Condensed Consolidated Statements of Cash Flows (Unaudited) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Cash flow from operating activities:
Net income	$ 217,551	$ 325,332
Adjustments to reconcile net income to cash flow from operating activities:
Depletion, depreciation, and amortization	239,805	253,184
Deferred income taxes	128,342	167,289
Stock-based compensation	15,671	15,249
Noncash fair value derivative adjustments	(33,516)	(87,686)
Loss on early extinguishment of debt	44,651	0
Amortization of debt issuance costs and discounts	7,139	7,347
Impairment of assets	0	17,515
Other, net	5,041	15,835
Changes in assets and liabilities, net of effects from acquisitions:
Accrued production receivable	(6,769)	35,466
Trade and other receivables	3,117	(10,769)
Other current and long-term assets	(9,171)	6,851
Accounts payable and accrued liabilities	86,969	28,256
Oil and natural gas production payable	20,222	(7,985)
Other liabilities	(12,308)	(33,264)
Net cash provided by operating activities	706,744	732,620
Cash flow used in investing activities:
Oil and natural gas capital expenditures	(486,163)	(574,008)
Acquisitions of oil and natural gas properties	(307)	(154,366)
CO2 capital expenditures	(44,708)	(53,313)
Pipelines and plants capital expenditures	(97,480)	(169,675)
Purchases of other assets	(22,825)	(10,748)
Net proceeds from sales of oil and natural gas properties and equipment	5,496	32,302
Proceeds from sale of short-term investments	0	83,545
Other	(19,586)	(2,961)
Net cash used in investing activities	(665,573)	(849,224)
Cash flow provided by (used in) financing activities:
Bank repayments	(970,000)	(400,000)
Bank borrowings	530,000	535,000
Repayment of senior subordinated notes	(651,270)	0
Premium paid on repayment of senior subordinated notes	(36,475)	0
Proceeds from issuance of senior subordinated notes	1,200,000	0
Costs of debt financing	(20,026)	(11)
Common stock repurchase program	(100,423)	0
Other	(15,623)	(8,965)
Net cash provided by (used in) financing activities	(63,817)	126,024
Net increase (decrease) in cash and cash equivalents	(22,646)	9,420
Cash and cash equivalents at beginning of period	98,511	18,693
Cash and cash equivalents at end of period	$ 75,865	$ 28,113