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Income taxes (Components of Deferred Income Taxes not Recognized in Repect to Deductible Temporary Differences) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Total deferred income taxes not recognized from temporary differences and unused tax losses as future utilization is not considered probable $ 140,804 $ 141,558
Temporary Differences - Property, Plant and Equipment [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred income taxes not recognized from temporary differences as future utilization is not considered probable 4,890 3,272
Mexican Tax Losses (expiring in 2018 - 2027) [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred income taxes not recognized from tax losses as future utilization is not considered probable 42,843 52,702
Canadien Tax Losses (expiring in 2029 - 2037) [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred income taxes not recognized from tax losses as future utilization is not considered probable 35,173 30,268
U.S. Tax Losses (expiring in 2020 - 2037) [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred income taxes not recognized from tax losses as future utilization is not considered probable 32,070 29,764
Provisions and Other [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred income taxes not recognized from temporary differences as future utilization is not considered probable 24,956 24,718
Deferred Mexican Mining Royalty [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred income taxes not recognized from temporary differences as future utilization is not considered probable $ 872 $ 834