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Income Tax Expense - Summary of Reconciliation of Effective and Nominal Tax Expense (Detail) - EUR (€)
€ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Major components of tax expense (income) [abstract]      
(Loss) before taxes € (147,692) € (114,531) € (44,674)
Theoretical group tax rate 33.33% 33.33% 33.33%
Nominal tax expense € 49,226 € 38,173 € 14,890
Permanent differences     6,089
Research tax credit 3,110 2,409 1,895
Share-based compensation (10,260) (11,451) (3,473)
Non recognition of deferred tax assets related to tax losses and temporary differences (41,453) (29,195) (19,211)
Other differences 622 64 € 190
Effective tax expense € 1 € 0  
Effective tax rate 0.00% 0.00% 0.00%