Consolidated Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Cash flows from operating activities:
Net income	$ 21,245	$ 16,293
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	40,671	39,718
Deferred income taxes	(2,714)	10,759
Gain on disposal of property and equipment	(4,747)	(4,775)
Stock-based compensation	1,122	609
Insurance and claims accruals, net of current portion	1,500	2,000
Other	(1,389)	478
Changes in certain working capital items:
Accounts receivable, net	5,511	(20,301)
Other current assets	16,807	1,666
Accounts payable	(1,811)	4,955
Other current liabilities	7,804	2,398
Net cash provided by operating activities	83,999	53,800
Cash flows from investing activities:
Additions to property and equipment	(95,954)	(37,615)
Retirements of property and equipment	13,405	17,561
Decrease in notes receivable	623	1,271
Net cash used in investing activities	(81,926)	(18,783)
Cash flows from financing activities:
Repayments of short-term debt	(80,000)	(20,000)
Proceeds from issuance of short-term debt	80,000	20,000
Change in net checks issued in excess of cash balances	736	0
Dividends on common stock	(3,642)	(3,632)
Stock options exercised	175	1,902
Excess tax benefits from exercise of stock options	10	317
Net cash used in financing activities	(2,721)	(1,413)
Effect of exchange rate fluctuations on cash	308	29
Net increase in cash and cash equivalents	(340)	33,633
Cash and cash equivalents, beginning of period	12,412	13,966
Cash and cash equivalents, end of period	12,072	47,599
Supplemental disclosures of cash flow information:
Interest paid	141	28
Income taxes paid	1,093	5,741
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	1,517	1,028
Property and equipment acquired included in accounts payable	13,265	2,485
Property and equipment disposed included in other receivables	$ 343	$ 678