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Investments (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Debt Securities, Available-for-sale [Abstract]      
Gain (Loss) on Investments $ 2,945 $ 1,356 $ (2,691)
Marketable Securities, Realized Gain (Loss) (7,964) (1,356) 2,691
Unrealized gain (loss) on equity securities (5,019) 0 $ 0
Equity securities      
Debt Securities, Available-for-sale [Line Items]      
Cost 14,135 21,771  
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax (1,398) (881)  
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax 2,852 7,355  
Available-for-sale Securities 15,589 28,245  
Debt Securities      
Debt Securities, Available-for-sale [Line Items]      
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax (53)    
Debt Securities, Available-for-sale, Unrealized Loss Position [Abstract]      
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0    
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value 0    
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 53    
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 2,217    
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 2,217    
Equity and debt securities      
Debt Securities, Available-for-sale [Line Items]      
Cost 16,406 28,448  
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax (1,452) (966)  
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax 2,852 7,355  
Available-for-sale Securities 17,806 34,837  
Metalla Royalty & Streaming Ltd. | Equity securities      
Debt Securities, Available-for-sale [Line Items]      
Cost 10,695 6,294  
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax 0 0  
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax 2,852 1,354  
Available-for-sale Securities 13,547 7,648  
Metalla Royalty & Streaming Ltd. | Debt Securities      
Debt Securities, Available-for-sale [Line Items]      
Cost 2,271 6,677  
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax (54) (85)  
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax 0 0  
Available-for-sale Securities   6,592  
Corvus Gold Inc. [Member] | Equity securities      
Debt Securities, Available-for-sale [Line Items]      
Cost   3,582  
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax   0  
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax   4,518  
Available-for-sale Securities   8,100  
Almaden Minerals, Ltd. [Member] | Equity securities      
Debt Securities, Available-for-sale [Line Items]      
Cost   3,125  
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax   (235)  
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax   0  
Available-for-sale Securities   2,890  
Northern Empire Resources Corp. | Equity securities      
Debt Securities, Available-for-sale [Line Items]      
Cost   4,489  
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax   0  
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax   1,077  
Available-for-sale Securities   5,566  
Rockhaven Resources, Ltd. [Member] | Equity securities      
Debt Securities, Available-for-sale [Line Items]      
Cost 2,064 2,064  
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax (452) (193)  
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax 0 0  
Available-for-sale Securities 1,612 1,871  
Other Investments [Member] | Equity securities      
Debt Securities, Available-for-sale [Line Items]      
Cost 1,376 1,479  
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax (946) (453)  
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax 0 405  
Available-for-sale Securities 430 1,431  
Kootenay Silver, Inc. [Member] | Equity securities      
Debt Securities, Available-for-sale [Line Items]      
Cost   738  
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax   0  
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax   1  
Available-for-sale Securities   739  
Level 3 | Equity and debt securities      
Debt Securities, Available-for-sale [Line Items]      
Available-for-sale Securities   $ 6,891  
Available-for-sale Securities | Level 3 | Equity and debt securities      
Debt Securities, Available-for-sale [Line Items]      
Available-for-sale Securities $ 2,217