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Equity-settled share-based payments - Narrative (Details) - USD ($)
$ in Thousands
 1 Months Ended 12 Months Ended
May 31, 2014
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2017
Dec. 31, 2004
Disclosure of terms and conditions of share-based payment arrangement [line items]            
Term of options granted   10 years        
Share-based compensation expense   $ 6,406 $ 4,316 $ 4,734    
Stock Options, 2004 Stock Incentive Option Plan            
Disclosure of terms and conditions of share-based payment arrangement [line items]            
Maximum term of options           10 years
Extended term on options 10 years          
Restricted Stock and Restricted Stock Units            
Disclosure of terms and conditions of share-based payment arrangement [line items]            
Vesting period   3 years        
Vesting rate (as a percent)   33.00%        
Maximum stock authorized (shares)   5,379,164        
Stock already granted (shares)   3,809,322        
Stock vested in future periods (shares)   1,396,221        
Restricted Stock Units            
Disclosure of terms and conditions of share-based payment arrangement [line items]            
Share-based compensation expense   $ 6,600 $ 4,400   $ 4,700