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Related Parties - Additional Information (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017
Mosaic | Strategic Research Collaboration to Develop Intravitreal Anti-Complement Factor 3 (C3) Products |  |
Related Party Transaction [Line Items] |  |
Expenses related to collaboration | $ 1,300 | $ 30

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