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Capital - Additional Information (Detail)
1 Months Ended 12 Months Ended
Jul. 20, 2015
€ / shares
shares
Oct. 22, 2014
€ / shares
$ / shares
shares
Dec. 09, 2011
Jul. 31, 2015
EUR (€)
shares
Oct. 31, 2014
EUR (€)
shares
Dec. 31, 2016
EUR (€)
shares
Dec. 31, 2015
EUR (€)
shares
Dec. 31, 2017
EUR (€)
€ / shares
shares
Dec. 31, 2014
shares
Disclosure of classes of share capital [line items]                  
Share Capital | €           € 2,465,000.00 € 2,421,000.00 € 2,499,082.20  
Number of share authorised               24,990,822  
Number of share issued               24,990,822  
Par value per share | € / shares               € 0.10  
Fees and commission expense related to share capital | €           € 0 € 18,300,000    
Number of shares issued during period shares           2,606,435 2,336,224 3,309,539  
Number of additional american depository shares issued 1,080,000                
Description of american depository shares Each ADS represents the right to receive one-half of one ordinary share.                
Number of ordinary shares per american depository shares | € / shares € 0.5                
Share split ratio     0.0667            
Common Shares [member]                  
Disclosure of classes of share capital [line items]                  
Number of share issued           24,648,828 24,205,129 24,990,822 19,160,661
Initial Public Offering [Member]                  
Disclosure of classes of share capital [line items]                  
Increase in share capital during period shares       4,140,000 3,074,686        
Increase in share capital through issuance of shares | €       € 414,000 € (307,468)        
Increase in share capital through issuance of shares gross | €       254,900,000 104,231,855        
Increase in share capital through issuance of shares net | €       236,900,000 93,403,561        
Fees and expenses related to share capital | €       18,000,000 10,828,294.23        
Number of common shares issued   2,673,641              
Initial Public Offering [Member] | Share premium [Member]                  
Disclosure of classes of share capital [line items]                  
Increase in share capital through issuance of shares | €       € 236,900,000 € 93,403,561        
Initial Public Offering [Member] | Common Shares [member]                  
Disclosure of classes of share capital [line items]                  
Number of shares issued during period shares 4,140,000 2,138,913              
Initial Public Offering [Member] | American Depositary Shares [Member]                  
Disclosure of classes of share capital [line items]                  
Par value per share | (per share) € 34.00 $ 21.64              
Number of shares issued during period shares 8,280,000 4,277,826              
Private Placements Offering [Member]                  
Disclosure of classes of share capital [line items]                  
Number of common shares issued   534,728              
Number of common shares issued per share | € / shares   $ 34