Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities:
Net earnings	$ 118,142	$ 102,702
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization	105,583	92,189
Share-based compensation	12,036	11,170
Loss on sale of revenue equipment and other	2,815	1,709
Deferred income taxes	3,412	1,928
Changes in operating assets and liabilities:
Trade accounts receivable	19,864	37,855
Other assets	(49,189)	35,063
Trade accounts payable	(10,650)	(26,901)
Income taxes payable or receivable	35,434	35,064
Claims accruals	12,543	(2,910)
Accrued payroll and other accrued expenses	11,628	(2,236)
Net cash provided by operating activities	261,618	285,633
Cash flows from investing activities:
Additions to property and equipment	(206,108)	(98,775)
Net proceeds from sale of equipment	27,063	7,768
Change in other assets	(299)	(3,467)
Net cash used in investing activities	(179,344)	(94,474)
Cash flows from financing activities:
Proceeds from revolving lines of credit and other	687,036	666,864
Payments on revolving lines of credit and other	(750,400)	(696,500)
Purchase of treasury stock		(129,761)
Stock repurchased for payroll taxes	(99)	(276)
Dividends paid	(26,341)	(25,602)
Net cash used in financing activities	(89,804)	(185,275)
Net change in cash and cash equivalents	(7,530)	5,884
Cash and cash equivalents at beginning of period	14,612	6,377
Cash and cash equivalents at end of period	7,082	12,261
Supplemental disclosure of cash flow information:
Interest	11,769	9,561
Income taxes	1,834	2,082
Noncash investing activities
Accruals for equipment received	$ 42,554	$ 25,879