Collaborations - Additional Information (Detail) - USD ($)	3 Months Ended		60 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue	$ 6,445,000	$ 12,916,000
Deferred revenue	68,373,000	25,659,000	$ 68,373,000	$ 73,931,000	$ 28,810,000
Regeneron Pharmaceuticals Inc. [Member] | Regeneron Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue additions			145,000,000.0
Collaboration revenue	$ 6,400,000	7,900,000	129,700,000
Aggregate transaction price remaining to be recognized, period	Through March 31, 2021
Payments due	$ 900,000	$ 4,800,000
Regeneron Pharmaceuticals Inc. [Member] | Regeneron Agreement [Member] | Research and Development Services [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue			35,600,000
Regeneron Pharmaceuticals Inc. [Member] | Regeneron Amendment [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate transaction price remaining to be recognized	68,400,000		68,400,000
Accounts receivable	1,000,000.0		1,000,000.0	2,100,000
Deferred revenue	$ 68,400,000		68,400,000	73,900,000
Novartis [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Strategic collaboration agreement amended date	2018-12
Novartis [Member] | Regeneron Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue	$ 5,000,000.0
Deferred revenue	0		0	0
Accounts receivable	$ 0		$ 0	$ 0