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Financial Instruments and Risk Management (Gold Option Contracts) (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2020
USD ($)
$ / oz
oz
Dec. 31, 2019
USD ($)
Disclosure of detailed information about hedged items [line items]    
Unrealized gains $ (0.7) $ 0.6
Gold Options [Member] | Commodity price risk [member]    
Disclosure of detailed information about hedged items [line items]    
Financial assets, at fair value   0.7
Gains (losses) on change in fair value of derivatives (8.1) (5.9)
Unrealized gains $ 0.7 $ 0.6
Gold Options [Member] | 2018 [member] | Commodity price risk [member]    
Disclosure of detailed information about hedged items [line items]    
Ounces subject to contract | oz 31,500  
Purchase Put Options [Member] | Gold Options [Member] | 2018 [member] | Commodity price risk [member]    
Disclosure of detailed information about hedged items [line items]    
Average realized price per ounce | $ / oz 1,730  
Sold Call Option [Member] | Gold Options [Member] | 2018 [member] | Commodity price risk [member]    
Disclosure of detailed information about hedged items [line items]    
Average realized price per ounce | $ / oz 2,074