Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities:
Net income	$ 66,071	$ 39,238
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	112,057	109,041
Deferred income taxes	16,269	6,252
Gain on disposal of property and equipment	(11,346)	(3,854)
Non-cash equity compensation	3,389	2,031
Insurance and claims accruals, net of current portion	14,081	(3,265)
Other	(2,795)	(8,206)
Changes in certain working capital items:
Accounts receivable, net	(28,960)	10,970
Other current assets	3,791	24,209
Accounts payable	10,745	1,759
Other current liabilities	(851)	318
Net cash provided by operating activities	182,451	178,493
Cash flows from investing activities:
Additions to property and equipment	(256,202)	(126,051)
Proceeds from sales of property and equipment	81,367	60,076
Issuance of notes receivable	(3,300)	0
Decrease in notes receivable	11,727	11,490
Net cash used in investing activities	(166,408)	(54,485)
Cash flows from financing activities:
Repayments of short-term debt	0	(45,000)
Repayments of long-term debt	0	(60,000)
Proceeds from issuance of long-term debt	20,000	0
Change in net checks issued in excess of cash balances	(7,842)	0
Dividends on common stock	(10,140)	(8,663)
Repurchases of common stock	(22,884)	0
Tax withholding related to net share settlements of restricted stock awards	(679)	(360)
Stock options exercised	248	988
Net cash used in financing activities	(21,297)	(113,035)
Effect of exchange rate fluctuations on cash	47	707
Net increase in cash, cash equivalents and restricted cash	(5,207)	11,680
Cash, cash equivalents and restricted cash, beginning of period	15,131	17,477
Cash, cash equivalents and restricted cash, end of period	9,924	29,157
Supplemental disclosures of cash flow information:
Interest paid	972	1,512
Income taxes paid	5,463	4,468
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	3,816	2,599
Change in fair value of interest rate swap	372	234
Property and equipment acquired included in accounts payable	7,697	4,214
Property and equipment disposed included in other receivables	$ 265	$ 103