Condensed Consolidated Statements of Cash Flows (Unaudited) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Cash flows from operating activities:
Net earnings	$ 226,377	$ 184,441
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization	170,120	158,312
Share-based compensation	21,979	17,838
Gain on sale of revenue equipment and other	(11,707)	(10,008)
Provision for deferred income taxes	5,146	41,977
Changes in operating assets and liabilities:
Trade accounts receivable	(92,773)	(73,447)
Other assets	20,835	31,930
Trade accounts payable	78,371	82,673
Income taxes payable or receivable	15,452	44,388
Claims accruals	5,830	5,018
Accrued payroll and other accrued expenses	(22,291)	5,087
Net cash provided by operating activities	417,339	488,209
Cash flows from investing activities:
Additions to property and equipment	(329,598)	(365,923)
Net proceeds from sale of equipment	50,561	41,657
Changes in other assets	(370)	350
Net cash used in investing activities	(279,407)	(323,916)
Cash flows from financing activities:
Proceeds from issuances of long-term debt		200,000
Payments on long-term debt	(50,000)	(200,000)
Proceeds from revolving lines of credit and other	1,174,531	736,644
Payments on revolving lines of credit and other	(1,227,543)	(623,172)
Purchase of treasury stock		(246,406)
Stock option exercises and other	(3,674)	2,318
Tax benefit of stock options exercised	18,288	13,470
Dividends paid	(49,325)	(47,051)
Net cash used in financing activities	(137,723)	(164,197)
Net change in cash and cash equivalents	209	96
Cash and cash equivalents at beginning of period	5,450	7,651
Cash and cash equivalents at end of period	5,659	7,747
Cash paid during the period for:
Interest	26,362	30,039
Income taxes	$ 99,646	$ 14,010