Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net income	$ 79,129	$ 123,714	$ 98,650
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	209,728	193,209	176,984
Deferred income taxes	44,632	38,442	5,038
Gain on disposal of property and equipment	(16,432)	(23,240)	(19,260)
Non-cash equity compensation	2,381	4,361	6,070
Insurance and claims accruals, net of current portion	(11,320)	1,750	(8,455)
Other	(3,370)	9,103	1,107
Changes in certain working capital items:
Accounts receivable, net	(10,349)	15,704	(35,080)
Other current assets	4,979	9,455	(25,926)
Accounts payable	(5,272)	7,256	(1,497)
Other current liabilities	18,291	(9,362)	8,934
Net cash provided by operating activities	312,397	370,392	206,565
Cash flows from investing activities:
Additions to property and equipment	(537,838)	(454,097)	(296,649)
Proceeds from sales of property and equipment	108,231	102,614	84,355
Decrease in notes receivable	19,353	19,517	14,390
Other	0	(3,580)	(5,583)
Net cash used in investing activities	(410,254)	(335,546)	(203,487)
Cash flows from financing activities:
Repayments of short-term debt	(20,000)	(10,000)	(10,000)
Proceeds from issuance of short-term debt	40,000	10,000	10,000
Repayments of long-term debt	(40,000)	0	(40,000)
Proceeds from issuance of long-term debt	125,000	0	75,000
Payment of notes payable	(3,117)	(3,117)	0
Dividends on common stock	(17,289)	(15,115)	(14,440)
Repurchases of common stock	0	(6,438)	(30,587)
Tax withholding related to net share settlements of restricted stock awards	(1,832)	(1,724)	(1,977)
Stock options exercised	370	846	7,012
Excess tax benefits from equity compensation	238	556	1,324
Net cash provided by (used in) financing activities	83,370	(24,992)	(3,668)
Effect of exchange rate fluctuations on cash	(384)	(625)	(484)
Net increase (decrease) in cash and cash equivalents	(14,871)	9,229	(1,074)
Cash and cash equivalents, beginning of period	31,833	22,604	23,678
Cash and cash equivalents, end of period	16,962	31,833	22,604
Supplemental disclosures of cash flow information:
Interest paid	2,470	1,978	820
Income taxes paid	4,673	35,205	76,849
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	25,449	36,060	14,385
Issuance of notes payable	0	0	6,233
Change in fair value of interest rate swap	337	242	(1,180)
Property and equipment acquired included in accounts payable	1,874	627	2,067
Property and equipment disposed included in other receivables	$ 155	$ 21	$ 0