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Employee Benefits - Schedule of Share-based Compensation Expenses Recognized as Employee Benefit Expenses (Detail) - EUR (€) € in Thousands | 3 Months Ended | 6 Months Ended
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Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018
Disclosure Of Share Of Equity Investment [line items] |  |  |  |
Stock award expense | € 1,806 | € 2,109 | € 3,058 | € 4,554
R&D costs [member] |  |  |  |
Disclosure Of Share Of Equity Investment [line items] |  |  |  |
Stock award expense | 715 | 714 | 1,482 | 1,525
Management and administration costs [member] |  |  |  |
Disclosure Of Share Of Equity Investment [line items] |  |  |  |
Stock award expense | 919 | 1,343 | 1,255 | 2,852
Other expenses [member] |  |  |  |
Disclosure Of Share Of Equity Investment [line items] |  |  |  |
Stock award expense | € 172 | € 52 | € 321 | € 177

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