Consolidated Statements of Cash Flows $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Cash flows from operating activities:
Net income	$ 169,078	$ 166,944	$ 168,148
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	263,286	249,527	230,151
Deferred income taxes	(10,233)	16,401	37,694
Gain on disposal of property and equipment	(11,271)	(21,557)	(24,898)
Non-cash equity compensation	8,903	8,077	7,394
Insurance and claims accruals, net of current portion	3,420	14,188	26,570
Other	13,641	(3,360)	(4,774)
Changes in certain working capital items:
Accounts receivable, net	(18,258)	15,081	(33,753)
Other current assets	(7,390)	975	(9,979)
Accounts payable	(2,483)	(7,537)	7,559
Other current liabilities	37,216	(12,095)	14,047
Net cash provided by operating activities	445,909	426,644	418,159
Cash flows from investing activities:
Additions to property and equipment	(413,065)	(420,748)	(519,872)
Proceeds from sales of property and equipment	146,824	136,873	170,900
Investment in equity securities	(5,000)	0	0
Decrease in notes receivable	7,966	11,566	20,898
Issuance of notes receivable	0	0	(3,300)
Net cash used in investing activities	(263,275)	(272,309)	(331,374)
Cash flows from financing activities:
Repayments of short-term debt	(90,000)	0	(40,000)
Proceeds from issuance of short-term debt	40,000	0	40,000
Repayments of long-term debt	(50,000)	(100,000)	(20,000)
Proceeds from issuance of long-term debt	0	275,000	70,000
Change in net checks issued in excess of cash balances	0	0	(21,539)
Dividends on common stock	(24,888)	(286,190)	(23,013)
Repurchases of common stock	(56,521)	(42,301)	(72,165)
Tax withholding related to net share settlements of restricted stock awards	(4,553)	(1,899)	(1,371)
Stock options exercised	0	171	476
Net cash used in financing activities	(185,962)	(155,219)	(67,612)
Effect of exchange rate fluctuations on cash	(780)	396	(374)
Net increase (decrease) in cash, cash equivalents and restricted cash	(4,108)	(488)	18,799
Cash, cash equivalents and restricted cash, beginning of period	33,442	33,930	15,131
Cash, cash equivalents and restricted cash, end of period	29,334	33,442	33,930
Supplemental disclosures of cash flow information:
Interest paid	4,415	6,441	2,690
Income taxes paid	54,173	49,599	11,355
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	3,441	6,764	13,140
Change in fair value of interest rate swaps	(5,238)	(651)	255
Property and equipment acquired included in accounts payable	12,250	21,138	16,748
Property and equipment disposed included in other receivables	30	18,600	674
Dividends accrued but not yet paid at end of period	6,114	6,232	6,340
Reconciliation of cash, cash equivalents and restricted cash
Cash and cash equivalents	29,334	26,418	33,930
Restricted cash included in other current assets	0	7,024	0
Cash, cash equivalents and restricted cash, end of period	$ 29,334	$ 33,442	$ 33,930