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Embedded Derivatives (Details)
 6 Months Ended
Jun. 30, 2018
oz
t
$ / oz
$ / t
Gold  
Embedded Derivative [Line Items]  
Under contract | oz 8,928
Average forward price | $ / oz 1,318
Silver  
Embedded Derivative [Line Items]  
Under contract | oz 891,195
Average forward price | $ / oz 16.61
Copper  
Embedded Derivative [Line Items]  
Under contract | t 740
Average forward price | $ / t 6,934
Lead  
Embedded Derivative [Line Items]  
Under contract | t 2,941
Average forward price | $ / t 2,440
Zinc  
Embedded Derivative [Line Items]  
Under contract | t 7,959
Average forward price | $ / t 3,263