Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash flows from operating activities:
Net earnings	$ 310,354	$ 257,006	$ 199,617
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization	229,166	213,943	197,062
Share-based compensation	29,715	26,841	21,397
Gain on sale of revenue equipment and other	(16,845)	(14,109)	(1,724)
Impairment on assets held for sale			180
Provision for deferred income taxes	20,795	117,711	40,101
Changes in operating assets and liabilities:
Trade accounts receivable	(54,532)	(60,292)	(40,848)
Income taxes receivable or payable	11,868	9,045	(21,433)
Other assets	(4,826)	1,298	(9,426)
Trade accounts payable	15,097	56,179	1,883
Claims accruals	5,078	9,722	14,096
Accrued payroll and other accrued expenses	2,174	18,348	27,173
Net cash provided by operating activities	548,044	635,692	428,078
Cash flows from investing activities:
Additions to property and equipment	(439,494)	(502,282)	(262,449)
Proceeds from sale of equipment	69,815	56,413	36,517
Change in other assets	85	340	(84)
Net cash used in investing activities	(369,594)	(445,529)	(226,016)
Cash flows from financing activities:
Proceeds from issuances of long-term debt		200,000	249,207
Payments on long-term debt	(50,000)	(200,000)
Proceeds from revolving lines of credit and other	1,605,674	1,097,657	1,058,805
Payments on revolving lines of credit and other	(1,618,233)	(1,000,696)	(1,218,420)
Purchase of treasury stock	(50,000)	(246,406)	(250,892)
Stock option exercises and other	(2,411)	3,762	6,802
Tax benefit of stock options exercised	20,090	15,562	12,154
Dividends paid	(83,431)	(62,243)	(59,910)
Net cash used in financing activities	(178,311)	(192,364)	(202,254)
Net increase/(decrease) in cash and cash equivalents	139	(2,201)	(192)
Cash and cash equivalents at beginning of year	5,450	7,651	7,843
Cash and cash equivalents at end of year	5,589	5,450	7,651
Supplemental disclosure of cash flow information:
Interest	27,070	30,733	25,548
Income taxes	$ 132,096	$ 16,377	$ 88,717