Collaboration Agreements - Summary of Activity and Reimbursement Amounts Related to Collaboration Agreement (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue		$ 86,455	$ 7,686	$ 6,308
Research and development expenses		356,235	326,163	283,166
Selling, general and administrative expenses		274,524	227,699	183,498
Biogen Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue					$ 1,100,000
Cost of revenues		504
Research and development expenses		94,325	86,028	84,221
Selling, general and administrative expenses		89,599	51,870	18,425
Reimbursement amounts to (from) collaboration [Abstract]
Collaboration revenue - related party	$ 8,000	(824)
Cost of revenues		504
Selling, general and administrative expenses		16,496	(2,230)	(11,282)
Collaboration Revenue - Related Party [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue		824
Collaboration Revenue - Related Party [Member] | Biogen Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue		824
Research and Development [Member] | Biogen Collaboration Agreement [Member]
Reimbursement amounts to (from) collaboration [Abstract]
Research and development expenses		$ (76,208)	$ (73,227)	$ (79,848)