Collaboration Agreements - Summary of Collaboration Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative revenue, revenue from contract with customer	$ 5,811	$ 4,098	$ 9,363	$ 8,209
Collaborative revenue, excluding revenue from contract with customer	36	99	88	193
Collaborative Revenue	5,847	4,197	9,451	8,402
Takeda Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative revenue, revenue from contract with customer	5,537	740	8,555	1,422
Sanofi Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative Revenue	310	3,457	896	6,980
Sanofi Collaboration Agreement | Alzheimer's Disease Services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative revenue, excluding revenue from contract with customer	36	99	88	193
Sanofi Collaboration Agreement | Retained Activities
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative revenue, revenue from contract with customer	$ 274	$ 3,358	$ 808	$ 6,787