Consolidated Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Cash flows from operating activities:
Net income	$ 14,339	$ 17,511
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	43,123	42,331
Deferred income taxes	(6,294)	(5,183)
Gain on disposal of property and equipment	(4,647)	(3,533)
Non-cash equity compensation	1,304	1,301
Insurance and claims accruals, net of current portion	(1,200)	2,450
Other	203	(213)
Changes in certain working capital items:
Accounts receivable, net	(12,647)	(7,877)
Other current assets	(2,650)	5,156
Accounts payable	8,405	6,411
Other current liabilities	19,273	18,252
Net cash provided by operating activities	59,209	76,606
Cash flows from investing activities:
Additions to property and equipment	(30,864)	(31,919)
Retirements of property and equipment	14,818	10,613
Decrease in notes receivable	2,766	2,197
Net cash used in investing activities	(13,280)	(19,109)
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	(3,636)	(3,662)
Repurchases of common stock	(12,880)	0
Tax withholding related to net share settlements of restricted stock awards	0	(157)
Stock options exercised	3,133	220
Excess tax benefits from equity compensation	218	4
Net cash used in financing activities	(13,165)	(53,595)
Effect of exchange rate fluctuations on cash	26	199
Net increase in cash and cash equivalents	32,790	4,101
Cash and cash equivalents, beginning of period	23,678	15,428
Cash and cash equivalents, end of period	56,468	19,529
Supplemental disclosures of cash flow information:
Interest paid	105	168
Income taxes paid	1,344	3,780
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	3,452	3,489
Property and equipment acquired included in accounts payable	507	25
Property and equipment disposed included in other receivables	$ 469	$ 1,016