Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Net income (loss)	$ 163,152	$ (976,177)	$ (4,385,448)
Adjustments to reconcile net income (loss) to cash flows from operating activities
Depletion, depreciation, and amortization	207,713	846,043	531,660
Write-down of oil and natural gas properties	0	810,921	4,939,600
Impairment of goodwill	0	0	1,261,512
Deferred income taxes	(95,779)	(543,385)	(1,932,179)
Stock-based compensation	15,154	14,995	30,604
Commodity derivatives expense (income)	77,576	127,944	(147,999)
Receipt (payment) on settlements of commodity derivatives	(47,795)	84,181	511,699
Gain on debt extinguishment	0	(115,095)	0
Debt issuance costs and discounts	6,191	17,006	9,121
Other, net	3,112	(2,161)	343
Changes in assets and liabilities, net of effects from acquisitions
Accrued production receivable	(21,398)	(24,290)	81,213
Trade and other receivables	(4,421)	35,923	67,047
Other current and long-term assets	(1,722)	(8,661)	241
Accounts payable and accrued liabilities	(24,710)	(34,240)	(55,234)
Oil and natural gas production payable	(3,997)	(6,752)	(40,833)
Other liabilities	(5,933)	(7,029)	(7,043)
Net cash provided by operating activities	267,143	219,223	864,304
Cash flows from investing activities
Oil and natural gas capital expenditures	(262,867)	(243,027)	(476,398)
Acquisitions of oil and natural gas properties	(88,886)	(1,310)	(21,876)
CO2 capital expenditures	(2,159)	(2,321)	(26,301)
Pipelines and plants capital expenditures	(2,540)	(2,666)	(31,728)
Net proceeds from sales of oil and natural gas properties and equipment	1,696	47,725	563
Other	(2,548)	(3,818)	5,555
Net cash used in investing activities	(357,304)	(205,417)	(550,185)
Cash flows from financing activities
Bank repayments	(1,589,000)	(1,730,500)	(1,862,000)
Bank borrowings	1,763,000	1,856,500	1,642,000
Interest payments on senior secured notes treated as a reduction of debt	(50,349)	(25,835)	0
Repayment or repurchases of senior subordinated notes	(2,503)	(76,708)	(485)
Pipeline financing and capital lease debt repayments	(27,462)	(28,849)	(33,642)
Cash dividends paid	(275)	(486)	(65,426)
Other	(4,798)	(9,134)	(14,907)
Net cash provided by (used in) financing activities	88,613	(15,012)	(334,460)
Net decrease in cash and cash equivalents	(1,548)	(1,206)	(20,341)
Cash and cash equivalents at beginning of year	1,606	2,812	23,153
Cash and cash equivalents at end of year	$ 58	$ 1,606	$ 2,812