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Employee Benefits - Additional Information (Detail)
€ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2019
EUR (€)
Employees
Jun. 30, 2018
EUR (€)
Employees
Jun. 30, 2019
EUR (€)
Employees
Jun. 30, 2018
EUR (€)
Employees
Disclosure of employee benefits [line Items]        
Received and recognized subsidies | € € 1.1 € 0.7 € 2.3 € 1.9
Number of employees 112 85 112 85
Netherlands [member]        
Disclosure of employee benefits [line Items]        
Number of employees 94 70 94 70
United States [member]        
Disclosure of employee benefits [line Items]        
Number of employees 18 15 18 15
Management and administration costs [member]        
Disclosure of employee benefits [line Items]        
Number of employees 29 18 29 18