Summary of Significant Accounting Policies (Details Textual) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Apr. 12, 2019	Jan. 01, 2019	Dec. 31, 2018	Apr. 20, 2018
Summary of Significant Accounting Policies
Total capitalized internal costs	$ 1,600	$ 1,400
Proved oil and gas properties	1,005,988				$ 925,865
Unproved oil and gas properties	64,797				56,715
Furniture, fixtures, and other equipment	3,668				3,520
Less - Accumulated depreciation, depletion, and amortization	(306,609)				(284,804)
Net Furniture, Fixtures and other equipment	767,844				701,296
Write-down of oil and gas properties	0	0
Allowance for doubtful accounts receivable, current	100				100
Accounts receivable, gross	21,400				36,900
Accounts receivable related to joint interest owners	4,500				5,600
Severance tax receivable	3,500				2,400
Other receivables	$ 1,900				1,600
Percentage of working interest in wells	100.00%
Total amount of supervision fees charged to wells	$ 1,300	1,100
Income Tax Expense (Benefit)	232	0
Oil and gas sales	72,064	$ 52,752
Trade accounts payable	20,728				32,683
Accrued operating expenses	3,258				3,549
Accrued payroll costs	2,066				4,785
Asset retirement obligation - current portion	284				302
Accrued non-income based taxes	2,533				3,583
Accrued corporate and legal fees	335				534
Other payables	2,774				3,485
Accounts payable and accrued liabilities	$ 31,978				48,921
Purchase of treasury stock (shares)	11,076	10,458
Operating Lease, Right-of-Use Asset	$ 2,024			$ 2,200	$ 0
Minimum [Member]
Summary of Significant Accounting Policies
Estimated useful lives of property	2 years
Maximum [Member]
Summary of Significant Accounting Policies
Estimated useful lives of property	20 years
Line of Credit [Member] | New Credit Facility [Member]
Summary of Significant Accounting Policies
Line of Credit Facility, Current Borrowing Capacity						$ 330,000
Line of Credit [Member] | Subsequent Event [Member] | New Credit Facility [Member]
Summary of Significant Accounting Policies
Line of Credit Facility, Current Borrowing Capacity			$ 410,000
Oil sales [Member]
Summary of Significant Accounting Policies
Oil and gas sales	$ 14,607	$ 11,439
Natural gas sales [Member]
Summary of Significant Accounting Policies
Oil and gas sales	51,304	35,753
NGL sales [Member]
Summary of Significant Accounting Policies
Oil and gas sales	$ 6,154	$ 5,560