Consolidated Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Cash flows from operating activities:
Net income	$ 43,351	$ 51,925
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	84,698	82,177
Deferred income taxes	(4,458)	(5,588)
Gain on disposal of property and equipment	(10,046)	(10,446)
Stock-based compensation	2,590	2,233
Insurance and claims accruals, net of current portion	1,900	(300)
Other	31	(449)
Changes in certain working capital items:
Accounts receivable, net	(12,857)	(923)
Other current assets	(3,654)	26,069
Accounts payable	9,165	(8,848)
Other current liabilities	188	2,948
Net cash provided by operating activities	110,908	138,798
Cash flows from investing activities:
Additions to property and equipment	(64,865)	(152,429)
Retirements of property and equipment	30,336	30,503
Decrease in notes receivable	4,439	2,371
Net cash used in investing activities	(30,090)	(119,555)
Cash flows from financing activities:
Repayments of short-term debt	(20,000)	(120,000)
Proceeds from issuance of short-term debt	0	120,000
Repayments of long-term debt	(30,000)	0
Change in net checks issued in excess of cash balances	0	(6,671)
Dividends on common stock	(7,326)	(7,285)
Repurchases of common stock	(15,065)	0
Tax withholding related to net share settlements of restricted stock awards	(166)	0
Stock options exercised	733	379
Excess tax benefits from exercise of stock options	24	22
Net cash used in financing activities	(71,800)	(13,555)
Effect of exchange rate fluctuations on cash	(198)	84
Net increase in cash and cash equivalents	8,820	5,772
Cash and cash equivalents, beginning of period	15,428	12,412
Cash and cash equivalents, end of period	24,248	18,184
Supplemental disclosures of cash flow information:
Interest paid	262	207
Income taxes paid	40,765	26,192
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	7,238	4,593
Property and equipment acquired included in accounts payable	3,886	9,541
Property and equipment disposed included in other receivables	$ 1,104	$ 587