Consolidated Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Cash flows from operating activities:
Net income	$ 39,971	$ 43,351
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	87,696	84,698
Deferred income taxes	(10,753)	(4,458)
Gain on disposal of property and equipment	(9,815)	(10,046)
Non-cash equity compensation	2,837	2,590
Insurance and claims accruals, net of current portion	350	1,900
Other	(420)	31
Changes in certain working capital items:
Accounts receivable, net	(29,658)	(12,857)
Other current assets	2,097	(3,654)
Accounts payable	11,238	9,165
Other current liabilities	(2,998)	188
Net cash provided by operating activities	90,545	110,908
Cash flows from investing activities:
Additions to property and equipment	(99,969)	(64,865)
Retirements of property and equipment	37,162	30,336
Decrease in notes receivable	8,076	4,439
Other	3,343	0
Net cash used in investing activities	(58,074)	(30,090)
Cash flows from financing activities:
Repayments of short-term debt	0	(20,000)
Repayments of long-term debt	0	(30,000)
Dividends on common stock	(7,255)	(7,326)
Repurchases of common stock	(25,602)	(15,065)
Tax withholding related to net share settlements of restricted stock awards	(31)	(166)
Stock options exercised	4,150	733
Excess tax benefits from equity compensation	284	24
Net cash used in financing activities	(28,454)	(71,800)
Effect of exchange rate fluctuations on cash	19	(198)
Net increase in cash and cash equivalents	4,036	8,820
Cash and cash equivalents, beginning of period	23,678	15,428
Cash and cash equivalents, end of period	27,714	24,248
Supplemental disclosures of cash flow information:
Interest paid	233	262
Income taxes paid	42,581	40,765
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	6,333	7,238
Issuance of notes payable	6,233	0
Property and equipment acquired included in accounts payable	9,407	3,886
Property and equipment disposed included in other receivables	$ 696	$ 1,104