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Long-term investments - Schedule of Long-Term Investments (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Disclosure of financial assets [line items]    
Investments $ 4,155 $ 9,744
Accumulated other comprehensive loss (146,153) (94,294)
Calibre Mining Corp.    
Disclosure of financial assets [line items]    
Investments 1,449 5,526
Accumulated other comprehensive loss (6,395) 2,012
RTG Mining Inc.    
Disclosure of financial assets [line items]    
Investments 1,094 2,588
Accumulated other comprehensive loss (12,306) 1,283
St. Augustine Gold & Copper Ltd.    
Disclosure of financial assets [line items]    
Investments 999 1,626
Accumulated other comprehensive loss (19,194) 91
Libero Copper Corp    
Disclosure of financial assets [line items]    
Investments 610 0
Accumulated other comprehensive loss (22) 0
Goldstone Resources Ltd.    
Disclosure of financial assets [line items]    
Investments 3 4
Accumulated other comprehensive loss (17) 2
Investments in Equity Instruments Designated At Fair Value Through Other Comprehensive Income    
Disclosure of financial assets [line items]    
Accumulated other comprehensive loss (37,934) 3,388
Cost    
Disclosure of financial assets [line items]    
Investments 42,089 41,457
Cost | Calibre Mining Corp.    
Disclosure of financial assets [line items]    
Investments 7,844 7,844
Cost | RTG Mining Inc.    
Disclosure of financial assets [line items]    
Investments 13,400 13,400
Cost | St. Augustine Gold & Copper Ltd.    
Disclosure of financial assets [line items]    
Investments 20,193 20,193
Cost | Libero Copper Corp    
Disclosure of financial assets [line items]    
Investments 632 0
Cost | Goldstone Resources Ltd.    
Disclosure of financial assets [line items]    
Investments $ 20 20
Total Impairment    
Disclosure of financial assets [line items]    
Investments   (35,101)
Total Impairment | Calibre Mining Corp.    
Disclosure of financial assets [line items]    
Investments   (4,330)
Total Impairment | RTG Mining Inc.    
Disclosure of financial assets [line items]    
Investments   (12,095)
Total Impairment | St. Augustine Gold & Copper Ltd.    
Disclosure of financial assets [line items]    
Investments   (18,658)
Total Impairment | Libero Copper Corp    
Disclosure of financial assets [line items]    
Investments   0
Total Impairment | Goldstone Resources Ltd.    
Disclosure of financial assets [line items]    
Investments   $ (18)