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Marketable Securities (Schedule of Marketable Securities) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Disclosure of financial assets [line items]    
Current financial assets $ 556 $ 1,579
Derivative Warrants [member] | Medgold Resources Corporation [member]    
Disclosure of financial assets [line items]    
Current financial assets   313
Derivative Warrants [member] | Prospero Silver Corporation [member]    
Disclosure of financial assets [line items]    
Current financial assets 1  
Equity investments [member] | Medgold Resources Corporation [member]    
Disclosure of financial assets [line items]    
Current financial assets   $ 1,266
Equity investments [member] | Prospero Silver Corporation [member]    
Disclosure of financial assets [line items]    
Current financial assets $ 555