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Financial Instruments - Fair Value Hierarchy (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Disclosure of Financial Assets and Liabilities [Line Items]    
Short-term investments $ 111,946 $ 117,776
Derivative financial assets 7,812 1,272
Derivative financial liabilities (367) 0
Financial assets 406,441 404,975
Level 1 [member]    
Disclosure of Financial Assets and Liabilities [Line Items]    
Derivative financial assets 0 0
Derivative financial liabilities 0 0
Financial assets 111,946 117,776
Level 2 [member]    
Disclosure of Financial Assets and Liabilities [Line Items]    
Derivative financial assets 7,812 1,272
Derivative financial liabilities (367) 0
Financial assets 42,529 50,039
Short-term investments | Level 1 [member]    
Disclosure of Financial Assets and Liabilities [Line Items]    
Short-term investments 111,946 117,776
Short-term investments | 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 35,084 48,767
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 $ 35,084 $ 48,767