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Share-based compensation - Options on EUR-denominated listed share (Detail) - Euro [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 491,914  
Option plans [Member]    
Share-based compensation [Line Items]    
Number of share options outstanding in share-based payment arrangement 491,914 934,707
Weighted average exercise price of share options outstanding in share-based payment arrangement € 17.10 € 19.14
Number of share options exercised in share-based payment arrangement 408,727  
Weighted average exercise price of share options exercised in share-based payment arrangement € 21.20  
Number of share options expired in share-based payment arrangement 34,066  
Weighted average exercise price of share options expired in share-based payment arrangement € 23.94  
Number of share options exercisable in share-based payment arrangement 491,914  
Weighted average exercise price of share options exercisable in share-based payment arrangement € 17.10