Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net income	$ 123,714	$ 98,650	$ 86,785
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	193,209	176,984	173,019
Deferred income taxes	38,442	5,038	(8,389)
Gain on disposal of property and equipment	(23,240)	(19,260)	(16,408)
Non-cash equity compensation	4,361	6,070	4,809
Insurance and claims accruals, net of current portion	1,750	(8,455)	6,400
Other	9,103	1,107	(541)
Changes in certain working capital items:
Accounts receivable, net	15,704	(35,080)	(20,514)
Other current assets	9,455	(25,926)	3,398
Accounts payable	7,256	(1,497)	2,793
Other current liabilities	(9,362)	8,934	1,105
Net cash provided by operating activities	370,392	206,565	232,457
Cash flows from investing activities:
Additions to property and equipment	(454,097)	(296,649)	(211,329)
Proceeds from sales of property and equipment	102,614	84,355	59,413
Decrease in notes receivable	19,517	14,390	10,679
Other	(3,580)	(5,583)	979
Net cash used in investing activities	(335,546)	(203,487)	(140,258)
Cash flows from financing activities:
Repayments of short-term debt	(10,000)	(10,000)	(20,000)
Proceeds from issuance of short-term debt	10,000	10,000	0
Repayments of long-term debt	0	(40,000)	(40,000)
Proceeds from issuance of long-term debt	0	75,000	10,000
Payment of notes payable	(3,117)	0	0
Dividends on common stock	(15,115)	(14,440)	(14,587)
Repurchases of common stock	(6,438)	(30,587)	(20,060)
Tax withholding related to net share settlements of restricted stock awards	(1,724)	(1,977)	(1,804)
Stock options exercised	846	7,012	2,548
Excess tax benefits from equity compensation	556	1,324	379
Net cash used in financing activities	(24,992)	(3,668)	(83,524)
Effect of exchange rate fluctuations on cash	(625)	(484)	(425)
Net increase (decrease) in cash and cash equivalents	9,229	(1,074)	8,250
Cash and cash equivalents, beginning of period	22,604	23,678	15,428
Cash and cash equivalents, end of period	31,833	22,604	23,678
Supplemental disclosures of cash flow information:
Interest paid	1,978	820	466
Income taxes paid	35,205	76,849	66,032
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	36,060	14,385	17,110
Issuance of notes payable	0	6,233	0
Change in fair value of interest rate swap	242	(1,180)	0
Property and equipment acquired included in accounts payable	627	2,067	5,403
Property and equipment disposed included in other receivables	$ 21	$ 0	$ 434