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Prepaid Sales (Details)
$ in Thousands
 1 Months Ended 12 Months Ended
Mar. 31, 2016
USD ($)
oz
Dec. 31, 2018
USD ($)
oz
Dec. 31, 2017
USD ($)
oz
Disclosure of disaggregation of revenue from contracts with customers [line items]      
Proceeds from prepaid sales   $ 0 $ 30,000
Deferred revenue earned   $ 60,000 $ 60,000
Gold and Silver Ores      
Disclosure of disaggregation of revenue from contracts with customers [line items]      
Prepaid sales volume delivered | oz   51,633 51,633
Deferred revenue earned   $ 60,000 $ 60,000
2016 Gold Delivery Contracts | Gold and Silver Ores      
Disclosure of disaggregation of revenue from contracts with customers [line items]      
Proceeds from prepaid sales $ 120,000    
Prepaid sales contract volume | oz 103,266    
2017 Gold Delivery Contract | Gold and Silver Ores      
Disclosure of disaggregation of revenue from contracts with customers [line items]      
Proceeds from prepaid sales     $ 30,000
Prepaid sales contract volume | oz     25,282
2016 and 2017 Gold Delivery Contracts | Gold and Silver Ores      
Disclosure of disaggregation of revenue from contracts with customers [line items]      
Prepaid sales contract volume | oz   25,282  
Deferred income   $ 30,000