Condensed Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 5,747	$ 5,589
Trade accounts receivable, net	529,333	466,011
Prepaid expenses and other	80,118	82,932
Total current assets	615,198	554,532
Property and equipment, at cost	2,982,697	2,904,706
Less accumulated depreciation	1,041,271	1,019,232
Net property and equipment	1,941,426	1,885,474
Other assets	29,495	24,635
Total assets	2,586,119	2,464,641
Current liabilities:
Current portion of long-term debt	200,000	100,000
Trade accounts payable	374,968	266,722
Claims accruals	49,850	47,442
Accrued payroll	54,485	70,647
Other accrued expenses	47,168	17,199
Deferred income taxes	750	750
Total current liabilities	727,221	502,760
Long-term debt	393,289	585,347
Other long-term liabilities	53,815	53,050
Deferred income taxes	536,022	531,624
Stockholders' equity	875,772	791,860
Total liabilities and stockholders' equity	$ 2,586,119	$ 2,464,641