Condensed Consolidated Statements of Cash Flows (Unaudited) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Cash flows from operating activities:
Net earnings	$ 161,045	$ 148,136
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization	123,013	112,347
Share-based compensation	17,977	16,389
Gain on sale of revenue equipment and other	(1,442)	(9,773)
Benefit from deferred income taxes	(3,473)	(13,121)
Changes in operating assets and liabilities:
Trade accounts receivable	(113,632)	(60,598)
Other assets	11,006	12,777
Trade accounts payable	97,473	50,424
Income taxes payable or receivable	2,120	20,334
Claims accruals	9,167	1,617
Accrued payroll and other accrued expenses	(9,683)	(17,953)
Net cash provided by operating activities	293,571	260,579
Cash flows from investing activities:
Additions to property and equipment	(238,494)	(214,748)
Net proceeds from sale of equipment	26,802	43,868
Changes in other assets	(1)	24
Net cash used in investing activities	(211,693)	(170,856)
Cash flows from financing activities:
Payments on long-term debt	(50,000)
Proceeds from revolving lines of credit and other	846,985	719,352
Payments on revolving lines of credit and other	(809,885)	(789,843)
Purchase of treasury stock	(64,723)
Stock option exercises and other	5,635	5,073
Tax benefit of stock options exercised	7,999	8,912
Dividends paid	(17,555)	(32,780)
Net cash used in financing activities	(81,544)	(89,286)
Net change in cash and cash equivalents	334	437
Cash and cash equivalents at beginning of period	5,589	5,450
Cash and cash equivalents at end of period	5,923	5,887
Cash paid during the period for:
Interest	12,695	14,324
Income taxes	$ 91,074	$ 74,525