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Financial Instruments - Fair Value Hierarchy (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Disclosure of Financial Assets and Liabilities [Line Items]    
Short-term investments $ 74,004 $ 51,590
Current derivative financial assets 640 1,092
Current derivative financial liabilities (51) (1,906)
Financial assets 294,875 324,054
Level 1 [member]    
Disclosure of Financial Assets and Liabilities [Line Items]    
Current derivative financial assets 0 0
Current derivative financial liabilities 0 0
Financial assets 74,004 51,590
Level 2 [member]    
Disclosure of Financial Assets and Liabilities [Line Items]    
Current derivative financial assets 640 1,092
Current derivative financial liabilities (51) (1,906)
Financial assets 41,392 51,138
Short-term investments [member] | Level 1 [member]    
Disclosure of Financial Assets and Liabilities [Line Items]    
Short-term investments 74,004 51,590
Short-term investments [member] | Level 2 [member]    
Disclosure of Financial Assets and Liabilities [Line Items]    
Short-term investments 0 0
Trade receivables from provisional concentrate sales [member]    
Disclosure of Financial Assets and Liabilities [Line Items]    
Financial assets 40,803 51,952
Trade receivables from provisional concentrate sales [member] | Level 1 [member]    
Disclosure of Financial Assets and Liabilities [Line Items]    
Financial assets 0 0
Trade receivables from provisional concentrate sales [member] | Level 2 [member]    
Disclosure of Financial Assets and Liabilities [Line Items]    
Financial assets $ 40,803 $ 51,952