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Income Taxes (Narrative) (Details) $ in Thousands | 12 Months Ended
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Dec. 31, 2019 USD ($) item | Dec. 31, 2018 USD ($) | Dec. 31, 2017
Income Taxes [Line Items] |  |  |
Deferred tax assets | $ 108,787 | $ 131,012 |
Valuation allowance | $ 84,628 | $ 90,935 |
Statutory tax rate | 21.00% | 21.00% | 35.00%
Expired foreign tax credits | $ 9,600 |  |
Interest or penalties accrued | $ 0 | $ 0 |
Production license agreement term extended by government | 5 years |  |
Deferred tax assets | $ 24,159 | $ 40,077 |
Prior Production Sharing Contract, Through September 17, 2018 [Member] |  |  |
Income Taxes [Line Items] |  |  |
Entitled percent for consortium after initial royalty percentage | 70.00% |  |
Production Sharing Contract, September 17, 2018 Through September 16, 2028 [Member] |  |  |
Income Taxes [Line Items] |  |  |
Entitled percent for consortium after initial royalty percentage | 80.00% |  |
Production Sharing Contract, After September 16, 2028 [Member] |  |  |
Income Taxes [Line Items] |  |  |
Entitled percent for consortium after initial royalty percentage | 70.00% |  |
United Kingdom Tax Authority [Member] |  |  |
Income Taxes [Line Items] |  |  |
Deferred tax assets | $ 8,700 |  |
Gabon Tax Authority [Member] |  |  |
Income Taxes [Line Items] |  |  |
Deferred tax assets | $ 15,900 |  |
Minimum [Member] |  |  |
Income Taxes [Line Items] |  |  |
Allocation of remaining profit production, government payments, percentage | 50.00% |  |
Number of contract extension periods | item | 2 |  |
Minimum [Member] | Domestic Tax Authority [Member] |  |  |
Income Taxes [Line Items] |  |  |
Foreign tax credit carryforward, expiration dates | Dec. 31, 2035 |  |
Minimum [Member] | Foreign Tax Authority [Member] |  |  |
Income Taxes [Line Items] |  |  |
Foreign tax credit carryforward, expiration dates | Dec. 31, 2020 |  |
Maximum [Member] |  |  |
Income Taxes [Line Items] |  |  |
Allocation of remaining profit production, government payments, percentage | 60.00% |  |
Maximum [Member] | Domestic Tax Authority [Member] |  |  |
Income Taxes [Line Items] |  |  |
Foreign tax credit carryforward, expiration dates | Dec. 31, 2037 |  |
Maximum [Member] | Foreign Tax Authority [Member] |  |  |
Income Taxes [Line Items] |  |  |
Foreign tax credit carryforward, expiration dates | Dec. 31, 2025 |  |

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