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Derivative Assets and Derivative Liabilities (Schedule of Derivative Gain (Loss) Settled and Open Positions) (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
USD ($)
$ / T
Dec. 31, 2016
USD ($)
$ / T
Zinc Commodity [member] | Option and Forward Contracts [member]    
Disclosure of financial assets and liabilities [line items]    
Tonnes settled 7,803  
Average settled price per tonne 2,894  
Settlement gains (losses) | $ $ (1,521)  
Open positions - tonnes 6,500 7,803
Unrealized gains (losses) | $ $ (2,957) $ 973
Zinc Commodity [member] | Option and Forward Contracts [member] | Bottom of range [member]    
Disclosure of financial assets and liabilities [line items]    
Price per tonne 2,500 2,650
Zinc Commodity [member] | Option and Forward Contracts [member] | Top of range [member]    
Disclosure of financial assets and liabilities [line items]    
Price per tonne 3,190 2,750
Lead Commodity [member] | Bottom of range [member]    
Disclosure of financial assets and liabilities [line items]    
Price per tonne 2,100  
Lead Commodity [member] | Top of range [member]    
Disclosure of financial assets and liabilities [line items]    
Price per tonne 2,689  
Lead Commodity [member] | Option and Forward Contracts [member]    
Disclosure of financial assets and liabilities [line items]    
Tonnes settled 4,465  
Average settled price per tonne 2,361  
Settlement gains (losses) | $ $ 19  
Open positions - tonnes 6,000  
Unrealized gains (losses) | $ $ (344)