Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities:
Net income	$ 113,585	$ 61,755
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	170,439	162,619
Deferred income taxes	25,114	6,492
Gain on disposal of property and equipment	(15,922)	(6,031)
Non-cash equity compensation	5,507	3,215
Insurance and claims accruals, net of current portion	21,100	(6,645)
Other	(5,081)	(8,755)
Changes in certain working capital items:
Accounts receivable, net	(20,737)	(18,344)
Other current assets	(9,535)	21,467
Accounts payable	11,742	7,107
Other current liabilities	6,301	351
Net cash provided by operating activities	302,513	223,231
Cash flows from investing activities:
Additions to property and equipment	(412,557)	(205,874)
Proceeds from sales of property and equipment	123,267	84,769
Issuance of notes receivable	(3,300)	(5,000)
Decrease in notes receivable	15,846	15,637
Net cash used in investing activities	(276,744)	(110,468)
Cash flows from financing activities:
Repayments of short-term debt	(40,000)	(45,000)
Proceeds from issuance of short-term debt	40,000	0
Repayments of long-term debt	(20,000)	(60,000)
Proceeds from issuance of long-term debt	70,000	0
Change in net checks issued in excess of cash balances	(18,240)	3,538
Dividends on common stock	(16,605)	(13,721)
Repurchases of common stock	(46,413)	0
Tax withholding related to net share settlements of restricted stock awards	(679)	(445)
Stock options exercised	476	2,339
Net cash used in financing activities	(31,461)	(113,289)
Effect of exchange rate fluctuations on cash	211	580
Net increase (decrease) in cash, cash equivalents and restricted cash	(5,481)	54
Cash, cash equivalents and restricted cash, beginning of period	15,131	17,477
Cash, cash equivalents and restricted cash, end of period	9,650	17,531
Supplemental disclosures of cash flow information:
Interest paid	1,699	2,004
Income taxes paid	6,332	15,819
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	8,054	4,058
Change in fair value of interest rate swap	377	334
Property and equipment acquired included in accounts payable	4,797	492
Property and equipment disposed included in other receivables	$ 1,007	$ 1,300