Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash flows from operating activities:
Net earnings	$ 146,606	$ 104,834
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization	137,545	130,095
Noncash lease expense	12,296	11,411
Share-based compensation	14,967	18,063
Loss on sale of revenue equipment and other	1,147	1,472
Deferred income taxes	30,996	15,470
Changes in operating assets and liabilities:
Trade accounts receivable	(94,759)	33,549
Other assets	46,385	(60,029)
Trade accounts payable	43,280	(5,392)
Income taxes payable or receivable	12,156	23,066
Claims accruals	2,242	(7,938)
Accrued payroll and other accrued expenses	11,797	(15,438)
Net cash provided by operating activities	364,658	249,163
Cash flows from investing activities:
Additions to property and equipment	(108,775)	(166,814)
Net proceeds from sale of equipment	22,921	38,078
Change in other assets	4	28
Net cash used in investing activities	(85,850)	(128,708)
Cash flows from financing activities:
Proceeds from revolving lines of credit and other	505	220,607
Payments on revolving lines of credit and other	0	(220,100)
Purchase of treasury stock	(5,239)	(75,193)
Stock repurchased for payroll taxes	(4,809)	(3,627)
Dividends paid	(29,597)	(28,688)
Net cash used in financing activities	(39,140)	(107,001)
Net change in cash and cash equivalents	239,668	13,454
Cash and cash equivalents at beginning of period	313,302	35,000
Cash and cash equivalents at end of period	552,970	48,454
Supplemental disclosure of cash flow information:
Interest	21,343	21,673
Income taxes	3,779	1,173
Noncash investing activities
Accruals for equipment received	$ 23,630	$ 22,395