Unproved Oil and Gas Properties, Full Cost Method (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Excluded from amortization base:
Drilling costs, and other operational related costs	$ 1,242	$ 14,999
Capitalized salary costs	1,579	2,034
Capitalized interest costs	677	346
Legal costs, license fees and other preparation costs	3,216	4,087
Other costs		229
Total unproved oil and gas properties, full cost method	6,714	21,695
Unproved Oil and Gas Properties [Member]
Excluded from amortization base:
Drilling costs, and other operational related costs	27,371
Capitalized salary costs	1,006
Legal costs, license fees and other preparation costs	2,039
Other costs	490
Total unproved oil and gas properties, full cost method	$ 30,906