Collaborative Arrangements - Takeda (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract liabilities, deferred revenue	$ 6,335	$ 4,791
Takeda
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract liabilities, deferred revenue	2,200	1,500
Revenue recognized, including opening balance	$ 7,400	$ 13,200
Takeda | Up-front Payment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract liabilities, deferred revenue			$ 8,500
Takeda | Research and Development Reimbursement, Three Initial Programs
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent receivable			10,500
Takeda | Research and Development Reimbursement, Fourth Program
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent receivable			8,300
Takeda | Milestone Payment Per Target Gene
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent receivable			$ 100,000