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Share-Based Compensation - Summary of Performance-Based RSU (Detail) - 2018 performance shares [member] - EUR (€)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of Input of Model Used for Performance-Based RSU [Line Items]    
Fair value at grant date (in euros) € 15.31 € 11.52
Share price at grant date (in euros) € 10.27 € 7.50
Dividend yield 0.00% 0.00%
Expected volatility 75.00% 75.00%
Risk-free interest rate (US government bond yield) 2.60% 1.51%
Model used Monte Carlo Monte Carlo