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Share-based compensation - Options on USD-denominated listed share (Detail) - US Dollar [Member]
12 Months Ended
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Share-based compensation - Text Details (Detail) [Line Items]    
Number of share options outstanding in share-based payment arrangement 1,633,868  
Option plans [Member]    
Share-based compensation - Text Details (Detail) [Line Items]    
Number of share options outstanding in share-based payment arrangement 1,633,868 3,309,766
Weighted average exercise price of share options outstanding in share-based payment arrangement $ 26.13 $ 28.41
Number of share options exercised in share-based payment arrangement 1,451,964  
Weighted average exercise price of share options exercised in share-based payment arrangement $ 29.91  
Number of share options expired in share-based payment arrangement 223,934  
Weighted average exercise price of share options expired in share-based payment arrangement $ 35.36  
Number of share options exercisable in share-based payment arrangement 1,633,868  
Weighted average exercise price of share options exercisable in share-based payment arrangement $ 26.13