Consolidated Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash flows from operating activities:
Net income	$ 103,034	$ 102,757	$ 80,039
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	166,957	158,634	152,242
Deferred income taxes	(9,068)	57,863	(11,921)
Gain on disposal of property and equipment	(20,533)	(21,290)	(5,907)
Stock-based compensation	4,602	2,599	1,461
Insurance and claims accruals, net of current portion	4,250	8,000	(250)
Other	(237)	(1,073)	592
Changes in certain working capital items:
Accounts receivable, net	7,579	(28,448)	(9,524)
Other current assets	17,581	(21,744)	(1,218)
Accounts payable	(20,172)	19,381	10,839
Other current liabilities	1,103	(12,199)	12,130
Net cash provided by operating activities	255,096	264,480	228,483
Cash flows from investing activities:
Additions to property and equipment	(284,942)	(302,340)	(176,057)
Retirements of property and equipment	60,015	70,142	57,024
Decrease in notes receivable	8,122	7,354	4,699
Net cash used in investing activities	(216,805)	(224,844)	(114,334)
Cash flows from financing activities:
Repayments of short-term debt	(160,000)	(50,000)	(50,000)
Proceeds from issuance of short-term debt	180,000	50,000	50,000
Proceeds from issuance of long-term debt	70,000	0	0
Change in net checks issued in excess of cash balances	(6,671)	6,671	0
Dividends on common stock	(124,391)	(50,969)	(130,676)
Tax withholding related to net share settlements of restricted stock awards	(674)	0	0
Stock options exercised	6,035	2,940	7,980
Excess tax benefits from stock-based compensation	150	605	3,422
Net cash used in financing activities	(35,551)	(40,753)	(119,274)
Effect of exchange rate fluctuations on cash	276	(437)	661
Net increase (decrease) in cash and cash equivalents	3,016	(1,554)	(4,464)
Cash and cash equivalents, beginning of period	12,412	13,966	18,430
Cash and cash equivalents, end of period	15,428	12,412	13,966
Supplemental disclosures of cash flow information:
Interest paid	245	85	47
Income taxes paid	67,968	28,802	56,050
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	10,564	9,172	4,607
Property and equipment acquired included in accounts payable	109	17,026	629
Property and equipment disposed included in other receivables	$ 311	$ 258	$ 573