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Income Taxes (Income Tax Years Subject To Examination By Major Tax Jurisdictions) (Details) | 12 Months Ended
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Dec. 31, 2018
United States | Minimum |
Income Tax Examination [Line Items] |
Income tax examination year under examination | 2009
United States | Maximum |
Income Tax Examination [Line Items] |
Income tax examination year under examination | 2018
Gabon | Minimum |
Income Tax Examination [Line Items] |
Income tax examination year under examination | 2014
Gabon | Maximum |
Income Tax Examination [Line Items] |
Income tax examination year under examination | 2018

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Tax year being audited in the income tax examination, in CCYY format. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | us-types:gYearListItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_IncomeTaxAuthorityAxis=country_GA
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |