Income Taxes - Summary of significant components of deferred tax assets and liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Self-insurance	$ 5,729	$ 5,055
Pension	9,617	7,261
State net operating loss carryforwards	1,445	1,663
Bad debt	1,392	1,291
Stock-based compensation	3,358	3,501
Inventory reserve	2,478	2,447
Impairment reserve	2,802
Foreign tax credits carryforwards	3,086
Federal net operating loss carryforwards	19,664
Basis differences in consolidated limited liability company	5,778
All others	2,191	1,690
Valuation allowance	(415)
Gross deferred tax assets	57,540	22,908
Deferred tax liabilities:
Depreciation	(87,647)	(71,647)
Goodwill amortization	(6,763)	(6,587)
Basis differences in consolidated limited liability company		(4,636)
Basis differences in joint ventures	(444)	(408)
All others	(5)	(5)
Gross deferred tax liabilities	(94,859)	(83,283)
Net deferred tax liabilities	$ (37,319)	$ (60,375)