Income Taxes - Summary of significant components of deferred tax assets and liabilities (Details 2) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Self-insurance	$ 5,055	$ 4,660
Pension	7,261	8,730
State net operating loss carryforwards	1,663	4,048
Bad debt	1,291	1,149
Accrued payroll	1,416	1,117
Stock-based compensation	3,501	3,612
Foreign tax credits	0	3,592
All others	3,166	1,681
Valuation allowance	(445)	(3,994)
Gross deferred tax assets	22,908	24,595
Deferred tax liabilities:
Depreciation	(71,647)	(53,119)
Goodwill amortization	(6,587)	(6,450)
Basis differences in consolidated limited liability company	(4,636)	(4,102)
Basis differences in joint ventures	(408)	(355)
All others	(5)	(6)
Gross deferred tax liabilities	(83,283)	(64,032)
Net deferred tax liabilities	$ (60,375)	$ (39,437)