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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% |  |  |  |  |

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