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Investments (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Equity securities    
Debt Securities, Available-for-sale [Line Items]    
Cost $ 15,223 $ 14,135
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax (1,403) (1,398)
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax 12,055 2,852
Available-for-sale Securities 25,875 15,589
Equity and debt securities    
Debt Securities, Available-for-sale [Line Items]    
Cost   16,406
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax   (1,452)
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax   2,852
Available-for-sale Securities   17,806
Metalla Royalty & Streaming Ltd. | Equity securities    
Debt Securities, Available-for-sale [Line Items]    
Cost 11,803 10,695
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax 0 0
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax 12,055 2,852
Available-for-sale Securities 23,858 13,547
Metalla Royalty & Streaming Ltd. | Debt Securities    
Debt Securities, Available-for-sale [Line Items]    
Cost   2,271
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax   (54)
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax   0
Available-for-sale Securities   2,217
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 (475) (452)
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax 0 0
Available-for-sale Securities 1,589 1,612
Other Investments [Member] | Equity securities    
Debt Securities, Available-for-sale [Line Items]    
Cost 1,356 1,376
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax (928) (946)
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax 0 0
Available-for-sale Securities 428 430
Level 3 | Equity and debt securities    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities   $ 2,217
Available-for-sale Securities | Level 3 | Equity and debt securities    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities $ 0