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Related Parties - Additional Information (Detail) - Mosaic - Intravitreal Anti-Complement Factor 3 - USD ($) $ in Millions		12 Months Ended
	Jan. 10, 2020	Dec. 31, 2019	Dec. 31, 2018
Subsequent Event
Related Party Transaction [Line Items]
Payment related to collaboration	$ 3.0
Collaboration Agreement
Related Party Transaction [Line Items]
Expenses related to collaboration		$ 1.1	$ 1.3
Biogen Collaboration | Subsequent Event
Related Party Transaction [Line Items]
Upfront license fee received from collaboration agreement	$ 15.0

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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