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Mineral Properties and Exploration and Evaluation Assets (Northwest Nevada Initiative Property) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of detailed information about property, plant and equipment [line items]    
Balance, January 1 $ 130,863  
Less: write-off 202 $ 1,143
Balance, December 31 133,664 130,863
Northwest Nevada Initiative [member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Balance, January 1 200  
Additions 1,410  
Subtotal carrying amount of property after expenditures 1,610  
Less: charged to exploration expenses (1,301)  
Less: transferred to accounts receivable (109)  
Decreases to property 200  
Less: write-off (200)  
Balance, December 31 $ 200