Collaboration Agreements - Summary of Collaboration Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue	$ 4,672	$ 13,604	$ 4,197	$ 4,205	$ 125,676	$ 1,195	$ 1,648	$ 641	$ 26,678	$ 129,160	$ 0
Deferred revenue												$ 0
Takeda Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue									5,926	5,677
Sanofi Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue									20,752	123,483
Sanofi Collaboration Agreement | CNS Program License
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue									0	73,932
Sanofi Collaboration Agreement | Peripheral Product
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue									10,000	47,148
Sanofi Collaboration Agreement | Alzheimer's Disease Services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue									358	60
Sanofi Collaboration Agreement | Retained Activities
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue									$ 10,394	$ 2,343