Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Cash flows from operating activities:
Net income	$ 20,092	$ 23,142
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	50,164	45,720
Deferred income taxes	6,410	(2,286)
Gain on disposal of property and equipment	(3,365)	(5,504)
Non-cash equity compensation	(315)	1,770
Insurance and claims accruals, net of current portion	(4,250)	5,400
Other	(5,006)	(190)
Changes in certain working capital items:
Accounts receivable, net	15,260	20,374
Other current assets	12,006	19,361
Accounts payable	813	9,220
Other current liabilities	(490)	3,979
Net cash provided by operating activities	91,319	120,986
Cash flows from investing activities:
Additions to property and equipment	(123,424)	(108,735)
Proceeds from sales of property and equipment	21,821	23,870
Decrease in notes receivable	3,394	3,777
Net cash used in investing activities	(98,209)	(81,088)
Cash flows from financing activities:
Dividends on common stock	(4,320)	(3,602)
Tax withholding related to net share settlements of restricted stock awards	(540)	(368)
Stock options exercised	99	703
Excess tax benefits from equity compensation	(19)	186
Net cash used in financing activities	(4,780)	(3,081)
Effect of exchange rate fluctuations on cash	247	(158)
Net increase (decrease) in cash and cash equivalents	(11,423)	36,659
Cash and cash equivalents, beginning of period	31,833	22,604
Cash and cash equivalents, end of period	20,410	59,263
Supplemental disclosures of cash flow information:
Interest paid	494	482
Income taxes paid	1,088	1,467
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	14,051	5,917
Change in fair value of interest rate swap	(735)	(87)
Property and equipment acquired included in accounts payable	7,240	585
Property and equipment disposed included in other receivables	$ 476	$ 68