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Related-party transactions - Summary (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Directors and senior management      
Disclosure of transactions between related parties [line items]      
Compensation selected employees $ (5,232) $ (5,232) $ (7,122)
Amounts payable, related party transactions (15,499) (15,499)  
Joint venture      
Disclosure of transactions between related parties [line items]      
Amounts payable, related party transactions 0 0  
Sales of goods 0 0 456
Services 0 0 210
Interest income $ 0 $ 0 $ 242