Income Taxes - Summary of significant components of deferred tax assets and liabilities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Deferred tax assets:
Self-insurance	$ 7,417	$ 6,344
Pension	9,688	8,922
State net operating loss carry forwards	1,165	1,671
Bad debts	3,489	3,044
Accrued payroll	2,038	2,099
Stock-based compensation	4,567	3,680
All others	274	1
Valuation allowance	(1,003)	(1,295)
Gross deferred tax assets	27,635	24,466
Deferred tax liabilities:
Depreciation	(168,717)	(166,190)
Goodwill amortization	(6,394)	(5,707)
All others	(1,754)	(1,314)
Gross deferred tax liabilities	(176,865)	(173,211)
Net deferred tax liabilities	$ (149,230)	$ (148,745)