Collaboration Agreements - Biogen (Details)		3 Months Ended		9 Months Ended
		Sep. 30, 2022 USD ($) milestone	Sep. 30, 2021 USD ($)	Sep. 30, 2022 USD ($) milestone	Sep. 30, 2021 USD ($)	Dec. 31, 2021 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense due to cost share payments to related party	[1]	$ 87,786,000	$ 71,559,000	$ 266,621,000	$ 197,477,000
Cost sharing payments due to related party		1,443,000		1,443,000		$ 0
Offset to research and development expense, related party cost reimbursement			1,200,000		5,300,000
Cost sharing reimbursements due from related party		0		0		1,226,000
Collaboration revenue from customers	[2]	184,000	5,285,000	94,805,000	36,143,000
Biogen Collaboration Agreement | Related Party
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract liability		290,800,000		290,800,000		293,700,000
Biogen Collaborative Arrangement | Biogen Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction price, change		0		0
Cost sharing payments due to related party		$ 1,400,000		$ 1,400,000
Offset to research and development expense, related party cost reimbursement			$ 1,200,000		$ 5,300,000
Cost sharing reimbursements due from related party						$ 1,200,000
Number of milestones achieved | milestone		0		0
Biogen Collaborative Arrangement | Biogen Collaboration Agreement | Related Party
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense due to cost share payments to related party		$ 1,400,000		$ 3,800,000
Biogen Collaborative Arrangement | Biogen Collaboration Agreement | Option Research Services | Related Party
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract liability		1,900,000		$ 1,900,000
Biogen Collaborative Arrangement | Biogen Collaboration Agreement | Option Research Services | Related Party | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Performance obligation, period				one year
Biogen Collaborative Arrangement | Biogen Collaboration Agreement | Product
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue from customers				$ 0
ROFN and Option Agreement | Biogen Collaboration Agreement | Material Right For An Option | Related Party
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract liability		$ 288,900,000		$ 288,900,000
ROFN and Option Agreement | Biogen Collaboration Agreement | Material Right For An Option | Related Party | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Performance obligation, period				one year

[1] Includes expense for cost sharing payments due to a related party of $1.4 million and $3.8 million for the three and nine months ended September 30, 2022, respectively, and an offset to expense from related-party cost sharing reimbursements of $1.2 million and $5.3 million for the three and nine months ended September 30, 2021, respectively.

[2] Includes related-party collaboration revenue from a customer of $0.2 million and $2.9 million for the three and nine months ended September 30, 2022, respectively, and $0.9 million and $2.5 million for the three and nine months ended September 30, 2021, respectively.