Quarterly report pursuant to Section 13 or 15(d)

Revenue (Revenues From Contracts With Customers) (Details)

v3.10.0.1
Revenue (Revenues From Contracts With Customers) (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Disaggregation of Revenue [Line Items]        
Revenue $ 24,426 $ 20,425 $ 52,071 $ 41,691
Glencore Oil Revenue [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from customer contracts 27,193 19,812 55,656 40,156
Gabonese Government Share Of Profit Oil [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from customer contracts 3,096 2,193 6,290
U.S. Oil And Gas Revenue [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from customer contracts   10   30
Carried Interest Recoupment [Member]        
Disaggregation of Revenue [Line Items]        
Other items reported in revenue not associated with customer contracts 705 570 1,356 1,387
Royalties [Member]        
Disaggregation of Revenue [Line Items]        
Other items reported in revenue not associated with customer contracts $ (3,472) $ (3,063) $ (7,134) $ (6,172)