Condensed Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Cash flows from operating activities:
Net income	$ 98,808	$ 78,927
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	60,351	52,068
(Gain) loss on sale of property and equipment	(2,332)	844
Deferred income taxes	2,067	(2,338)
Other operating activities, net	(5,030)	14,921
Net cash provided by operating activities	153,864	144,422
Cash flows from investing activities:
Purchase of property and equipment	(156,354)	(210,019)
Proceeds from sale of property and equipment	8,613	3,098
Net cash used in investing activities	(147,741)	(206,921)
Cash flows from financing activities:
Proceeds from issuance of long-term debt	0	412
Principal payments under long-term debt agreements	(36,290)	(36,778)
Net proceeds (payments) from revolving line of credit	24,680	34,030
Net cash provided by (used in) financing activities	(11,610)	(2,336)
(Decrease) increase in cash and cash equivalents	(5,487)	(64,835)
Cash and cash equivalents at beginning of period	12,857	75,850
Cash and cash equivalents at end of period	7,370	11,015
Supplemental disclosure of noncash investing activities:
Acquisition of property and equipment by capital lease	$ 0	$ 1,094