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Operating Lease - Additional Information (Detail) - EUR (€)
€ in Millions
 3 Months Ended 6 Months Ended
May 01, 2018
Nov. 01, 2016
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Disclosure of operating lease [abstract]              
Rental expense € 0.5       € 0.4    
Operating lease term of lease agreement   5 years     2 years    
Leases expiration period         Fourth quarter of 2021    
Rent and service charges expense     € 0.3 € 0.2 € 0.5 € 0.3  
Deposits     € 0.1   € 0.1   € 0.1