Consolidated Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Cash flows from operating activities:
Net income	$ 17,511	$ 21,245
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	42,331	40,671
Deferred income taxes	(5,183)	(2,714)
Gain on disposal of property and equipment	(3,533)	(4,747)
Stock-based compensation	1,301	1,122
Insurance and claims accruals, net of current portion	2,450	1,500
Other	(213)	(1,389)
Changes in certain working capital items:
Accounts receivable, net	(7,877)	5,511
Other current assets	5,156	16,807
Accounts payable	6,411	(1,811)
Other current liabilities	18,252	7,804
Net cash provided by operating activities	76,606	83,999
Cash flows from investing activities:
Additions to property and equipment	(31,919)	(95,954)
Retirements of property and equipment	10,613	13,405
Decrease in notes receivable	2,197	623
Net cash used in investing activities	(19,109)	(81,926)
Cash flows from financing activities:
Repayments of short-term debt	(20,000)	(80,000)
Proceeds from issuance of short-term debt	0	80,000
Repayments of long-term debt	(30,000)	0
Change in net checks issued in excess of cash balances	0	736
Dividends on common stock	(3,662)	(3,642)
Tax withholding related to net share settlements of restricted stock awards	(157)	0
Stock options exercised	220	175
Excess tax benefits from exercise of stock options	4	10
Net cash used in financing activities	(53,595)	(2,721)
Effect of exchange rate fluctuations on cash	199	308
Net increase (decrease) in cash and cash equivalents	4,101	(340)
Cash and cash equivalents, beginning of period	15,428	12,412
Cash and cash equivalents, end of period	19,529	12,072
Supplemental disclosures of cash flow information:
Interest paid	168	141
Income taxes paid	3,780	1,093
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	3,489	1,517
Property and equipment acquired included in accounts payable	25	13,265
Property and equipment disposed included in other receivables	$ 1,016	$ 343