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)	$ 8,525	$ (206,735)	$ (961,791)
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities
Depreciation, depletion and amortization	118,818	92,948	244,750
Exploration expense	31,417	30,853	95,849
Pension benefit costs			259
Stock-based compensation	9,301	6,216	4,635
Impairment of proved oil and gas properties		17,665	691,334
Accretion of asset retirement obligations	544	391	1,623
(Gain) loss on derivative instruments	(45,365)	52,338	(56,021)
Net cash receipts (payments) on settled derivatives	(2,224)	38,696	37,074
(Gain) loss on sale of assets	(179)	6,936	(4,737)
(Gain) loss on sale of business			(400)
(Gain) loss on early extinguishment of debt		(14,489)	59,392
Deferred income taxes		540	(74,481)
Interest not paid in cash			1,232
Amortization of deferred financing costs	2,098	1,962	1,991
Amortization of debt discount	1,324	1,362	1,886
Changes in operating assets and liabilities:
Accounts receivable	(31,780)	(21,277)	20,209
Other assets	1,863	(1,795)	445
Accounts payable and accrued liabilities	18,404	794	17,050
Net cash provided by operating activities	112,746	6,405	80,299
CASH FLOWS FROM INVESTING ACTIVITIES
Capital expenditures for oil and gas properties	(291,779)	(167,355)	(475,659)
Capital expenditures for other property and equipment	(2,007)	(1,164)	(1,748)
Proceeds from sale of assets	1,317	79,201	40,139
Net cash used in investing activities	(292,469)	(89,318)	(437,268)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of long-term debt			538,466
Debt issuance costs	(1,750)	30	(13,362)
Repayments of long-term debt	(453)	(24,045)	(485,317)
Proceeds from issuance of common stock		124,361	440,000
Equity issuance costs	(44)	(548)	(5,930)
Employee tax withholding for settlement of equity compensation awards	(2,035)	(61)
Net cash provided by (used in) financing activities	(4,282)	99,737	473,857
Net decrease in cash and cash equivalents	(184,005)	16,824	116,888
Cash and cash equivalents at beginning of period	201,229	184,405	67,517
Cash and cash equivalents at end of period	17,224	201,229	184,405
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION
Cash paid for interest	47,362	48,483	39,808
Cash paid for income taxes			505
SUPPLEMENTAL DISCLOSURE OF NON-CASH ACTIVITIES
Asset retirement obligations incurred, including changes in estimate	679	1,014	3,435
Additions of other property through debt financing	183		888
Additions to oil and natural gas properties - changes in accounts payable, accrued liabilities, and accrued capital expenditures	22,264	$ 8,583	(142,665)
Assets held for sale	$ (262)		(35,289)
Interest paid-in-kind			$ 14,786