deferred gas costs	32	153
Deferred Income Tax Liability - Deferred Gas Costs	1,139	2,574
Deferred Income Tax Liability, Natural Gas Conversion Costs	3,987	2,760
Other	2,641	2,662
Total deferred income tax liabilities	170,118	151,053
Deferred income tax assets:
Pension and other employee benefits	3,711	4,698
Environmental costs	1,710	1,744
Net operating loss carryforwards	2,010	1,625
Self-insurance	151	164
Storm reserve liability	0	717
Other	5,716	6,255
Total deferred income tax assets	13,298	15,203
Deferred Income Tax Liabilities, Net	$ 156,820	$ 135,850