Statements Of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net income	$ 304,690	$ 267,514	$ 206,113
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	165,343	146,466	127,072
(Gain) loss on sale of property and equipment	(3,592)	(716)	(5,743)
Deferred income taxes	43,642	25,544	32,736
Changes in assets and liabilities
Customer and other receivables, net	(8,672)	(54,443)	(30,063)
Prepaid expenses and other assets	(6,097)	(4,316)	1,910
Accounts payable	21,460	8,526	(8,103)
Compensation, benefits and other accrued liabilities	14,699	13,672	17,714
Claims and insurance accruals	11,549	7,225	6,952
Income taxes, net	11,511	(36,758)	(12,027)
Other liabilities	(653)	18,960	14,105
Net cash provided by operating activities	553,880	391,674	350,666
Cash flows from investing activities:
Purchase of property and equipment	(462,059)	(367,680)	(295,606)
Proceeds from sale of property and equipment	24,442	21,866	11,235
Net cash used in investing activities	(437,617)	(345,814)	(284,371)
Cash flows from financing activities:
Principal payments under long-term debt agreements	(37,778)	(35,715)	(38,978)
Net (payments) proceeds on revolving line of credit	12,317	0	(10,000)
Payments for Repurchase of Common Stock	(114,117)	(5,532)	0
Net cash (used in) provided by financing activities	(139,578)	(41,247)	(48,978)
(Decrease) increase in cash and cash equivalents	(23,315)	4,613	17,317
Cash and cash equivalents at beginning of year	34,787	30,174	12,857
Cash and cash equivalents at end of year	11,472	34,787	30,174
Income taxes paid	130,058	176,221	102,448
Interest paid	8,414	9,710	11,585
Capitalized interest	$ 2,526	$ 2,884	$ 1,731