Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Cash Flows from Operating Activities:
Net Income (Loss)	$ 7,080	$ 12,884	$ 17,867	$ 46,834	$ 71,971
Adjustments to reconcile net income to net cash provided by operating activities -
Depreciation, depletion, and amortization	18,766	11,832	44,994	32,375
Accretion of asset retirement obligation	87	582	330	1,722	2,322
Deferred income taxes			549	0
Stock-based compensation expenses			4,240	4,538
Loss (gain) on derivatives			30,707	(14,465)
Cash settlements on derivatives			(5,671)	(2,503)
Settlements of asset retirement obligations			(159)	(2,245)
Debt issuance cost write-down			0	2,401
Other Noncash Income (Expense)			4,114	760
(Increase) decrease in accounts receivable and other current assets			(6,533)	(3,884)
Increase (decrease) in accounts payable and accrued liabilities			(2,612)	1,605
Increase (decrease) in accrued interest			198	(86)
Net Cash Provided by (Used in) Operating Activities			88,024	67,052
Cash Flows from Investing Activities:
Additions to property and equipment			(163,151)	(141,636)
Proceeds from the sale of property and equipment			27,940	653
Payments on property sale obligations			(7,036)	0
Transfer of company funds from restricted cash			(222)	0
Transfer of company funds to restricted cash			0	26
Net Cash Provided by (Used in) Investing Activities			(142,469)	(140,957)
Cash Flows from Financing Activities:
Proceeds from bank borrowings			192,300	349,000
Payments of bank borrowings			(141,300)	(297,000)
Net proceeds from issuances of common stock			709	39,180
Purchase of treasury shares			(418)	(618)
Payments of debt issuance costs			(330)	(4,073)
Net Cash Provided by (Used in) Financing Activities			50,961	86,489
Net increase (decrease) in Cash, Cash Equivalents and Restricted Cash			(3,484)	12,584
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 4,542	$ 13,081	4,542	13,081	$ 8,026	$ 497
Supplemental Disclosures of Cash Flows Information:
Cash paid during period for interest, net of amounts capitalized			17,620	11,390
Changes in capital accounts payable and capital accruals			54,060	9,375
Increase (decrease) in other long-term liabilities for capital expenditures			$ (3,750)	$ 0