Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities:
Net earnings	$ 269,794	$ 200,571
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization	213,006	185,238
Share-based compensation	26,001	23,110
Loss on sale of revenue equipment and other	6,459	3,393
Deferred income taxes	10,996	(37,357)
Changes in operating assets and liabilities:
Trade accounts receivable	(98,855)	(3,754)
Other assets	(26,693)	47,274
Trade accounts payable	33,716	13,269
Income taxes payable or receivable	54,392	11,370
Claims accruals	12,150	4,329
Accrued payroll and other accrued expenses	28,067	(3,996)
Net cash provided by operating activities	529,033	443,447
Cash flows from investing activities:
Additions to property and equipment	(411,744)	(176,660)
Net proceeds from sale of equipment	57,613	11,568
Changes in other assets	(1,293)	(3,464)
Net cash used in investing activities	(355,424)	(168,556)
Cash flows from financing activities:
Proceeds from revolving lines of credit and other	1,421,328	1,247,227
Payments on revolving lines of credit and other	(1,492,600)	(1,291,200)
Purchase of treasury stock	(50,027)	(179,813)
Stock repurchased for payroll taxes and other	852	669
Dividends paid	(52,583)	(50,877)
Net cash used in financing activities	(173,030)	(273,994)
Net change in cash and cash equivalents	579	897
Cash and cash equivalents at beginning of period	14,612	6,377
Cash and cash equivalents at end of period	15,191	7,274
Supplemental disclosure of cash flow information:
Interest	19,027	14,477
Income taxes	27,762	122,718
Noncash investing activities
Accruals for equipment received	$ 56,645	$ 35,252