Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net income	$ 168,148	$ 202,889	$ 79,129
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	230,151	217,639	209,728
Deferred income taxes	37,694	(100,948)	44,632
Gain on disposal of property and equipment	(24,898)	(6,798)	(16,432)
Non-cash equity compensation	7,394	4,546	2,381
Insurance and claims accruals, net of current portion	26,570	(5,605)	(11,320)
Other	(4,774)	(11,957)	(3,370)
Changes in certain working capital items:
Accounts receivable, net	(33,753)	(42,802)	(10,349)
Other current assets	(9,979)	20,173	2,245
Accounts payable	7,559	5,831	(5,272)
Other current liabilities	14,047	(140)	18,291
Net cash provided by operating activities	418,159	282,828	309,663
Cash flows from investing activities:
Additions to property and equipment	(519,872)	(316,343)	(537,838)
Proceeds from sales of property and equipment	170,900	117,498	108,231
Decrease in notes receivable	20,898	20,037	19,353
Issuance of notes receivable	(3,300)	(5,000)	0
Net cash used in investing activities	(331,374)	(183,808)	(410,254)
Cash flows from financing activities:
Repayments of short-term debt	(40,000)	(45,000)	(20,000)
Proceeds from issuance of short-term debt	40,000	0	40,000
Repayments of long-term debt	(20,000)	(60,000)	(40,000)
Proceeds from issuance of long-term debt	70,000	0	125,000
Payment of notes payable	0	0	(3,117)
Change in net checks issued in excess of cash balances	(21,539)	21,539	0
Dividends on common stock	(23,013)	(18,784)	(17,289)
Repurchases of common stock	(72,165)	0	0
Tax withholding related to net share settlements of restricted stock awards	(1,371)	(1,632)	(1,832)
Stock options exercised	476	2,461	370
Excess tax benefits from equity compensation	0	0	238
Net cash provided by (used in) financing activities	(67,612)	(101,416)	83,370
Effect of exchange rate fluctuations on cash	(374)	50	(384)
Net increase (decrease) in cash, cash equivalents and restricted cash	18,799	(2,346)	(17,605)
Cash, cash equivalents and restricted cash, beginning of period	15,131	17,477	35,082
Cash, cash equivalents and restricted cash, end of period	33,930	15,131	17,477
Supplemental disclosures of cash flow information:
Interest paid	2,690	2,491	2,470
Income taxes paid	11,355	22,088	4,673
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	13,140	5,816	25,449
Change in fair value of interest rate swap	255	599	337
Property and equipment acquired included in accounts payable	16,748	3,227	1,874
Property and equipment disposed included in other receivables	$ 674	$ 654	$ 155