Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash flow from operating activities:
Consolidated net income	$ 525,360	$ 573,333	$ 285,527
Adjustments to reconcile consolidated net income to cash flow from operating activities:
Depletion, depreciation and amortization	507,538	409,196	434,307
Deferred income taxes	255,743	342,463	160,349
Gain on sale of interests in Genesis	0	0	(101,537)
Stock-based compensation	29,310	33,190	35,366
Non-cash fair value derivative adjustments	13,159	(50,008)	(55,445)
Loss on early extinguishment of debt	0	16,131	0
Amortization of debt issuance costs and discounts	14,695	16,954	17,876
Impairment of assets	17,515	22,951	0
Other, net	16,804	(4,302)	(2,144)
Changes in assets and liabilities, net of effects from acquisitions:
Accrued production receivable	36,234	(74,781)	2,426
Trade and other receivables	45,836	(55,470)	24,977
Other current and long-term assets	7,688	(15,817)	(4,119)
Accounts payable and accrued liabilities	5,828	(35,462)	48,549
Oil and natural gas production payable	(23,460)	54,391	15,565
Other liabilities	(41,359)	(27,955)	(5,886)
Net cash provided by operating activities	1,410,891	1,204,814	855,811
Cash flow used for investing activities:
Oil and natural gas capital expenditures	(1,122,615)	(1,082,853)	(671,574)
Acquisitions of oil and natural gas properties	(156,082)	(35,305)	(25,672)
Cash paid in Encore Merger and Riley Ridge acquisitions	0	(199,263)	(947,241)
Cash received in Bakken Exchange Transaction	281,669	0	0
CO2 capital expenditures	(131,043)	(84,789)	(93,556)
Pipelines and plants capital expenditures	(330,417)	(236,133)	(207,536)
Purchases of other assets	(25,765)	(28,838)	(28,684)
Net proceeds from sale of interests in Genesis	0	0	162,619
Net proceeds from sales of oil and natural gas properties and equipment	34,750	69,370	1,458,029
Net proceeds from sale of short-term investments	83,545	0	0
Other	(10,883)	(8,147)	(1,165)
Net cash used for investing activities	(1,376,841)	(1,605,958)	(354,780)
Cash flow provided by (used for) financing activities:
Bank repayments	(1,555,000)	(330,000)	(1,530,000)
Bank borrowings	1,870,000	715,000	1,114,000
Repayment of senior subordinated notes	0	(525,000)	(609,424)
Premium paid on repayment of senior subordinated notes	0	(13,137)	(7,213)
Net proceeds from issuance of senior subordinated notes	0	400,000	1,000,000
Costs of debt financing	(34)	(13,123)	(76,251)
ENP distributions to noncontrolling interest	0	0	(36,738)
Common stock repurchase program	(251,480)	(195,227)	0
Other	(17,718)	(545)	5,873
Net cash provided by (used for) financing activities	45,768	37,968	(139,753)
Net increase (decrease) in cash and cash equivalents	79,818	(363,176)	361,278
Cash and cash equivalents at beginning of year	18,693	381,869	20,591
Cash and cash equivalents at end of year	$ 98,511	$ 18,693	$ 381,869