Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net earnings	$ 102,702	$ 100,098
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization	92,189	88,352
Share-based compensation	11,170	10,972
Loss on sale of revenue equipment and other	1,709	30
Deferred income taxes	1,928	15,257
Changes in operating assets and liabilities:
Trade accounts receivable	37,855	(26,743)
Other assets	35,063	22,154
Trade accounts payable	(26,901)	41,168
Income taxes payable or receivable	35,064	41,803
Claims accruals	(2,910)	3,020
Accrued payroll and other accrued expenses	(2,236)	8,785
Net cash provided by operating activities	285,633	304,896
Cash flows from investing activities:
Additions to property and equipment	(98,775)	(156,786)
Net proceeds from sale of equipment	7,768	42,328
Change in other assets	(3,467)	(2)
Net cash used in investing activities	(94,474)	(114,460)
Cash flows from financing activities:
Proceeds from revolving lines of credit and other	666,864	313,729
Payments on revolving lines of credit and other	(696,500)	(378,397)
Purchase of treasury stock	(129,761)	(100,000)
Stock option exercises and other		42
Stock repurchased for payroll taxes	(276)	(621)
Tax benefit of stock options exercised		47
Dividends paid	(25,602)	(24,810)
Net cash used in financing activities	(185,275)	(190,010)
Net change in cash and cash equivalents	5,884	426
Cash and cash equivalents at beginning of period	6,377	5,566
Cash and cash equivalents at end of period	12,261	5,992
Supplemental disclosure of cash flow information:
Interest	9,561	8,797
Income taxes	$ 2,082	$ 3,046