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Shareholders' Equity - Additional Information - Equity (Detail) $ / shares in Units, € in Thousands, $ in Millions |  |  |  |  | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | ---
Dec. 21, 2018 EUR (€) | Dec. 21, 2018 USD ($) | Dec. 20, 2018 $ / shares shares | Feb. 15, 2018 EUR (€) | Feb. 15, 2018 USD ($) | Feb. 13, 2018 $ / shares shares | Jun. 30, 2019 EUR (€) shares | Jun. 30, 2018 EUR (€) shares | Dec. 31, 2018 EUR (€) | Dec. 31, 2018 USD ($) | Dec. 31, 2018 USD ($)
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |  |  |  |  |  |  |  |  |
Aggregate purchase price | € |  |  |  |  |  |  | € 1 | € 44,731 |  |  |
Investors [member] |  |  |  |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |  |  |  |  |  |  |  |  |
Common shares issued |  |  |  |  |  | 3,099,997 |  |  |  |  |
Aggregate purchase price |  |  |  | € 44,800 | $ 55.8 |  |  |  |  |  |
Purchase price | $ / shares |  |  |  |  |  | $ 18.0 |  |  |  |  |
Regeneron Pharmaceuticals, INC [member] |  |  |  |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |  |  |  |  |  |  |  |  |
Common shares issued |  |  | 600,000 |  |  |  |  |  |  |  |
Aggregate purchase price | € 13,100 | $ 15.0 |  |  |  |  |  |  |  |  |
Purchase price | $ / shares |  |  | $ 25.00 |  |  |  |  |  |  |  |
Total proceeds received |  |  |  |  |  |  |  |  | € 13,100 | $ 15.0 |
Aggregate value of common shares issued |  |  |  |  |  |  |  |  | 6,000 |  | $ 6.9
Gain on litigation settlement |  |  |  |  |  |  |  |  | € 7,100 | $ 8.1 |
Ordinary shares [member] |  |  |  |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |  |  |  |  |  |  |  |  |
Number of shares fully issued and paid |  |  |  |  |  |  | 23,393,287 | 22,632,800 |  |  |

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