Income Taxes - Summary of significant components of deferred tax assets and liabilities (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deferred tax assets:
Self-insurance	$ 7,675	$ 7,247
Pension	12,555	8,018
State net operating loss carryforwards	451	484
Bad debts	5,755	4,748
Accrued payroll	2,833	2,019
Stock-based compensation	5,583	5,183
Tangible property regulations 481(a)		7,665
All others	2,121	1,541
Valuation allowance	(2)	(83)
Gross deferred tax assets	36,971	36,822
Deferred tax liabilities:
Depreciation	(172,813)	(165,960)
Goodwill amortization	(7,974)	(7,094)
All Others	(3,739)	(2,759)
Gross deferred tax liabilities	(184,526)	(175,813)
Net deferred tax liabilities	$ (147,555)	$ (138,991)