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Oil And Natural Gas Properties And Equipment (Details) $ in Thousands |  |  | 9 Months Ended |
--- | --- | --- | --- | ---
Sep. 26, 2018 USD ($) | Sep. 17, 2018 item | Sep. 30, 2018 USD ($) item | Sep. 30, 2017 USD ($) | Sep. 25, 2018 USD ($)
Property Plant And Equipment [Line Items] |  |  |  |  |
Reduction in VAT receivable |  |  | $ 658 | $ 2,783 |
Allocated to proved leasehold cost |  |  | 10,900 |  |
Allocated to unproved leasehold cost |  |  | 6,700 |  |
VAT receivable, net of valuation allowance |  |  | $ 4,200 |  |
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 |  |
Entitled percent for consortium after initial royalty percentage |  |  | 70.00% |  |
Monthly royalty rate, based on production at the published price |  |  | 13.00% |  |
Future entitled percent for consortium after initial royalty percentage |  |  | 80.00% |  |
Period of increased cost recovery percentage |  |  | 10 years |  |
Additional increase to working interest ownership transferred from company, percent |  |  | 0.80% |  |
Office And Miscellaneous Equipment [Member] |  |  |  |  |
Property Plant And Equipment [Line Items] |  |  |  |  |
Estimated useful life |  |  | 5 years |  |
Leasehold Improvements [Member] | Maximum [Member] |  |  |  |  |
Property Plant And Equipment [Line Items] |  |  |  |  |
Estimated useful life |  |  | 7 years |  |
Leasehold Improvements [Member] | Minimum [Member] |  |  |  |  |
Property Plant And Equipment [Line Items] |  |  |  |  |
Estimated useful life |  |  | 5 years |  |
Etame Field [Member] |  |  |  |  |
Property Plant And Equipment [Line Items] |  |  |  |  |
Percent of participating interest |  |  | 33.575% |  |
Number of exploitation areas | item |  | 3 |  |  |
Period of agreement for exploitation areas |  | 10 years |  |  |
Number of contract extension periods | item |  | 2 |  |  |
Production license agreement term extended by government |  | 5 years |  |  |
Gabon [Member] |  |  |  |  |
Property Plant And Equipment [Line Items] |  |  |  |  |
Contractual commitment, gross |  |  |  |  | $ 65,000
Contractual obligation |  |  |  |  | $ 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 |  |
Gabon [Member] | Government Of Gabon [Member] |  |  |  |  |
Property Plant And Equipment [Line Items] |  |  |  |  |
Working interest ownership, percentage |  |  | 7.50% |  |
Additional increase to working interest ownership, percent |  |  | 2.50% |  |

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