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Cost of Sales (Schedule of Cost of Sales) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of detailed information about property, plant and equipment [line items]    
Direct mining costs $ 93,663 $ 78,621
Salaries and benefits 11,299 10,096
Workers' Participation 7,350 5,715
Depletion and depreciation 42,104 32,717
Royalties 4,135 2,500
Cost of sales 158,551 129,649
Caylloma M&I Property [member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Direct mining costs 35,476 32,047
Salaries and benefits 6,013 5,399
Workers' Participation 1,545 973
Depletion and depreciation 9,175 7,958
Royalties 1,283 873
Cost of sales 53,492 47,250
San Jose M&I Property [member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Direct mining costs 58,187 46,574
Salaries and benefits 5,286 4,697
Workers' Participation 5,805 4,742
Depletion and depreciation 32,929 24,759
Royalties 2,852 1,627
Cost of sales $ 105,059 $ 82,399