Collaboration Agreements - Summary of Collaboration Revenue (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative revenue, revenue from contract with customer	[1]	$ 294,123	$ 52,480	$ 329,264	$ 94,621
Total Collaboration Revenue		294,123	52,480	329,264	94,621
Takeda Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total Collaboration Revenue		0	12,000	10,000	51,916
Revenue recognized included in the contract liability balance at the beginning of the period			0		27,900
Takeda Collaboration Agreement | Collaboration Agreement Services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative revenue, revenue from contract with customer		0	12,000	0	51,916
Takeda Collaboration Agreement | PTV:PGRN Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative revenue, revenue from contract with customer		0	0	10,000	0
Sanofi Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total Collaboration Revenue		0	40,000	25,000	40,000
Sanofi Collaboration Agreement | CNS Program License
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative revenue, revenue from contract with customer		0	40,000	25,000	40,000
Biogen
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total Collaboration Revenue		294,123	480	294,264	2,705
Biogen | ATV:Abeta Program License
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative revenue, revenue from contract with customer		293,912	0	293,912	0
Biogen | Option Research Services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative revenue, revenue from contract with customer		211	$ 480	352	$ 2,705
Collaborative Arrangement, ATV:Abeta
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized included in the contract liability balance at the beginning of the period		$ 288,900		$ 288,900

[1] Includes related-party collaboration revenue from customers of $294.1 million and $294.3 million for the three and six months ended June 30, 2023, respectively, and $0.5 million and $2.7 million for the three and six months ended June 30, 2022, respectively.