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Employee Benefits - Additional Information (Detail)
€ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2018
EUR (€)
Employees
Jun. 30, 2017
EUR (€)
Employees
Jun. 30, 2018
EUR (€)
Employees
Jun. 30, 2017
EUR (€)
Employees
Disclosure of employee benefits [line Items]        
Received and recognized subsidies | € € 0.7 € 1.0 € 1.9 € 2.0
Number of employees 85 64 85 64
Netherlands [member]        
Disclosure of employee benefits [line Items]        
Number of employees 70 55 70 55
United States [member]        
Disclosure of employee benefits [line Items]        
Number of employees 15 9 15 9
Management and administration costs [member]        
Disclosure of employee benefits [line Items]        
Number of employees 18 13 18 13