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Change in Accounting Principle - Other Income Related to the Sale of Exploration Properties (Details) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2017
Dec. 31, 2017
Dec. 31, 2018
Aug. 31, 2016
Change in Accounting Principle            
Accumulated Deficit $ (513,475,000)       $ (511,124,000)  
Gain on sale of assets 39,000 $ 1,115,000        
Trading securities     $ 238,000 $ 238,000    
Available for sale equity unrealized losses       $ 40,000    
Retained earnings and other comprehensive income adjustment   (40,000)        
Trading securities loss 105,000 6,000        
Sale, not discontinued operations | Zacatecas            
Change in Accounting Principle            
Total consideration 1,500,000         $ 1,500,000
Gain on sale of assets $ 115,000          
Sale, not discontinued operations | Zacatecas | ASU 2014-09 Revenue from Contracts with Customers            
Change in Accounting Principle            
Gain on sale of assets   $ 49,000        
Sale, not discontinued operations | Zacatecas | Previously Reported            
Change in Accounting Principle            
Gain on sale of assets     $ 49,000      
Sale, not discontinued operations | Zacatecas | Cumulative Effect Adjustment | ASU 2014-09 Revenue from Contracts with Customers            
Change in Accounting Principle            
Accumulated Deficit         $ (49,000)