Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Cash flows from operating activities:
Net earnings	$ 318,768	$ 226,532
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization	276,916	260,393
Noncash lease expense	25,411	23,553
Share-based compensation	31,466	34,065
Loss on sale of revenue equipment and other	2,359	2,442
Deferred income taxes	36,945	621
Changes in operating assets and liabilities:
Trade accounts receivable	(169,521)	61,193
Other assets	85,657	(26,446)
Trade accounts payable	49,273	10,526
Income taxes payable or receivable	(26,359)	46,964
Claims accruals	7,359	(9,060)
Accrued payroll and other accrued expenses	30,474	9,167
Net cash provided by operating activities	668,748	639,950
Cash flows from investing activities:
Additions to property and equipment	(306,727)	(342,309)
Net proceeds from sale of equipment	46,016	77,695
Changes in other assets	(88)	88
Net cash used in investing activities	(260,799)	(264,526)
Cash flows from financing activities:
Proceeds from revolving lines of credit and other	1,011	221,113
Payments on revolving lines of credit and other	0	(220,100)
Purchase of treasury stock	(86,580)	(75,193)
Stock repurchased for payroll taxes and other	(3,463)	(4,415)
Dividends paid	(61,301)	(57,175)
Net cash used in financing activities	(150,333)	(135,770)
Net change in cash and cash equivalents	257,616	239,654
Cash and cash equivalents at beginning of period	313,302	35,000
Cash and cash equivalents at end of period	570,918	274,654
Supplemental disclosure of cash flow information:
Interest	24,341	29,771
Income taxes	92,368	31,319
Noncash investing activities
Accruals for equipment received	$ 50,606	$ 22,344