Income Taxes (Income Tax Years Subject To Examination By Major Tax Jurisdictions) (Details)	12 Months Ended
	Dec. 31, 2016
United States [Member] | Minimum [Member]
Income Tax Examination [Line Items]
Income tax examination year under examination	2009
United States [Member] | Maximum [Member]
Income Tax Examination [Line Items]
Income tax examination year under examination	2016
Gabon [Member] | Minimum [Member]
Income Tax Examination [Line Items]
Income tax examination year under examination	2007
Gabon [Member] | Maximum [Member]
Income Tax Examination [Line Items]
Income tax examination year under examination	2016