Unproved Oil and Gas Properties, Full Cost Method (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Exclusive of non-cash amounts	$ 3,625	$ 4,478	$ 238