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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, 2020
Dec. 31, 2019
Dec. 31, 2018
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   $ 4,316 $ 4,734 $ 4,728  
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   $ 4,500 $ 4,800 $ 4,900