Collaboration Agreements - Summary of Collaboration Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative revenue, revenue from contract with customer	$ 3,552	$ 4,111
Collaborative revenue, excluding revenue from contract with customer	52	94
Collaborative Revenue	3,604	4,205
Takeda Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative revenue, revenue from contract with customer	3,018	682
Sanofi Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative Revenue	586	3,523
Sanofi Collaboration Agreement \| Alzheimer's Disease Services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative revenue, excluding revenue from contract with customer	52	94
Sanofi Collaboration Agreement \| Retained Activities
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative revenue, revenue from contract with customer	$ 534	$ 3,429

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Amount of revenue from collaborative arrangement. Excludes revenue from contract with customer under Topic 606.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 808
-SubTopic 10
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=118644919&loc=SL5834089-161433

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 808
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6931272&loc=SL5834143-161434

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Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
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-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
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-Name Accounting Standards Codification
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