Condensed Consolidated Statements of Cash Flows (Unaudited) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Cash flow from operating activities:
Net income	$ 319,605	$ 410,699
Adjustments to reconcile net income to cash flow from operating activities:
Depletion, depreciation, and amortization	365,400	390,119
Deferred income taxes	169,634	216,959
Stock-based compensation	23,774	22,662
Noncash fair value derivative adjustments	46,296	(19,757)
Loss on early extinguishment of debt	44,651	0
Amortization of debt issuance costs and discounts	10,581	11,021
Impairment of assets	0	17,515
Other, net	(3,570)	15,087
Changes in assets and liabilities, net of effects from acquisitions:
Accrued production receivable	(34,910)	3,221
Trade and other receivables	(756)	11,010
Other current and long-term assets	(1,199)	8,218
Accounts payable and accrued liabilities	52,672	(30,127)
Oil and natural gas production payable	31,111	5,014
Other liabilities	(11,080)	(35,515)
Net cash provided by operating activities	1,012,209	1,026,126
Cash flow used in investing activities:
Oil and natural gas capital expenditures	(688,270)	(848,618)
Acquisitions of oil and natural gas properties	(1,896)	(155,636)
Bakken exchange transaction	(10,385)	0
CO2 capital expenditures	(72,929)	(93,945)
Pipelines and plants capital expenditures	(136,654)	(231,459)
Purchases of other assets	(29,680)	(18,666)
Net proceeds from sales of oil and natural gas properties and equipment	6,312	33,973
Proceeds from sale of short-term investments	0	83,545
Other	(18,201)	(7,166)
Net cash used in investing activities	(951,703)	(1,237,972)
Cash flow provided by (used in) financing activities:
Bank repayments	(1,170,000)	(970,000)
Bank borrowings	780,000	1,210,000
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)	(17)
Common stock repurchase program	(215,197)	(16,747)
Other	(19,501)	(6,049)
Net cash provided by (used in) financing activities	(132,469)	217,187
Net increase (decrease) in cash and cash equivalents	(71,963)	5,341
Cash and cash equivalents at beginning of period	98,511	18,693
Cash and cash equivalents at end of period	$ 26,548	$ 24,034