Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net earnings	$ 489,585	$ 686,263	$ 432,090
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization	435,893	383,518	361,510
Share-based compensation	47,369	38,291	40,625
Loss on sale of revenue equipment and other	12,107	7,370	5,490
Advance deposit impairment		20,240
Deferred income taxes	101,591	(248,764)	50,414
Changes in operating assets and liabilities:
Trade accounts receivable	(130,931)	(166,111)	(120,994)
Income taxes receivable or payable	(41,071)	(45,542)	60,956
Other current assets	(6,133)	69,462	(37,101)
Trade accounts payable	98,037	85,237	60,818
Claims accruals	21,580	25,021	5,524
Accrued payroll and other accrued expenses	59,814	168	(5,189)
Net cash provided by operating activities	1,087,841	855,153	854,143
Cash flows from investing activities:
Additions to property and equipment	(995,650)	(526,928)	(638,430)
Proceeds from sale of equipment	110,165	16,413	153,174
Business acquisition		(136,879)
Change in other assets	(1,288)	(3,888)	(132)
Net cash used in investing activities	(886,773)	(651,282)	(485,388)
Cash flows from financing activities:
Proceeds from revolving lines of credit and other	3,204,715	2,716,155	1,715,427
Payments on revolving lines of credit and other	(3,137,900)	(2,612,501)	(1,724,365)
Purchase of treasury stock	(150,338)	(179,813)	(249,760)
Stock option exercises and other	967	1,100	1,341
Stock repurchased for payroll taxes	(20,530)	(19,215)	(18,641)
Tax benefit of stock options exercised and restricted shares issued			7,044
Dividends paid	(104,994)	(101,362)	(98,990)
Net cash used in financing activities	(208,080)	(195,636)	(367,944)
Net increase/(decrease) in cash and cash equivalents	(7,012)	8,235	811
Cash and cash equivalents at beginning of year	14,612	6,377	5,566
Cash and cash equivalents at end of year	7,600	14,612	6,377
Supplemental disclosure of cash flow information:
Interest	39,901	28,785	24,800
Income taxes	83,822	190,783	143,634
Noncash investing activities
Accruals for equipment received	$ 49,390	$ 53,026	$ 13,522