Consolidated Statements of Cash Flows $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Cash flows from operating activities:
Net income	$ 166,944	$ 168,148	$ 202,889
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	249,527	230,151	217,639
Deferred income taxes	16,401	37,694	(100,948)
Gain on disposal of property and equipment	(21,557)	(24,898)	(6,798)
Non-cash equity compensation	8,077	7,394	4,546
Insurance and claims accruals, net of current portion	14,188	26,570	(5,605)
Other	(3,360)	(4,774)	(11,957)
Changes in certain working capital items:
Accounts receivable, net	15,081	(33,753)	(42,802)
Other current assets	975	(9,979)	20,173
Accounts payable	(7,537)	7,559	5,831
Other current liabilities	(12,095)	14,047	(140)
Net cash provided by operating activities	426,644	418,159	282,828
Cash flows from investing activities:
Additions to property and equipment	(420,748)	(519,872)	(316,343)
Proceeds from sales of property and equipment	136,873	170,900	117,498
Decrease in notes receivable	11,566	20,898	20,037
Issuance of notes receivable	0	(3,300)	(5,000)
Net cash used in investing activities	(272,309)	(331,374)	(183,808)
Cash flows from financing activities:
Repayments of short-term debt	0	(40,000)	(45,000)
Proceeds from issuance of short-term debt	0	40,000	0
Repayments of long-term debt	(100,000)	(20,000)	(60,000)
Proceeds from issuance of long-term debt	275,000	70,000	0
Change in net checks issued in excess of cash balances	0	(21,539)	21,539
Dividends on common stock	(286,190)	(23,013)	(18,784)
Repurchases of common stock	(42,301)	(72,165)	0
Tax withholding related to net share settlements of restricted stock awards	(1,899)	(1,371)	(1,632)
Stock options exercised	171	476	2,461
Net cash used in financing activities	(155,219)	(67,612)	(101,416)
Effect of exchange rate fluctuations on cash	396	(374)	50
Net increase (decrease) in cash, cash equivalents and restricted cash	(488)	18,799	(2,346)
Cash, cash equivalents and restricted cash, beginning of period	33,930	15,131	17,477
Cash, cash equivalents and restricted cash, end of period	33,442	33,930	15,131
Supplemental disclosures of cash flow information:
Interest paid	6,441	2,690	2,491
Income taxes paid	49,599	11,355	22,088
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	6,764	13,140	5,816
Change in fair value of interest rate swaps	(651)	255	599
Property and equipment acquired included in accounts payable	21,138	16,748	3,227
Property and equipment disposed included in other receivables	18,600	674	654
Dividends accrued but not yet paid at end of period	6,232	6,340	5,069
Reconciliation of cash, cash equivalents and restricted cash
Cash and cash equivalents	26,418	33,930	13,626
Restricted cash included in other current assets	7,024	0	1,505
Cash, cash equivalents and restricted cash, end of period	$ 33,442	$ 33,930	$ 15,131