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Share-based payments - Additional information (Details)
12 Months Ended
Dec. 31, 2018
USD ($)
Options
Dec. 31, 2017
Options
Option plans    
Number of share options outstanding 2,330,000 2,073,518
Restricted stock units (RSU)    
Option plans    
Number of share options outstanding 813,019 545,378
Number of share options vested 126,067  
Number of share options unvested 686,952  
Weighted average price for share options exercised | $ $ 0  
Employee Stock Option Plan (ESOP)    
Option plans    
Number of share options outstanding 1,516,981 1,528,140
Number of share options vested 1,516,981