Condensed Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Cash flows from operating activities:
Net income	$ 40,553	$ 31,095
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	29,834	25,544
Loss on sale of property and equipment	(47)	(474)
Deferred income taxes	(3,736)	(13,743)
Other operating activities, net	3,164	(34,957)
Net cash provided by operating activities	63,534	78,327
Cash flows from investing activities:
Purchase of property and equipment	(26,949)	(89,393)
Proceeds from sale of property and equipment	815	628
Net cash used in investing activities	(26,134)	(88,765)
Cash flows from financing activities:
Proceeds from issuance of long-term debt	0	6
Principal payments under long-term debt agreements	(11,074)	(11,199)
Net proceeds (payments) from revolving line of credit	(10,000)	0
Net cash provided by financing activities	(21,074)	(11,193)
(Decrease) increase in cash and cash equivalents	16,326	(21,631)
Cash and cash equivalents at beginning of period	12,857	75,850
Cash and cash equivalents at end of period	29,183	54,219
Supplemental disclosure of noncash investing activities:
Acquisition of property and equipment by capital lease	$ 0	$ 211