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Mineral Properties, Utah (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Property, Plant and Equipment [Line Items]      
Gain on disposal of mineral property, net $ 358 $ 150 $ 1,958
Utah Sale [Member] | Sold      
Property, Plant and Equipment [Line Items]      
Proceeds from sale of gold project   $ 150  
Net smelter return royalty (as a percent)   2.00%  
Gain on disposal of mineral property, net   $ 150