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Significant Judgements in Applying Accounting Policies Disclosure of significant accounting policies (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2009
Dec. 31, 2016
Disclosure of detailed information about property, plant and equipment [line items]      
Number of shares issuable upon conversion of convertible debenture (shares) 363,854    
Deferred credit $ 20,788   $ 20,788
Aquiline [member]      
Disclosure of detailed information about property, plant and equipment [line items]      
Number of shares issuable upon conversion of convertible debenture (shares)   363,854  
Deferred credit 20,800   20,800
Deferred Stripping Costs [member] | Dolores mine [member]      
Disclosure of detailed information about property, plant and equipment [line items]      
Deferred stripping costs $ 47,700   $ 40,300