Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Cash Flows from Operating Activities:
Net Income (Loss)	$ 2,319	$ 16,241	$ 10,787	$ 33,951	$ 71,971
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities
Adjustments to reconcile net income to net cash provided by operating activities -
Depreciation, depletion, and amortization	13,096	10,828	26,228	20,543
Accretion of asset retirement obligation	84	576	243	1,140	2,322
Deferred income taxes			(328)	0
Stock-based compensation expenses			2,675	3,136
Loss (gain) on derivatives			17,107	(16,068)
Cash settlements on derivatives			(1,935)	(2,586)
Settlements of asset retirement obligations			(144)	(1,894)
Debt issuance cost write-down			0	2,401
Other Noncash Income (Expense)			3,374	482
(Increase) decrease in accounts receivable and other current assets			2,332	(1,486)
Increase (decrease) in accounts payable and accrued liabilities			(8,439)	4,437
Increase (decrease) in accrued interest			491	(90)
Net Cash Provided by (Used in) Operating Activities			53,047	43,966
Cash Flows from Investing Activities:
Additions to property and equipment			(84,097)	(85,655)
Proceeds from the sale of property and equipment			26,924	460
Payments on property sale obligations			(6,042)	0
Transfer of company funds from restricted cash			0	15
Net Cash Provided by (Used in) Investing Activities			(63,215)	(85,210)
Cash Flows from Financing Activities:
Proceeds from bank borrowings			122,300	300,000
Payments of bank borrowings			(113,300)	(287,000)
Net proceeds from issuances of common stock			708	39,244
Purchase of treasury shares			(418)	(618)
Payments of debt issuance costs			(317)	(4,073)
Net Cash Provided by (Used in) Financing Activities			8,973	47,553
Net increase (decrease) in Cash, Cash Equivalents and Restricted Cash			(1,195)	6,309
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 6,831	$ 6,806	6,831	6,806	$ 8,026	$ 497
Supplemental Disclosures of Cash Flows Information:
Cash paid during period for interest, net of amounts capitalized			10,926	8,847
Changes in capital accounts payable and capital accruals			35,299	5,356
Increase (decrease) in other long-term liabilities for capital expenditures			$ (2,500)	$ 0