Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
CASH FLOWS FROM OPERATING ACTIVITIES
Net income (loss)	$ 18,826	$ 8,525	$ (206,735)
Adjustments to reconcile net income (loss) to net cash provided by operating activities
Depreciation, depletion and amortization	134,277	118,818	92,948
Exploration expense	28,324	31,417	30,853
Stock-based compensation	7,891	9,301	6,216
Impairment of proved oil and gas properties			17,665
Accretion of asset retirement obligations	663	544	391
(Gain) loss on derivative instruments	21,169	(45,365)	52,338
Net cash receipts (payments) on settled derivatives	(26,985)	(2,224)	38,696
(Gain) loss on sale of assets	(1,815)	(179)	6,936
(Gain) loss on early extinguishment of debt			(14,489)
Deferred income taxes			540
Amortization of deferred financing costs	2,256	2,098	1,962
Amortization of debt discount	1,327	1,324	1,362
Changes in operating assets and liabilities:
Accounts receivable	(42,879)	(31,780)	(21,277)
Other assets	(2,192)	1,863	(1,795)
Accounts payable and accrued liabilities	84,231	18,404	794
Net cash provided by operating activities	225,093	112,746	6,405
CASH FLOWS FROM INVESTING ACTIVITIES
Capital expenditures for oil and gas properties	(275,601)	(291,779)	(167,355)
Capital expenditures for other property and equipment	(1,007)	(2,007)	(1,164)
Proceeds from sale of assets	10,358	1,317	79,201
Net cash used in investing activities	(266,250)	(292,469)	(89,318)
CASH FLOWS FROM FINANCING ACTIVITIES
Debt issuance costs	(497)	(1,750)	30
Repayments of long-term debt	(506)	(453)	(24,045)
Proceeds from issuance of common stock			124,361
Proceeds from credit facility	32,500
Equity issuance costs	(344)	(44)	(548)
Employee tax withholding for settlement of equity compensation awards	(1,261)	(2,035)	(61)
Net cash provided by (used in) financing activities	29,892	(4,282)	99,737
Net increase (decrease) in cash and cash equivalents	(11,265)	(184,005)	16,824
Cash and cash equivalents at beginning of period	17,224	201,229	184,405
Cash and cash equivalents at end of period	5,959	17,224	201,229
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION
Cash paid for interest	51,101	47,362	48,483
SUPPLEMENTAL DISCLOSURE OF NON-CASH ACTIVITIES
Asset retirement obligations incurred, including changes in estimate	418	679	1,014
Additions of other property through debt financing	173	183
Additions to oil and natural gas properties - changes in accounts payable, accrued liabilities, and accrued capital expenditures	(15,269)	22,264	$ 8,583
Assets held for sale		$ (262)
Asset acquisition through stock issuance	$ 90,020