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Share-Based Compensation - Summary of PSUs (Detail) - Performance share units (PSUs) [member] - EUR (€) | 12 Months Ended
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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%

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