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Fair Value Measurement (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure, Total $ 16,692 $ 13,143
Net allowance for doubtful accounts 1,400  
Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash bank deposits 8,527 7,762
Gold and silver rounds/bullion 3,650 3,637
Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Receivables from provisional concentrate sales $ 4,515 $ 1,744