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Shareholders' equity - Share capital (Details) - EUR (€)
1 Months Ended 6 Months Ended 12 Months Ended
Jun. 28, 2017
Nov. 30, 2017
Jun. 30, 2017
Dec. 31, 2017
Jun. 30, 2018
Dec. 31, 2016
Oct. 02, 2015
Shareholders' Equity              
Number of shares issued         0    
Authorized share capital         € 7,200,000    
Number of shares outstanding     24,105,249 36,425,014 36,425,014 23,346,856  
Maximum aggregate offering of ordinary shares             € 200,000,000
Par value per share         € 0.04    
Treasury shares       4,503,149 4,490,378    
Shares issued in the period     € 3,223,000        
Ordinary              
Shareholders' Equity              
Number of shares authorised         90,000,000    
Preferred              
Shareholders' Equity              
Number of shares authorised         90,000,000    
Market offering program              
Shareholders' Equity              
Number of shares issued       976,477      
Transaction cost       € 127,000      
Net proceeds       € 4,138,000      
Institutional investors              
Shareholders' Equity              
Number of shares issued 1,200,000            
Share price € 5.00            
Gross proceeds € 5,278,000            
Transaction cost 414,000            
Net proceeds € 4,864,000            
Underwritten public offering and direct offering              
Shareholders' Equity              
Number of shares issued   6,397,498          
Share price   € 3.25          
Gross proceeds   € 17,671,000          
Transaction cost   988,000          
Net proceeds   € 16,683,000          
Cantor Fitzgerald & Co | Ordinary              
Shareholders' Equity              
Maximum aggregate offering of ordinary shares             € 60,000,000