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Share-based payments - Summary of Share Based Payments Expense (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Share based payments $ 6,989 $ 11,218
Employee stock option plan [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Share based payments 3,392 6,736
Restricted Shares Without Performance Criteria [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Share based payments 1,425 2,716
Restricted Shares With Performance Criteria [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Share based payments 175 0
Deferred share units plans [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Share based payments (277) (1,023)
Performance share unit [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Share based payments $ 2,274 $ 2,789