Dispositions (Summarized Results Of Discontinued Operations) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loss from discontinued operations	$ (52)	$ (176)
Discontinued Operations [Member] | Joint Operating With Republic Of Angola [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
General and administrative expense	32	171
Total operating costs and expenses	32	171
Operating loss	(32)	(171)
Other, net	(20)	(2)
Total other expense	(20)	(2)
Loss from discontinued operations before income taxes	(52)	(173)
Income tax expense		3
Loss from discontinued operations	$ (52)	$ (176)