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Shareholders' Equity - Additional Information - Share Based Payment Arrangements (Detail) | 6 Months Ended |  |
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Jun. 30, 2019 EUR (€) shares | Jun. 30, 2018 EUR (€) | Jan. 01, 2019 shares | Dec. 31, 2018 EUR (€) shares
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |  |
Share based payment expense | € 3,100,000 | € 4,600,000 |  |
Option granted | shares | 1,009,671 |  |  |
Weighted average exercise price | € 13.27 |  |  | € 14.62
Weighted-average remaining contractual life | 7 years 3 months 18 days |  |  |
Weighted-average share price | € 11.02 |  |  |
Number of share options outstanding in share-based payment arrangement | shares | 3,282,510 |  |  | 2,633,039
Top of range [member] |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |  |
Weighted average exercise price | € 1.93 |  |  |
Bottom of range [member] |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |  |
Weighted average exercise price | € 27.47 |  |  |
2016 Plan [member] |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |  |
Description of options vested | Options granted under the 2016 Plan are exercisable once vested. |  |  |
Vesting period | 4 years |  |  |
Percentage of initial vesting period | 25.00% |  |  |
Percentage of remaining vesting options | 75.00% |  |  |
Period of remaining option vesting period | 36 monthly installments |  |  |
Percentage of vesting period | 100.00% |  |  |
Description of option lapse | Options will lapse on the tenth anniversary of the date of grant. |  |  |
Vesting period of Restricted Stock Units vested | 4 years |  |  |
Number of common share entitled to receive | shares | 1 |  |  |
Option granted | shares | 1,009,671 |  |  |
Option grant fair value | € 10,300,000 |  |  |
Increase in authorized share | shares |  |  | 1,469,785 | 934,359
Non-Executive Compensation Program [member] |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |  |
Percentage of initial vesting period | 33.00% |  |  |
Percentage of remaining vesting options | 67.00% |  |  |
Period of remaining option vesting period | 24 substantially equal monthly installments |  |  |
Vesting period | Three-year period |  |  |
Period of option will be vested and exercised | 12 substantially equal monthly installments |  |  |
Description of grant award | Following the vesting commencement date, such that the subsequent award shall be fully vested on the first anniversary of the date of grant. |  |  |

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