Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net income	$ 16,019	$ 20,092
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	55,336	50,164
Deferred income taxes	3,433	6,410
Gain on disposal of property and equipment	(1,392)	(3,365)
Non-cash equity compensation	896	(315)
Insurance and claims accruals, net of current portion	(2,915)	(4,250)
Other	(3,897)	(5,006)
Changes in certain working capital items:
Accounts receivable, net	14,337	15,260
Other current assets	288	12,006
Accounts payable	(1,869)	813
Other current liabilities	(5,289)	(490)
Net cash provided by operating activities	74,947	91,319
Cash flows from investing activities:
Additions to property and equipment	(42,659)	(123,424)
Proceeds from sales of property and equipment	28,065	21,821
Decrease in notes receivable	7,206	3,394
Net cash used in investing activities	(7,388)	(98,209)
Cash flows from financing activities:
Repayments of short-term debt	(20,000)	0
Repayments of long-term debt	(30,000)	0
Dividends on common stock	(4,330)	(4,320)
Tax withholding related to net share settlements of restricted stock awards	(341)	(540)
Stock options exercised	524	99
Excess tax benefits from equity compensation	0	(19)
Net cash used in financing activities	(54,147)	(4,780)
Effect of exchange rate fluctuations on cash	430	247
Net increase (decrease) in cash and cash equivalents	13,842	(11,423)
Cash and cash equivalents, beginning of period	16,962	31,833
Cash and cash equivalents, end of period	30,804	20,410
Supplemental disclosures of cash flow information:
Interest paid	876	494
Income taxes paid	378	1,088
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	872	14,051
Change in fair value of interest rate swap	214	(735)
Property and equipment acquired included in accounts payable	671	7,240
Property and equipment disposed included in other receivables	$ 11	$ 476