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Cash and Cash Equivalents - Summary of Cash and Cash Equivalents by Geographic Area (Detail) - EUR (€)
€ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of geographical areas [line items]      
Cash € 39,799 € 62,131 € 55,035
Europe [member]      
Disclosure of geographical areas [line items]      
Cash 21,168 40,479  
China [member]      
Disclosure of geographical areas [line items]      
Cash 16,572 18,290  
North America [member]      
Disclosure of geographical areas [line items]      
Cash 1,317 2,857  
South America [member]      
Disclosure of geographical areas [line items]      
Cash 575 318  
Other [member]      
Disclosure of geographical areas [line items]      
Cash € 167 € 187