Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities:
Net earnings	$ 200,571	$ 205,109
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization	185,238	178,716
Share-based compensation	23,110	22,915
Loss on sale of revenue equipment and other	3,393	1,030
Benefit from deferred income taxes	(37,357)	2,354
Changes in operating assets and liabilities:
Trade accounts receivable	(3,754)	(75,703)
Other assets	47,274	33,501
Trade accounts payable	13,269	11,267
Income taxes payable or receivable	11,370	72,041
Claims accruals	4,329	11,408
Accrued payroll and other accrued expenses	(3,996)	11,500
Net cash provided by operating activities	443,447	474,138
Cash flows from investing activities:
Additions to property and equipment	(176,660)	(351,507)
Net proceeds from sale of equipment	11,568	93,549
Changes in other assets	(3,464)	(7)
Net cash used in investing activities	(168,556)	(257,965)
Cash flows from financing activities:
Proceeds from revolving lines of credit and other	1,247,227	676,687
Payments on revolving lines of credit and other	(1,291,200)	(738,198)
Purchase of treasury stock	(179,813)	(100,000)
Stock option exercises and other	1,100	1,342
Stock repurchased for payroll taxes	(431)	(975)
Tax benefit of stock options exercised		280
Dividends paid	(50,877)	(49,598)
Net cash used in financing activities	(273,994)	(210,462)
Net change in cash and cash equivalents	897	5,711
Cash and cash equivalents at beginning of period	6,377	5,566
Cash and cash equivalents at end of period	7,274	11,277
Supplemental disclosure of cash flow information:
Interest	14,477	12,855
Income taxes	$ 122,718	$ 49,039