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Summary Of Significant Accounting Policies (Rollforward Analysis Of The Allowance Against Accounts Receivable Balance) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Charged to costs and expenses $ (452) $ (1,222) $ (2,699)
Gabon [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Balance at beginning of year (5,211) (4,221) (2,400)
Charged to costs and expenses (452) (1,222) (2,699)
Reclassification related to Sojitz acquisition (694)    
Foreign currency gain (loss) (676) 232 878
Balance at end of period $ (7,033) $ (5,211) $ (4,221)