Consolidated Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Cash flows from operating activities:
Net income	$ 77,053	$ 73,389
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	124,078	119,161
Deferred income taxes	(7,920)	35,169
Gain on disposal of property and equipment	(15,798)	(16,454)
Stock-based compensation	3,374	1,877
Insurance and claims accruals, net of current portion	(50)	7,000
Other	1,366	(844)
Changes in certain working capital items:
Accounts receivable, net	954	(23,241)
Other current assets	21,355	(1)
Accounts payable	(10,235)	6,729
Other current liabilities	4,538	(2,446)
Net cash provided by operating activities	198,715	200,339
Cash flows from investing activities:
Additions to property and equipment	(226,499)	(208,469)
Retirements of property and equipment	46,252	54,869
Decrease in notes receivable	4,114	4,218
Net cash used in investing activities	(176,133)	(149,382)
Cash flows from financing activities:
Repayments of short-term debt	(140,000)	(20,000)
Proceeds from issuance of short-term debt	140,000	20,000
Change in net checks issued in excess of cash balances	(6,671)	0
Dividends on common stock	(10,929)	(10,911)
Tax withholding related to net share settlements of restricted stock awards	(146)	0
Stock options exercised	379	2,626
Excess tax benefits from exercise of stock options	22	599
Net cash used in financing activities	(17,345)	(7,686)
Effect of exchange rate fluctuations on cash	388	(352)
Net increase in cash and cash equivalents	5,625	42,919
Cash and cash equivalents, beginning of period	12,412	13,966
Cash and cash equivalents, end of period	18,037	56,885
Supplemental disclosures of cash flow information:
Interest paid	225	43
Income taxes paid	49,324	23,864
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	7,286	6,053
Property and equipment acquired included in accounts payable	7,991	4,023
Property and equipment disposed included in other receivables	$ 411	$ 1,548