Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Joint interest billings and other, allowance	$ 134	$ 138
Office and other equipment, accumulated depreciation	$ 2,490	$ 2,093
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Class A Common Stock
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	28,696,321	27,584,638
Common stock, shares outstanding	28,696,321	27,584,638
Class B Common Stock
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	50,000,000	50,000,000
Common stock, shares issued	35,452,178	36,052,169
Common stock, shares outstanding	35,452,178	36,052,169