Condensed Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 5,923	$ 5,589
Trade accounts receivable, net	579,643	466,011
Prepaid expenses and other	65,996	82,932
Total current assets	651,562	554,532
Property and equipment, at cost	3,053,616	2,904,706
Less accumulated depreciation	1,077,390	1,019,232
Net property and equipment	1,976,226	1,885,474
Other assets	28,528	24,635
Total assets	2,656,316	2,464,641
Current liabilities:
Current portion of long-term debt	200,000	100,000
Trade accounts payable	364,195	266,722
Claims accruals	56,608	47,442
Accrued payroll	61,267	70,647
Other accrued expenses	16,286	17,199
Deferred income taxes	750	750
Total current liabilities	699,106	502,760
Long-term debt	474,132	585,347
Other long-term liabilities	52,688	53,050
Deferred income taxes	528,151	531,624
Stockholders' equity	902,239	791,860
Total liabilities and stockholders' equity	$ 2,656,316	$ 2,464,641