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Contingencies and Capital Commitments (Narrative) (Details) - USD ($)
 1 Months Ended 12 Months Ended
Oct. 27, 2015
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2011
Dec. 31, 2010
Dec. 11, 2015
Disclosure of other provisions [line items]              
Guarantees with government current year     $ 3,179,000        
Guarantees with government next year     4,990,000        
Guarantees with government in two years     6,928,000        
Tax expense (income)     38,646,000 $ 29,252,000      
Value added tax payables     1,285,000 1,866,000      
MEXICO | Tax contingent liability [member]              
Disclosure of other provisions [line items]              
Tax benefit assessment $ 198,000            
General import tax 30,000            
Value added tax payables 90,000            
Custom management tax 5,000            
Tax fines $ 94,000            
Tax effect security bond             $ 211,000
PERU | Tax contingent liability [member]              
Disclosure of other provisions [line items]              
Tax expense (income)         $ 1,750,000 $ 1,750,000  
Income tax interest and penaltie         573,000 573,000  
Income tax letter bond         $ 838,000 $ 838,000  
Income tax expense, interest and penalties, accrued     0 0      
Lindero Project [member]              
Disclosure of other provisions [line items]              
Capital commitments     5,715,000        
Provision for credit commitments [member]              
Disclosure of other provisions [line items]              
Undiscounted closure costs   $ 9,230,000          
Progressive closure activities   3,646,000          
Final closure activities   4,971,000          
Post closure activities   $ 613,000          
Bank letter of guarantee     $ 4,990,000 $ 3,179,000