Condensed Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 4,532	$ 69,538
Receivables, net	381,634	302,974
Oil and gas well equipment and supplies	66,902	81,029
Deferred income taxes	18,111	8,477
Derivative instruments	7,956
Prepaid expenses	7,120	7,420
Other current assets	286	699
Total current assets	486,541	470,137
Oil and gas properties at cost, using the full cost method of accounting:
Proved properties	12,097,102	11,258,748
Unproved properties and properties under development, not being amortized	567,178	645,078
Gross oil and gas properties	12,664,280	11,903,826
Less - accumulated depreciation, depletion and amortization	(7,166,038)	(6,899,057)
Net oil and gas properties	5,498,242	5,004,769
Fixed assets, net	135,367	152,605
Goodwill	620,232	620,232
Derivative instruments	2,395
Other assets, net	53,593	57,409
Total assets	6,796,370	6,305,152
Current liabilities:
Accounts payable	82,690	103,653
Accrued liabilities	416,111	392,909
Derivative instruments	59
Revenue payable	172,956	149,300
Total current liabilities	671,816	645,862
Long-term debt	892,000	750,000
Deferred income taxes	1,260,836	1,121,353
Other liabilities	292,721	313,201
Total liabilities	3,117,373	2,830,416
Stockholders' equity:
Preferred stock, $0.01 par value, 15,000,000 shares authorized, no shares issued
Common stock, $0.01 par value, 200,000,000 shares authorized, 86,503,109 and 86,595,976 shares issued, respectively	865	866
Paid-in capital	1,948,381	1,939,628
Retained earnings	1,729,178	1,533,768
Accumulated other comprehensive income	573	474
Total stockholders' equity	3,678,997	3,474,736
Total liabilities and stockholders' equity	$ 6,796,370	$ 6,305,152