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Share-based compensation - Options on USD-denominated listed share (Detail) - US Dollar [Member]
12 Months Ended
Dec. 31, 2020
$ / shares
Dec. 31, 2019
$ / shares
Share-based compensation [Line Items]    
Number of share options outstanding in share-based payment arrangement 387,177  
Option plans [Member]    
Share-based compensation [Line Items]    
Number of share options outstanding in share-based payment arrangement 387,177 928,975
Weighted average exercise price of share options outstanding in share-based payment arrangement $ 23.72 $ 26.26
Number of share options exercised in share-based payment arrangement 478,798  
Weighted average exercise price of share options exercised in share-based payment arrangement $ 27.73  
Number of share options expired in share-based payment arrangement 63,000  
Weighted average exercise price of share options expired in share-based payment arrangement $ 30.16  
Number of share options exercisable in share-based payment arrangement 387,177  
Weighted average exercise price of share options exercisable in share-based payment arrangement $ 23.72