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Oil and Natural Gas Properties and Equipment (Narrative) (Details) |  |  | 3 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Sep. 26, 2018 USD ($) | Sep. 17, 2018 USD ($) item | Jun. 30, 2019 USD ($) | Jun. 30, 2019 USD ($) item | Jun. 30, 2018 USD ($) | Dec. 31, 2017 | Dec. 31, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Reduction in VAT receivable |  |  |  | $ 718,000 | $ (249,000) |  |
Number of drilled wells required by agreement | item |  |  |  | 2 |  |  |
Number of appraisal wells required by agreement | item |  |  |  | 2 |  |  |
Estimated costs of technical studies, net |  |  |  | $ 400,000 |  |  |
Undeveloped Acreage |  |  | $ 23,771,000 | $ 23,771,000 |  |  | $ 23,771,000
Period of development |  |  |  | 25 years |  |  |
Monthly royalty rate, based on production at the published price |  |  |  | 13.00% |  |  |
Asset impairments |  |  | 0 |  |  |  |
Additional increase to working interest ownership, percent |  |  |  | 0.80% |  |  |
Etame Field [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Percent of participating interest |  |  |  | 33.575% |  |  |
Number of exploitation areas | item |  | 3 |  |  |  |  |
Period of agreement for exploitation areas |  | 10 years |  | 10 years |  |  |
Number of contract extension periods | item |  | 2 |  |  |  |  |
Production license agreement term extended by government |  | 5 years |  |  |  |  |
VAT receivable, net of valuation allowance |  |  | 4,200,000 | $ 4,200,000 |  |  |
Allocated to proved leasehold cost |  |  | 22,500,000 | 22,500,000 |  |  |
Allocated to unproved leasehold cost |  |  | 13,700,000 | $ 13,700,000 |  |  |
Number of drilled wells required by agreement | item |  |  |  | 2 |  |  |
Number of appraisal wells required by agreement | item |  |  |  | 2 |  |  |
Estimated costs of wells, gross amount |  |  |  | $ 61,200,000 |  |  |
Estimated costs of wells |  |  |  | 20,500,000 |  |  |
Estimated costs of technical studies |  |  |  | 1,300,000 |  |  |
Deferred tax assets, basis difference in fixed assets |  |  | 18,600,000 | 18,600,000 |  |  |
Deferred tax liabilities, basis difference in fixed assets |  |  | 18,600,000 | 18,600,000 |  |  |
Capitalized costs, signing bonus |  |  | 6,700,000 | 6,700,000 |  |  |
Capitalized costs, tax impact |  |  | 7,100,000 | $ 7,100,000 |  |  |
Block P Offshore Equatorial Guinea [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Working interest ownership, percentage |  |  |  | 31.00% |  |  |
Undeveloped Acreage |  |  | $ 10,000,000.0 | $ 10,000,000.0 |  |  |
Gabon |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Contractual commitment, gross |  | $ 65,000,000.0 |  |  |  |  |
Contractual obligation |  | $ 21,800,000 |  |  |  |  |
Cash paid for signing bonus, gross amount | $ 35,000,000.0 |  |  |  |  |  |
Payment of signing bonus, allocated to proved and unproved property | 11,800,000 |  |  |  |  |  |
Reduction in VAT receivable, gross amount | 25,000,000.0 |  |  |  |  |  |
Reduction in VAT receivable | 8,400,000 |  |  |  |  |  |
Accrued liabilities, end of drilling activities, gross amount | 5,000,000.0 |  |  |  |  |  |
Accrued liabilities, end of drilling activities | $ 1,700,000 |  |  |  |  |  |
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% |  |  |
Monthly royalty rate, based on production at the published price |  |  |  | 13.00% |  |  |
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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