Consolidated Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013
Cash flows from operating activities:
Net income	$ 65,941	$ 64,610
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	131,878	127,310
Deferred income taxes	(16,206)	(8,418)
Gain on disposal of property and equipment	(14,267)	(12,701)
Non-cash equity compensation	4,350	3,829
Insurance and claims accruals, net of current portion	(1,075)	2,700
Other	(3,481)	(1,239)
Changes in certain working capital items:
Accounts receivable, net	(32,759)	(25,725)
Other current assets	1,592	568
Accounts payable	24,356	13,899
Other current liabilities	4,882	6,258
Net cash provided by operating activities	165,211	171,091
Cash flows from investing activities:
Additions to property and equipment	(206,354)	(149,889)
Retirements of property and equipment	59,515	43,421
Decrease in notes receivable	11,135	7,095
Other	(3,343)	979
Net cash used in investing activities	(139,047)	(98,394)
Cash flows from financing activities:
Repayments of short-term debt	0	(20,000)
Repayments of long-term debt	(40,000)	(30,000)
Proceeds from issuance of long-term debt	75,000	10,000
Dividends on common stock	(10,853)	(10,961)
Repurchases of common stock	(30,587)	(20,060)
Tax withholding related to net share settlements of restricted stock awards	(96)	(560)
Stock options exercised	4,169	969
Excess tax benefits from equity compensation	295	53
Net cash used in financing activities	(2,072)	(70,559)
Effect of exchange rate fluctuations on cash	(172)	(216)
Net increase in cash and cash equivalents	23,920	1,922
Cash and cash equivalents, beginning of period	23,678	15,428
Cash and cash equivalents, end of period	47,598	17,350
Supplemental disclosures of cash flow information:
Interest paid	304	361
Income taxes paid	59,070	54,423
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	9,427	12,701
Issuance of notes payable	6,233	0
Change in fair value of interest rate swap	(431)	0
Property and equipment acquired included in accounts payable	460	2,696
Property and equipment disposed included in other receivables	$ 351	$ 696