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Management of Financial Risks - Additional Information (Detail) - EUR (€)
€ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Disclosure Of Financial Risk Management [line items]        
Cash and cash equivalents € 137,880 € 256,473 € 323,381 € 114,555
Operating losses and cash out flows from operation (147,693) (114,531) (44,674)  
Accumulated deficit and reserves (279,300)      
Net cash used in operating activities € (114,314) € (59,538) € (26,763)  
Top of range [member]        
Disclosure Of Financial Risk Management [line items]        
Percentage of purchases and other external expenses made in U.S. dollars 12.00% 12.00% 12.00%