Annual report pursuant to Section 13 and 15(d)

Oil And Natural Gas Properties And Equipment (Narrative) (Details)

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Oil And Natural Gas Properties And Equipment (Narrative) (Details)
$ in Thousands
 12 Months Ended
Sep. 26, 2018
USD ($)
Dec. 31, 2018
USD ($)
item
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Sep. 30, 2018
USD ($)
Sep. 25, 2018
USD ($)
Jun. 30, 2016
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Contractual obligation             $ 5,100
Reduction in VAT receivable   $ (777) $ (3,025) $ (1,937)      
VAT receivable, net of valuation allowance   4,200          
Allocated to proved leasehold cost   22,500          
Allocated to unproved leasehold cost   $ 13,700          
Number of drilled wells required by agreement | item   2          
Number of appraisal wells required by agreement | item   2          
Estimated costs of wells, gross amount   $ 61,200          
Estimated costs of wells   20,500          
Estimated costs of technical studies   1,300          
Estimated costs of technical studies, net   $ 400          
Future working interest of carried partner percentage   10.00%          
Monthly royalty rate, based on production at the published price   13.00%          
Undeveloped Acreage   $ 23,771 10,000   $ 23,771    
Additional increase to working interest ownership, percent   0.80%          
Inventory write-off   $ 400          
Block P Offshore Equatorial Guinea [Member]              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Undeveloped Acreage   $ 10,000          
Working interest ownership, percentage   31.00%          
Period of development   25 years          
Etame Field [Member]              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Percent of participating interest   33.575%          
Period of agreement for exploitation areas   10 years          
Number of exploitation areas | item   3          
Number of contract extension periods | item   2          
Production license agreement term extended by government   5 years          
Gabon              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Contractual commitment, gross           $ 65,000  
Contractual obligation   $ 1,100 $ 1,200 $ 1,700   $ 21,800  
Cash paid for signing bonus, gross amount $ 35,000            
Payment of signing bonus, allocated to proved and unproved property 11,800            
Reduction in VAT receivable, gross amount 25,000            
Reduction in VAT receivable $ 8,400            
Accrued liabilities, end of drilling activities, gross amount   5,000          
Accrued liabilities, end of drilling activities   $ 1,700          
Working interest ownership, percentage     7.50%        
Additional increase to working interest ownership, percent   2.50%          
Prior Production Sharing Contract, Through September 17, 2018 [Member]              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Entitled percent for consortium after initial royalty percentage   70.00%          
Production Sharing Contract, September 17, 2018 Through September 16, 2028 [Member]              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Entitled percent for consortium after initial royalty percentage   80.00%          
Production Sharing Contract, After September 16, 2028 [Member]              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Entitled percent for consortium after initial royalty percentage   70.00%