Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Cash flows from operating activities:
Net income	$ 38,398	$ 54,990
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	101,068	93,984
Deferred income taxes	23,621	(3,097)
Gain on disposal of property and equipment	(10,147)	(12,191)
Non-cash equity compensation	1,011	3,755
Insurance and claims accruals, net of current portion	(8,470)	7,100
Other	(5,212)	2,186
Changes in certain working capital items:
Accounts receivable, net	11,915	7,904
Other current assets	5,711	28,584
Accounts payable	3,627	16,254
Other current liabilities	9,500	(5,897)
Net cash provided by operating activities	171,022	193,572
Cash flows from investing activities:
Additions to property and equipment	(313,080)	(216,187)
Proceeds from sales of property and equipment	52,554	56,781
Decrease in notes receivable	8,297	7,188
Net cash used in investing activities	(252,229)	(152,218)
Cash flows from financing activities:
Proceeds from issuance of long-term debt	70,000	0
Change in net checks issued in excess of cash balances	6,941	0
Dividends on common stock	(8,643)	(7,207)
Repurchases of common stock	0	(6,438)
Tax withholding related to net share settlements of restricted stock awards	(559)	(393)
Stock options exercised	185	763
Excess tax benefits from equity compensation	(16)	199
Payment of notes payable	(3,117)	(3,117)
Net cash provided by (used in) financing activities	64,791	(16,193)
Effect of exchange rate fluctuations on cash	(180)	(274)
Net increase (decrease) in cash and cash equivalents	(16,596)	24,887
Cash and cash equivalents, beginning of period	31,833	22,604
Cash and cash equivalents, end of period	15,237	47,491
Supplemental disclosures of cash flow information:
Interest paid	1,072	1,066
Income taxes paid	4,238	17,862
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	18,743	12,549
Change in fair value of interest rate swap	(905)	237
Property and equipment acquired included in accounts payable	11,904	8,967
Property and equipment disposed included in other receivables	$ 606	$ 0