Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 202,889	$ 79,129	$ 123,714
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	217,639	209,728	193,209
Deferred income taxes	(100,948)	44,632	38,442
Gain on disposal of property and equipment	(6,798)	(16,432)	(23,240)
Non-cash equity compensation	4,546	2,381	4,361
Insurance and claims accruals, net of current portion	(5,605)	(11,320)	1,750
Other	(11,957)	(3,370)	9,103
Changes in certain working capital items:
Accounts receivable, net	(42,802)	(10,349)	15,704
Other current assets	19,183	4,979	9,455
Accounts payable	5,831	(5,272)	7,256
Other current liabilities	(140)	18,291	(9,362)
Net cash provided by operating activities	281,838	312,397	370,392
Cash flows from investing activities:
Additions to property and equipment	(316,343)	(537,838)	(454,097)
Proceeds from sales of property and equipment	117,498	108,231	102,614
Decrease in notes receivable	20,037	19,353	19,517
Issuance of notes receivable	(5,000)	0	0
Other	0	0	(3,580)
Net cash used in investing activities	(183,808)	(410,254)	(335,546)
Cash flows from financing activities:
Repayments of short-term debt	(45,000)	(20,000)	(10,000)
Proceeds from issuance of short-term debt	0	40,000	10,000
Repayments of long-term debt	(60,000)	(40,000)	0
Proceeds from issuance of long-term debt	0	125,000	0
Payment of notes payable	0	(3,117)	(3,117)
Change in net checks issued in excess of cash balances	21,539	0	0
Dividends on common stock	(18,784)	(17,289)	(15,115)
Repurchases of common stock	0	0	(6,438)
Tax withholding related to net share settlements of restricted stock awards	(1,632)	(1,832)	(1,724)
Stock options exercised	2,461	370	846
Excess tax benefits from equity compensation	0	238	556
Net cash provided by (used in) financing activities	(101,416)	83,370	(24,992)
Effect of exchange rate fluctuations on cash	50	(384)	(625)
Net increase (decrease) in cash and cash equivalents	(3,336)	(14,871)	9,229
Cash and cash equivalents, beginning of period	16,962	31,833	22,604
Cash and cash equivalents, end of period	13,626	16,962	31,833
Supplemental disclosures of cash flow information:
Interest paid	2,491	2,470	1,978
Income taxes paid	22,088	4,673	35,205
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	5,816	25,449	36,060
Change in fair value of interest rate swap	599	337	242
Property and equipment acquired included in accounts payable	3,227	1,874	627
Property and equipment disposed included in other receivables	$ 654	$ 155	$ 21