Statements Of Cash Flows (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash flows from operating activities:
Net income	$ 169,452,000	$ 139,470,000	$ 75,651,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	110,743,000	90,820,000	80,362,000
Loss on sale of property and equipment	78,000	1,263,000	758,000
Deferred income taxes	17,682,000	43,348,000	7,624,000
Changes in assets and liabilities
Customer and other receivables, net	(5,410,000)	(40,414,000)	(36,169,000)
Prepaid expenses and other assets	(7,956,000)	(2,952,000)	(13,314,000)
Accounts payable	2,795,000	12,875,000	(4,247,000)
Compensation, benefits and other accrued liabilities	13,559,000	17,626,000	22,476,000
Claims and insurance accruals	7,458,000	6,696,000	3,831,000
Income taxes, net	9,264,000	3,224,000	(3,613,000)
Other liabilities	10,391,000	5,978,000	7,163,000
Net cash provided by operating activities	328,056,000	277,934,000	140,522,000
Cash flows from investing activities:
Purchase of property and equipment	(373,193,000)	(250,768,000)	(106,347,000)
Proceeds from sale of property and equipment	12,018,000	5,436,000	2,604,000
Net cash used in investing activities	(361,175,000)	(245,332,000)	(103,743,000)
Cash flows from financing activities:
Proceeds from issuance of long-term debt	412,000	96,010,000	0
Principal payments under long-term debt agreements	(40,284,000)	(40,382,000)	(36,681,000)
Net (payments) proceeds on revolving line of credit	10,000,000	(66,230,000)	1,183,000
Proceeds from stock issuance, net of issuance costs	0	48,400,000	0
Other financing activities, net	(2,000)	0	(2,000)
Net cash (used in) provided by financing activities	(29,874,000)	37,798,000	(35,500,000)
(Decrease) increase in cash and cash equivalents	(62,993,000)	70,400,000	1,279,000
Cash and cash equivalents at beginning of year	75,850,000	5,450,000	4,171,000
Cash and cash equivalents at end of year	12,857,000	75,850,000	5,450,000
Income taxes paid	74,932,000	34,579,000	44,893,000
Interest paid	13,728,000	14,011,000	13,561,000
Capitalized interest	1,963,000	895,000	682,000
Supplemental disclosure of noncash investing and financing activities:
Acquisition of property and equipment by capital lease	$ 1,094,000	$ 8,570,000	$ 1,183,000