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Fair Value Measurement (Details) - USD ($)
$ in Thousands
 6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure, Total $ 32,036 $ 29,086
Fair value of non-current investment in equity securities 200  
Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash bank deposits 26,645 22,390
Gold and silver rounds/bullion 3,664 3,812
Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Receivables from provisional concentrate sales $ 1,727 $ 2,884