Consolidated Statements Of Cash Flows (USD $) In Thousands	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010
Cash flows from operating activities:
Net income	$ 43,811	$ 31,766
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	78,964	75,756
Deferred income taxes	22,721	(17,276)
Gain on disposal of property and equipment	(10,422)	(1,649)
Stock-based compensation	1,147	507
Insurance claims accruals, net of current portion	4,500	3,000
Other	(144)	1,014
Changes in certain working capital items:
Accounts receivable, net	(22,986)	(11,065)
Other current assets	746	12,468
Accounts payable	5,595	5,725
Other current liabilities	(6,902)	11,170
Net cash provided by operating activities	117,030	111,416
Cash flows from investing activities:
Additions to property and equipment	(140,766)	(82,372)
Retirements of property and equipment	34,826	30,057
Decrease in notes receivable	2,828	2,230
Net cash used in investing activities	(103,112)	(50,085)
Cash flows from financing activities:
Repayments of short-term debt	(20,000)	(10,000)
Proceeds from issuance of short-term debt	20,000	10,000
Dividends on common stock	(7,271)	(7,198)
Stock options exercised	2,385	4,761
Excess tax benefits from exercise of stock options	471	2,024
Net cash used in financing activities	(4,415)	(413)
Effect of foreign exchange rate fluctuations on cash	14	(155)
Net increase in cash and cash equivalents	9,517	60,763
Cash and cash equivalents, beginning of period	13,966	18,430
Cash and cash equivalents, end of period	23,483	79,193
Supplemental disclosures of cash flow information:
Interest paid	38	12
Income taxes paid	20,341	30,910
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of revenue equipment	$ 2,165	$ 2,415