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Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 USD ($) / $ | Dec. 31, 2019 USD ($) / $ | Dec. 31, 2018 USD ($) | Jun. 20, 2026
Organization And Accounting Policies [Line Items] |  |  |  |
Receivable balance, gross, noncurrent | $ 13.4 |  |  |
Value added tax receivable, net | $ 4.5 |  |  |
Exchange rate | / $ | 534.8 | 585.7 |  |
Interest costs capitalized | $ 0.0 | $ 0.0 | $ 0.0 |
Gain (loss) on foreign currency transaction | $ (0.2) | $ (0.2) | $ 0.1 |
Office and Miscellaneous Equipment [Member] |  |  |  |
Organization And Accounting Policies [Line Items] |  |  |  |
Estimated useful life of assets | 5 years |  |  |
Leasehold Improvements [Member] | Minimum [Member] |  |  |  |
Organization And Accounting Policies [Line Items] |  |  |  |
Estimated useful life of assets | 5 years |  |  |
Leasehold Improvements [Member] | Maximum [Member] |  |  |  |
Organization And Accounting Policies [Line Items] |  |  |  |
Estimated useful life of assets | 7 years |  |  |
Prior Production Sharing Contract, Through September 17, 2018 [Member] | Government Of Gabon [Member] |  |  |  |
Organization And Accounting Policies [Line Items] |  |  |  |
Monthly royalty rate, based on production at the published price | 13.00% |  |  |
Production Sharing Contract, September 17, 2018 Through September 16, 2028 [Member] | Consortium [Member] |  |  |  |
Organization And Accounting Policies [Line Items] |  |  |  |
Monthly royalty rate, based on production at the published price | 13.00% |  |  |
Etame Marine Block [Member] | Government Of Gabon [Member] |  |  |  |
Organization And Accounting Policies [Line Items] |  |  |  |
Monthly royalty rate, based on production at the published price | 13.00% |  |  |
Working interest ownership, percentage | 7.50% |  |  |
Etame Marine Block [Member] | Prior Production Sharing Contract, Through September 17, 2018 [Member] | Consortium [Member] |  |  |  |
Organization And Accounting Policies [Line Items] |  |  |  |
Working interest ownership, percentage | 7.50% |  |  |
Etame Marine Block [Member] | Prior Production Sharing Contract, Through September 17, 2018 [Member] | Government Of Gabon [Member] |  |  |  |
Organization And Accounting Policies [Line Items] |  |  |  |
Working interest ownership, percentage |  |  | 7.50% |
Etame Marine Block [Member] | Scenario, Forecast [Member] | Government Of Gabon [Member] |  |  |  |
Organization And Accounting Policies [Line Items] |  |  |  |
Working interest ownership, percentage |  |  |  | 10.00%
Etame Marine Block [Member] | Scenario, Forecast [Member] | Production Sharing Contract, September 17, 2018 Through September 16, 2028 [Member] | Government Of Gabon [Member] |  |  |  |
Organization And Accounting Policies [Line Items] |  |  |  |
Working interest ownership, percentage |  |  |  | 10.00%

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