Income Taxes - Summary of significant components of deferred tax assets and liabilities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Deferred tax assets:
Self-insurance	$ 7,247	$ 7,417
Pension	8,018	9,688
State net operating loss carryforwards	484	1,165
Bad debts	4,748	3,489
Accrued payroll	2,019	2,038
Stock-based compensation	5,183	4,567
Tangible property regulations 481(a)	7,665
All others	1,541	274
Valuation allowance	(83)	(1,003)
Gross deferred tax assets	36,822	27,635
Deferred tax liabilities:
Depreciation	(165,960)	(168,717)
Goodwill amortization	(7,094)	(6,394)
All Others	(2,759)	(1,754)
Gross deferred tax liabilities	(175,813)	(176,865)
Net deferred tax liabilities	$ (138,991)	$ (149,230)