Income Taxes - Summary of significant components of deferred tax assets and liabilities (Details 2) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Self-insurance	$ 4,660	$ 5,907
Pension	8,730	11,995
State net operating loss carryforwards	4,048	1,455
Bad debt	1,149	991
Accrued payroll	1,117	857
Stock-based compensation	3,612	5,847
Foreign tax credits	3,592	0
All others	1,681	2,483
Valuation allowance	(3,994)	(356)
Gross deferred tax assets	24,595	29,179
Deferred tax liabilities:
Depreciation	(53,119)	(95,606)
Goodwill amortization	(6,450)	(9,340)
Basis differences in consolidated limited liability company	(4,102)	(5,281)
Basis differences in joint ventures	(355)	(396)
All others	(6)	(22)
Gross deferred tax liabilities	(64,032)	(110,645)
Net deferred tax liabilities	$ (39,437)	$ (81,466)