Quarterly report pursuant to Section 13 or 15(d)

Revenue (Revenues From Contracts With Customers) (Details)

v3.8.0.1
Revenue (Revenues From Contracts With Customers) (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Disaggregation of Revenue [Line Items]    
Revenue $ 27,645 $ 21,266
Glencore Oil Revenue [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from customer contracts 28,463 20,344
Gabonese Government Share Of Profit Oil [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from customer contracts 2,193 3,194
U.S. Oil And Gas Revenue [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from customer contracts   20
Carried Interest Recoupment [Member]    
Disaggregation of Revenue [Line Items]    
Other items reported in revenue not associated with customer contracts 651 817
Royalties [Member]    
Disaggregation of Revenue [Line Items]    
Other items reported in revenue not associated with customer contracts $ (3,662) $ (3,109)