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Financial Instruments and Risk Management (Gold Option Contracts) (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
$ / oz
oz
Dec. 31, 2016
USD ($)
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)
Gold Options [Member] | Commodity price risk [member]    
Disclosure of detailed information about hedged items [line items]    
Financial assets, at fair value 0.5 3.3
Gains (losses) on change in fair value of derivatives (0.6) (1.2)
Unrealized gains $ 2.6 $ 3.3
Gold Options [Member] | 2017 [member] | Commodity price risk [member]    
Disclosure of detailed information about hedged items [line items]    
Ounces subject to contract | oz 50,100  
Purchase Put Options [Member] | Gold Options [Member] | 2017 [member] | Commodity price risk [member]    
Disclosure of detailed information about hedged items [line items]    
Average realized price per ounce | $ / oz 1,270  
Sold Call Option [Member] | Gold Options [Member] | 2017 [member] | Commodity price risk [member]    
Disclosure of detailed information about hedged items [line items]    
Average realized price per ounce | $ / oz 1,444