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Share-based compensation - Accelerate! options (Detail)
12 Months Ended
Dec. 31, 2018
EUR (€)
Jan. 01, 2018
EUR (€)
Euro [Member]    
Share-based compensation - Text Details (Detail) [Line Items]    
Number of share options outstanding in share-based payment arrangement 1,648,720  
US Dollar [Member]    
Share-based compensation - Text Details (Detail) [Line Items]    
Number of share options outstanding in share-based payment arrangement 1,633,868  
Accelerate options [Member] | Euro [Member]    
Share-based compensation - Text Details (Detail) [Line Items]    
Number of share options outstanding in share-based payment arrangement 296,750 481,200
Weighted average exercise price of share options outstanding in share-based payment arrangement € 16.57 € 16.06
Number of share options exercised in share-based payment arrangement 179,450  
Weighted average exercise price of share options exercised in share-based payment arrangement € 15.24  
Number of share options forfeited in share-based payment arrangement 5,000  
Weighted average exercise price of share options forfeited in share-based payment arrangement € 15.24  
Number of share options exercisable in share-based payment arrangement 296,750  
Weighted average exercise price of share options exercisable in share-based payment arrangement € 16.57  
Accelerate options [Member] | US Dollar [Member]    
Share-based compensation - Text Details (Detail) [Line Items]    
Number of share options outstanding in share-based payment arrangement 123,300 170,800
Weighted average exercise price of share options outstanding in share-based payment arrangement € 20.02 € 20.02
Number of share options exercised in share-based payment arrangement 47,500  
Weighted average exercise price of share options exercised in share-based payment arrangement € 20.02  
Number of share options forfeited in share-based payment arrangement 0  
Number of share options exercisable in share-based payment arrangement 123,300  
Weighted average exercise price of share options exercisable in share-based payment arrangement € 20.02