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Non-controlling interests - Additional Information (Details) - USD ($)
 12 Months Ended
May 11, 2020
Dec. 31, 2020
Dec. 31, 2019
Disclosure of subsidiaries [line items]      
Payments from changes in ownership interests in subsidiaries that do not result in loss of control $ 7,500,000 $ 7,500,000 $ 0
Carrying amount of NCI   40,873,000 59,304,000
Non-controlling interests   (7,180,000) (6,901,000)
Contributed surplus      
Disclosure of subsidiaries [line items]      
Acquisition of non-controlling interest, without change in control   $ 4,172,000 $ 0
Hellas Gold SA ("Hellas")      
Disclosure of subsidiaries [line items]      
NCI percentage 5.00% 0.00% 5.00%
Carrying amount of NCI   $ 0 $ 13,362,000
Non-controlling interests   (1,691,000) (5,388,000)
Non-Material Subsidiaries      
Disclosure of subsidiaries [line items]      
Carrying amount of NCI   3,353,000 3,039,000
Non-controlling interests   $ (106,000) $ (247,000)