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Fair Value Measurements (Schedule of Fair Value of Financial Instruments) (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Assets $ 1,055,338 $ 936,065
Liabilities 329,568 300,639
At fair value [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial assets 27,368  
Financial assets, at fair value   33,642
Financial liabilities, at fair value (1,260)  
At fair value [member] | Fair value hedges [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial assets 1,059  
Financial liabilities (1,084)  
At fair value [member] | Financial liabilities at fair value through profit or loss, category [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities, at fair value   (894)
At fair value [member] | Financial liabilities at fair value through profit or loss, category [member] | Fair value hedges [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities   (894)
At fair value [member] | Financial assets at fair value through profit or loss, category [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial assets 26,309 33,642
Financial liabilities, at fair value (176)  
At fair value [member] | Level 1 of fair value hierarchy [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial assets, at fair value 1,059  
At fair value [member] | Level 2 of fair value hierarchy [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial assets, at fair value 26,309 33,642
Financial liabilities, at fair value (1,260)  
At fair value [member] | Level 2 of fair value hierarchy [member] | Financial liabilities at fair value through profit or loss, category [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities, at fair value   (894)
At fair value [member] | Interest rate swap liability [Member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities, at fair value (1,084)  
At fair value [member] | Interest rate swap liability [Member] | Fair value hedges [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities (1,084)  
At fair value [member] | Interest rate swap liability [Member] | Financial liabilities at fair value through profit or loss, category [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities, at fair value   (894)
At fair value [member] | Interest rate swap liability [Member] | Financial liabilities at fair value through profit or loss, category [member] | Fair value hedges [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities   (894)
At fair value [member] | Interest rate swap liability [Member] | Level 2 of fair value hierarchy [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities, at fair value (1,084)  
At fair value [member] | Interest rate swap liability [Member] | Level 2 of fair value hierarchy [member] | Financial liabilities at fair value through profit or loss, category [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities, at fair value   (894)
At fair value [member] | Metal Forward Sales Contracts [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities, at fair value (124)  
At fair value [member] | Metal Forward Sales Contracts [member] | Financial assets at fair value through profit or loss, category [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities, at fair value (124)  
At fair value [member] | Metal Forward Sales Contracts [member] | Level 2 of fair value hierarchy [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities, at fair value (124)  
At fair value [member] | Fuel Forward Contracts Liability [Member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities, at fair value (52)  
At fair value [member] | Fuel Forward Contracts Liability [Member] | Financial assets at fair value through profit or loss, category [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities, at fair value (52)  
At fair value [member] | Fuel Forward Contracts Liability [Member] | Level 2 of fair value hierarchy [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities, at fair value (52)  
At fair value [member] | Investments in Equity Securities [Member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial assets 1,059  
At fair value [member] | Investments in Equity Securities [Member] | Fair value hedges [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial assets 1,059  
At fair value [member] | Investments in Equity Securities [Member] | Level 1 of fair value hierarchy [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial assets, at fair value 1,059  
At fair value [member] | Trade Receivables Concentrate Sales [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial assets 26,309  
Financial assets, at fair value   33,642
At fair value [member] | Trade Receivables Concentrate Sales [member] | Financial assets at fair value through profit or loss, category [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial assets 26,309 33,642
At fair value [member] | Trade Receivables Concentrate Sales [member] | Level 2 of fair value hierarchy [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial assets, at fair value 26,309 33,642
Not measured at fair value in statement of financial position but for which fair value is disclosed [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Assets 136,006 85,823
Liabilities 225,216 227,171
Financial assets   85,823
Financial assets, at fair value 136,006  
Financial liabilities (66,600) (75,561)
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Financial assets at amortised cost, category [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial assets 136,006 85,823
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Financial assets at amortised cost, category [member] | Financial liabilities at amortised cost, category [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities (225,216) (227,171)
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Level 2 of fair value hierarchy [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Liabilities 198,315  
Financial liabilities, at fair value   (153,933)
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Trade Payables [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Liabilities 26,140 37,357
Financial liabilities (26,140) (37,357)
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Trade Payables [member] | Financial assets at amortised cost, category [member] | Financial liabilities at amortised cost, category [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities (26,140) (37,357)
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Payroll Payable [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Liabilities 17,676 15,801
Financial liabilities (17,676) (15,801)
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Payroll Payable [member] | Financial assets at amortised cost, category [member] | Financial liabilities at amortised cost, category [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities (17,676) (15,801)
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Share Units Payable [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Liabilities   5,075
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Share Units Payable [member] | Financial assets at amortised cost, category [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities, at fair value   (5,075)
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Share Units Payable [member] | Level 2 of fair value hierarchy [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities, at fair value   (5,075)
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Credit Facilities [Member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Liabilities 119,850 109,430
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Credit Facilities [Member] | Financial assets at amortised cost, category [member] | Financial liabilities at amortised cost, category [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities (119,850) (109,430)
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Credit Facilities [Member] | Level 2 of fair value hierarchy [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Liabilities 120,000  
Financial liabilities, at fair value   (110,000)
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Convertible Debentures [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Liabilities   37,105
Financial liabilities (38,766)  
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Convertible Debentures [member] | Financial assets at amortised cost, category [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities, at fair value   (37,105)
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Convertible Debentures [member] | Financial assets at amortised cost, category [member] | Financial liabilities at amortised cost, category [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities (38,766)  
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Convertible Debentures [member] | Level 2 of fair value hierarchy [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities (78,315)  
Financial liabilities, at fair value   (38,858)
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Other Payables [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Liabilities 22,784 22,403
Financial liabilities (22,784) (22,403)
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Other Payables [member] | Financial assets at amortised cost, category [member] | Financial liabilities at amortised cost, category [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial liabilities (22,784) (22,403)
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Cash and Cash Equivalent [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Assets 131,898 83,404
Financial assets   83,404
Financial assets, at fair value 131,898  
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Cash and Cash Equivalent [member] | Financial assets at amortised cost, category [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial assets 131,898 83,404
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Other Receivables [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Assets 4,108 2,419
Financial assets   2,419
Financial assets, at fair value 4,108  
Not measured at fair value in statement of financial position but for which fair value is disclosed [member] | Other Receivables [member] | Financial assets at amortised cost, category [member]    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [line items]    
Financial assets $ 4,108 $ 2,419