Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Cash Flows from Operating Activities:
Net Income (Loss)	$ 16,053	$ 8,466
Adjustments to reconcile net income to net cash provided by operating activities -
Depreciation, depletion, and amortization	21,805	13,131
Accretion of asset retirement obligation	83	159	$ 419
Deferred income taxes	134	0
Stock-based compensation expenses	1,692	1,359
Loss (gain) on derivatives	4,022	6,355
Cash settlements on derivatives	1,077	977
Settlements of asset retirement obligations	(31)	(120)
Other Noncash Income (Expense)	564	129
(Increase) decrease in accounts receivable and other current assets	14,401	4,005
Increase (decrease) in accounts payable and accrued liabilities	(9,105)	(9,497)
Increase (decrease) in accrued interest	(100)	192
Net Cash Provided by (Used in) Operating Activities	50,595	25,156
Cash Flows from Investing Activities:
Additions to property and equipment	(86,555)	(33,753)
Proceeds from the sale of property and equipment	(91)	26,969
Payments on property sale obligations	(1,278)	(6,042)
Net Cash Provided by (Used in) Investing Activities	(87,924)	(12,826)
Cash Flows from Financing Activities:
Proceeds from bank borrowings	132,000	35,100
Payments of bank borrowings	(96,000)	(55,100)
Net proceeds from issuances of common stock	0	708
Purchase of treasury shares	(260)	(290)
Payments of debt issuance costs	0	(317)
Net Cash Provided by (Used in) Financing Activities	35,740	(19,899)
Net increase (decrease) in Cash, Cash Equivalents and Restricted Cash	(1,589)	(7,569)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	876	457	$ 2,465	$ 8,026
Supplemental Disclosures of Cash Flows Information:
Cash paid during period for interest, net of amounts capitalized	8,303	5,170
Changes in capital accounts payable and capital accruals	1,487	12,177
Increase (decrease) in other long-term liabilities for capital expenditures	$ 0	$ (1,250)