Condensed Consolidated Balance Sheets (Unaudited) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Current assets
Cash and cash equivalents	$ 7,892	$ 12,187
Accrued production receivable	286,984	262,047
Trade and other receivables, net	72,664	78,295
Derivative assets	0	5
Deferred tax assets	45,264	52,754
Other current assets	10,780	9,271
Total current assets	423,584	414,559
Oil and natural gas properties (using full cost accounting)
Proved properties	9,126,401	8,945,326
Unevaluated properties	801,314	780,481
CO2 properties	1,126,579	1,117,167
Pipelines and plants	2,217,265	2,209,560
Other property and equipment	463,796	466,969
Less accumulated depletion, depreciation, amortization and impairment	(3,803,378)	(3,668,225)
Net property and equipment	9,931,977	9,851,278
Derivative assets	541	9,942
Goodwill	1,283,590	1,283,590
Other assets	228,266	229,368
Total assets	11,867,958	11,788,737
Current liabilities
Accounts payable and accrued liabilities	329,103	410,543
Oil and gas production payable	178,593	174,677
Derivative liabilities	89,185	53,822
Current maturities of long-term debt	36,383	36,157
Total current liabilities	633,264	675,199
Long-term liabilities
Long-term debt, net of current portion	3,512,041	3,260,625
Asset retirement obligations	116,524	119,888
Derivative liabilities	8,144	3,413
Deferred tax liabilities	2,422,004	2,399,294
Other liabilities	28,073	28,912
Total long-term liabilities	6,086,786	5,812,132
Commitments and contingencies (Note 7)
Stockholders' equity
Preferred stock, $.001 par value, 25,000,000 shares authorized, none issued and outstanding	0	0
Common stock, $.001 par value, 600,000,000 shares authorized; 411,058,201 and 409,215,573 issued, respectively	411	409
Paid-in capital in excess of par	3,199,379	3,186,714
Retained earnings	2,880,911	2,844,432
Accumulated other comprehensive loss	(261)	(276)
Treasury stock, at cost, 59,236,568 and 46,710,896 shares, respectively	(932,532)	(729,873)
Total stockholders' equity	5,147,908	5,301,406
Total liabilities and stockholders' equity	$ 11,867,958	$ 11,788,737