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Financial Instruments and Risk Management (Market Price Risk) (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
$ / oz
Disclosure of detailed information about hedged items [line items]    
Financial assets, at fair value $ 0.4  
Gains (losses) on change in fair value of derivatives 2.3 $ 1.6
Unrealized gains $ 0.3 (1.6)
Market risk [member]    
Disclosure of detailed information about hedged items [line items]    
Potential percentage change in value 1000.00%  
Impact on other comprehensive income $ 3.5 1.4
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 $ 54.3 $ 48.2
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   1,146
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   1,349