Condensed Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Cash flows from operating activities:
Net income	$ 78,927	$ 60,956
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	52,068	43,134
Loss on sale of property and equipment	(844)	(783)
Deferred income taxes	(2,338)	15,125
Other operating activities, net	(14,921)	818
Net cash provided by operating activities	144,422	119,180
Cash flows from investing activities:
Purchase of property and equipment	(210,019)	(142,445)
Proceeds from sale of property and equipment	3,098	4,054
Net cash used in investing activities	(206,921)	(138,391)
Cash flows from financing activities:
Proceeds from issuance of long-term debt	412	96,514
Principal payments under long-term debt agreements	(36,778)	(36,259)
Net proceeds (payments) from revolving line of credit	34,030	(66,230)
Proceeds from stock issuance, net of issuance costs	0	48,400
Net cash provided by (used in) financing activities	(2,336)	42,425
(Decrease) increase in cash and cash equivalents	(64,835)	23,214
Cash and cash equivalents at beginning of period	75,850	5,450
Cash and cash equivalents at end of period	11,015	28,664
Supplemental disclosure of noncash investing activities:
Acquisition of property and equipment by capital lease	$ 1,094	$ 0