Annual report pursuant to Section 13 and 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 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disaggregation of Revenue [Line Items]                      
Royalties                 $ (15,076) $ (11,558) $ (9,263)
Total revenue, net $ 27,606 $ 25,266 $ 24,426 $ 27,645 $ 17,156 $ 18,178 $ 20,425 $ 21,266 104,943 77,025 59,784
Sales Under COSPA [Member]                      
Disaggregation of Revenue [Line Items]                      
Revenue from customer contracts                 104,891 74,693 59,475
Gabonese Government Share Of Profit Oil and Profit Oil Taken In-Kind [Member]                      
Disaggregation of Revenue [Line Items]                      
Revenue from customer contracts                 2,193 11,638 9,248
Total revenue, net                 9,385    
U.S. Oil And Gas Revenue [Member]                      
Disaggregation of Revenue [Line Items]                      
Revenue from customer contracts                 5 47 $ 324
Carried Interest Recoupment [Member]                      
Disaggregation of Revenue [Line Items]                      
Total revenue, net                 $ 3,545 $ 2,205