Income Taxes - Summary of significant components of deferred tax assets and liabilities (Details 2) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets:
Self-insurance	$ 7,274	$ 7,675
Pension	12,048	12,555
State net operating loss carry forwards	370	451
Bad debts	4,041	5,755
Accrued payroll	1,330	2,833
Stock-based compensation	5,885	5,583
All others	4,704	2,121
Valuation allowance	(276)	(2)
Gross deferred tax assets	35,376	36,971
Deferred tax liabilities:
Depreciation	(137,606)	(172,813)
Goodwill amortization	(8,887)	(7,974)
All others	(4,378)	(3,739)
Gross deferred tax liabilities	(150,871)	(184,526)
Net deferred tax liabilities	$ (115,495)	$ (147,555)