Condensed Consolidated Statements of Cash Flows (Unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Cash flows from operating activities:
Net earnings	$ 73,349	$ 67,684
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization	60,730	56,120
Share-based compensation	9,137	8,101
Gain on sale of revenue equipment and other	(1,307)	(5,709)
Deferred income taxes	4,398	(11,540)
Changes in operating assets and liabilities:
Trade accounts receivable	(63,322)	(36,227)
Other assets	(4,166)	(4,152)
Trade accounts payable	108,246	52,061
Income taxes payable or receivable	37,379	49,621
Claims accruals	2,408	(433)
Accrued payroll and other accrued expenses	(20,093)	(29,254)
Net cash provided by operating activities	206,759	146,272
Cash flows from investing activities:
Additions to property and equipment	(125,537)	(112,719)
Net proceeds from sale of equipment	10,479	28,937
Change in other assets	(12)	16
Net cash used in investing activities	(115,070)	(83,766)
Cash flows from financing activities:
Payments on long-term debt	(50,000)
Proceeds from revolving lines of credit and other	302,643	334,093
Payments on revolving lines of credit and other	(345,600)	(383,304)
Stock option exercises and other	360	1,218
Tax benefit of stock options exercised	1,066	2,136
Dividends paid		(16,386)
Net cash used in financing activities	(91,531)	(62,243)
Net change in cash and cash equivalents	158	263
Cash and cash equivalents at beginning of period	5,589	5,450
Cash and cash equivalents at end of period	5,747	5,713
Cash paid during the period for:
Interest	10,846	12,526
Income taxes	$ 2,047	$ 1,196