Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities:
Net income	$ 267,514	$ 206,113	$ 169,452
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	146,466	127,072	110,743
(Gain) loss on sale of property and equipment	(716)	(5,743)	78
Deferred income taxes	25,544	32,736	17,682
Changes in assets and liabilities
Customer and other receivables, net	(54,443)	(30,063)	(5,410)
Prepaid expenses and other assets	(4,316)	1,910	(7,956)
Accounts payable	8,526	(8,103)	2,795
Compensation, benefits and other accrued liabilities	13,672	17,714	13,559
Claims and insurance accruals	7,225	6,952	7,458
Income taxes, net	(36,758)	(12,027)	9,264
Other liabilities	18,960	14,105	10,391
Net cash provided by operating activities	391,674	350,666	328,056
Cash flows from investing activities:
Purchase of property and equipment	(367,680)	(295,606)	(373,193)
Proceeds from sale of property and equipment	21,866	11,235	12,018
Net cash used in investing activities	(345,814)	(284,371)	(361,175)
Cash flows from financing activities:
Proceeds from issuance of long-term debt	0	0	412
Principal payments under long-term debt agreements	(35,715)	(38,978)	(40,284)
Net (payments) proceeds on revolving line of credit	0	(10,000)	10,000
Payments for Repurchase of Common Stock	(5,532)	0	0
Other financing activities, net	0	0	(2)
Net cash (used in) provided by financing activities	(41,247)	(48,978)	(29,874)
(Decrease) increase in cash and cash equivalents	4,613	17,317	(62,993)
Cash and cash equivalents at beginning of year	30,174	12,857	75,850
Cash and cash equivalents at end of year	34,787	30,174	12,857
Income taxes paid	176,221	102,448	74,932
Interest paid	9,710	11,585	13,728
Capitalized interest	$ 2,884	$ 1,731	$ 1,963