Condensed Consolidated Statements of Cash Flows (Unaudited) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Cash flows from operating activities:
Net earnings	$ 250,518	$ 226,377
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization	187,304	170,120
Share-based compensation	23,602	21,979
Gain on sale of revenue equipment and other	(4,625)	(11,707)
Provision for deferred income taxes	15,798	5,146
Changes in operating assets and liabilities:
Trade accounts receivable	(118,903)	(92,773)
Other assets	17,890	20,835
Trade accounts payable	37,396	78,371
Income taxes payable or receivable	2,120	15,452
Claims accruals	14,381	5,830
Accrued payroll and other accrued expenses	(14,253)	(22,291)
Net cash provided by operating activities	411,228	417,339
Cash flows from investing activities:
Additions to property and equipment	(372,564)	(329,598)
Net proceeds from sale of equipment	38,940	50,561
Changes in other assets	(35)	(370)
Net cash used in investing activities	(333,659)	(279,407)
Cash flows from financing activities:
Payments on long-term debt	(100,000)	(50,000)
Proceeds from revolving lines of credit and other	1,372,752	1,174,531
Payments on revolving lines of credit and other	(1,264,438)	(1,227,543)
Purchase of treasury stock	(64,723)
Stock option exercises and other	(9,192)	(3,674)
Tax benefit of stock options exercised	23,637	18,288
Dividends paid	(35,226)	(49,325)
Net cash used in financing activities	(77,190)	(137,723)
Net change in cash and cash equivalents	379	209
Cash and cash equivalents at beginning of period	5,589	5,450
Cash and cash equivalents at end of period	5,968	5,659
Cash paid during the period for:
Interest	23,514	26,362
Income taxes	$ 111,404	$ 99,646