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Share-Based Compensation - Summary of PSUs (Detail) - Performance share units (PSUs) [member] - EUR (€)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of Input of Model Used for Performance-Based RSU [Line Items]    
Fair value at grant date € 11.52  
Share price at grant date € 7.50  
Expected volatility 75.00%  
Risk-free interest rate (U.S. government bond yield) 1.51%  
Model used Monte Carlo Monte Carlo
Bottom of range [member]    
Disclosure of Input of Model Used for Performance-Based RSU [Line Items]    
Fair value at grant date   € 6.76
Share price at grant date   € 4.46
Expected volatility   69.00%
Risk-free interest rate (U.S. government bond yield)   0.76%
Top of range [member]    
Disclosure of Input of Model Used for Performance-Based RSU [Line Items]    
Fair value at grant date   € 9.86
Share price at grant date   € 5.61
Expected volatility   71.00%
Risk-free interest rate (U.S. government bond yield)   1.27%