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Financial Instruments and Risk Management (Market Price Risk) (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
$ / oz
Disclosure of detailed information about hedged items [line items]    
Unrealized gains $ (0.7) $ 0.6
Market risk [member]    
Disclosure of detailed information about hedged items [line items]    
Potential percentage change in value 1000.00%  
Impact on other comprehensive income $ 4.3 2.1
Gold Options [Member] | Market risk [member]    
Disclosure of detailed information about hedged items [line items]    
Potential percentage change in value 10.00%  
Impact on pre-tax earnings $ 73.8 $ 66.6
Bottom of range [member] | Gold Options [Member] | Market risk [member]    
Disclosure of detailed information about hedged items [line items]    
Average realized price per ounce | $ / oz   1,474
Top of range [member] | Gold Options [Member] | Market risk [member]    
Disclosure of detailed information about hedged items [line items]    
Average realized price per ounce | $ / oz   2,067