Income Taxes - Summary of significant components of deferred tax assets and liabilities (Details 2) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Self-insurance	$ 5,907	$ 7,274
Pension	11,995	12,048
State net operating loss carryforwards	1,455	370
Bad debt	991	4,041
Accrued payroll	857	1,330
Stock-based compensation	5,847	5,885
All others	2,483	4,704
Valuation allowance	(356)	(276)
Gross deferred tax assets	29,179	35,376
Deferred tax liabilities:
Depreciation	(95,606)	(137,606)
Goodwill amortization	(9,340)	(8,887)
Basis differences in variable interest entities	(5,281)	(4,876)
Basis differences in joint ventures	(396)	518
All others	(22)	(20)
Gross deferred tax liabilities	(110,645)	(150,871)
Net deferred tax liabilities	$ (81,466)	$ (115,495)