Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Cash Flows from Operating Activities:
Net Income (Loss)	$ (35,943)	$ (6,896)	$ (27,514)	$ (318,730)
Adjustments to reconcile net income to net cash provided by operating activities -
Depreciation, depletion, and amortization	16,054	13,975	45,485	51,130
Write-down of oil and gas properties	0	0	0	355,948
Accretion of asset retirement obligation	77	90	226	263	$ 354
Deferred income taxes			0	21,087
Stock-based compensation expenses			3,450	3,503
Loss (gain) on derivatives			152,879	(66,884)
Cash settlement (paid) received on derivatives			(28,976)	76,150
Settlements of asset retirement obligations			(151)	(27)
Write off of Deferred Debt Issuance Cost			229	459
Other Noncash Income (Expense)			1,883	2,436
(Increase) decrease in accounts receivable and other current assets			(20,941)	7,413
Increase (decrease) in accounts payable and accrued liabilities			7,215	(3,981)
Increase (Decrease) in Income Taxes Payable			0	(480)
Increase (decrease) in accrued interest			(137)	(505)
Net Cash Provided by (Used in) Operating Activities			133,648	127,782
Cash Flows from Investing Activities:
Additions to property and equipment			(98,219)	(102,713)
Acquisition of oil and gas properties			(13,219)	(3,441)
Proceeds from Sale of Property, Plant, and Equipment			0	(4,752)
Payments on property sale obligations			(1,084)	(426)
Net Cash Provided by (Used in) Investing Activities			(112,522)	(101,828)
Cash Flows from Financing Activities:
Proceeds from bank borrowings			195,000	71,000
Payments of bank borrowings			(227,000)	(97,000)
Proceeds from Issuance of Common Stock			12,756	0
Purchase of treasury shares			(612)	(90)
Payments of Debt Issuance Costs			(2,400)	0
Net Cash Provided by (Used in) Financing Activities			(22,256)	(26,090)
Net increase (decrease) in Cash, Cash Equivalents and Restricted Cash			(1,130)	(136)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 988	$ 1,222	988	1,222	$ 2,118	$ 1,358
Supplemental Disclosures of Cash Flows Information:
Cash paid during period for interest, net of amounts capitalized			20,277	22,290
Changes in capital accounts payable and capital accruals			11,393	(25,641)
Non-cash equity consideration for acquisitions			$ (10,023)	$ 0