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 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Disaggregation of Revenue [Line Items]        
Revenue $ 25,266 $ 18,178 $ 77,337 $ 59,869
Glencore Oil Revenue [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from customer contracts 18,931 17,757 74,587 57,913
Gabonese Government Share Of Profit Oil [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from customer contracts 2,749 2,193 9,039
U.S. Oil And Natural Gas Revenue [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from customer contracts   16   46
Gabonese Government Share Of Profit Oil Taken In-Kind [Member]        
Disaggregation of Revenue [Line Items]        
Other items reported in revenue not associated with customer contracts 9,385   9,385  
Carried Interest Recoupment [Member]        
Disaggregation of Revenue [Line Items]        
Other items reported in revenue not associated with customer contracts 573 431 1,929 1,818
Royalties [Member]        
Disaggregation of Revenue [Line Items]        
Other items reported in revenue not associated with customer contracts $ (3,623) $ (2,775) $ (10,757) $ (8,947)