Consolidated Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Cash flows from operating activities:
Net income	$ 51,925	$ 43,811
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	82,177	78,964
Deferred income taxes	(5,588)	22,721
Gain on disposal of property and equipment	(10,446)	(10,422)
Stock-based compensation	2,233	1,147
Insurance and claims accruals, net of current portion	(300)	4,500
Other	(449)	(144)
Changes in certain working capital items:
Accounts receivable, net	(923)	(22,986)
Other current assets	26,069	746
Accounts payable	(8,848)	5,595
Other current liabilities	2,948	(6,902)
Net cash provided by operating activities	138,798	117,030
Cash flows from investing activities:
Additions to property and equipment	(152,429)	(140,766)
Retirements of property and equipment	30,503	34,826
Decrease in notes receivable	2,371	2,828
Net cash used in investing activities	(119,555)	(103,112)
Cash flows from financing activities:
Repayments of short-term debt	(120,000)	(20,000)
Proceeds from issuance of short-term debt	120,000	20,000
Change in net checks issued in excess of cash balances	(6,671)	0
Dividends on common stock	(7,285)	(7,271)
Stock options exercised	379	2,385
Excess tax benefits from exercise of stock options	22	471
Net cash used in financing activities	(13,555)	(4,415)
Effect of exchange rate fluctuations on cash	84	14
Net increase in cash and cash equivalents	5,772	9,517
Cash and cash equivalents, beginning of period	12,412	13,966
Cash and cash equivalents, end of period	18,184	23,483
Supplemental disclosures of cash flow information:
Interest paid	207	38
Income taxes paid	26,192	20,341
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	4,593	2,165
Property and equipment acquired included in accounts payable	9,541	3,372
Property and equipment disposed included in other receivables	$ 587	$ 4,536