Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
CASH FLOWS FROM OPERATING ACTIVITIES
Net income (loss)	$ (21,660)	$ 38,341
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities
Depreciation, depletion and amortization	63,916	51,341
Exploration expense	13,763	9,192
Stock-based compensation	3,960	4,429
Accretion of asset retirement obligations	317	252
(Gain) loss on derivative instruments	20,792	(43,274)
Net cash receipts (payments) on settled derivatives	(2,347)	(6,633)
(Gain) loss on sale of assets	(1,820)	1
Amortization of deferred financing costs	1,114	1,035
Amortization of debt discount	663	661
Changes in operating assets and liabilities:
Accounts receivable	(35,420)	3,992
Other assets	(1,397)	308
Accounts payable and accrued liabilities	8,956	5,796
Net cash provided by operating activities	50,837	65,441
CASH FLOWS FROM INVESTING ACTIVITIES
Capital expenditures for oil and gas properties	(124,045)	(166,012)
Capital expenditures for other property and equipment	(768)	(537)
Proceeds from sale of assets	10,348	544
Net cash used in investing activities	(114,465)	(166,005)
CASH FLOWS FROM FINANCING ACTIVITIES
Debt issuance costs	(56)	(1,296)
Repayments of long-term debt	(181)	(222)
Proceeds from credit facility	60,000
Equity issuance costs	(170)	(40)
Employee tax withholding for settlement of equity compensation awards	(1,140)	(2,032)
Net cash provided by (used in) financing activities	58,453	(3,590)
Net increase (decrease) in cash and cash equivalents	(5,175)	(104,154)
Cash and cash equivalents at beginning of period	17,224	201,229
Cash and cash equivalents at end of period	12,049	97,075
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION
Cash paid for interest	24,809	23,502
SUPPLEMENTAL DISCLOSURE OF NON-CASH ACTIVITIES
Asset retirement obligations incurred, including changes in estimate	208	540
Additions of other property through debt financing	174
Additions to oil and natural gas properties - changes in accounts payable, accrued liabilities, and accrued capital expenditures	26,257	$ 11,674
Asset acquisition through stock issuance	$ 90,020