Collaborations - Additional Information (Detail) - USD ($)		1 Months Ended			3 Months Ended		9 Months Ended		66 Months Ended
		Sep. 30, 2021	Jul. 31, 2021	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Jul. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue	[1]				$ 7,204,000	$ 22,220,000	$ 20,199,000	$ 51,399,000
Deferred revenue		$ 119,782,000			119,782,000	79,614,000	119,782,000	79,614,000	$ 119,782,000		$ 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					6,700,000	22,200,000	$ 18,700,000	46,400,000	141,900,000
Aggregate transaction price remaining to be recognized, period							Through September 30, 2021
Payments due					2,100,000	$ 1,200,000	$ 3,900,000	$ 9,800,000
Regeneron Pharmaceuticals Inc. [Member] | Regeneron Agreement [Member] | Research and Development Services [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue									36,600,000
Regeneron Pharmaceuticals Inc. [Member] | Regeneron Amendment [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate transaction price remaining to be recognized		57,100,000			57,100,000		57,100,000		57,100,000
Accounts receivable		2,100,000			2,100,000		2,100,000		2,100,000		2,100,000
Deferred revenue		57,100,000			57,100,000		57,100,000		57,100,000		73,900,000
Regeneron Pharmaceuticals Inc. [Member] | Hemophilia Co/Co Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue					1,100,000		$ 2,100,000
Novartis [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Strategic collaboration agreement amended date							2018-12
One time payment to novartis					10,000,000.0		$ 10,000,000.0
Novartis [Member] | Regeneron Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue		300,000		$ 5,000,000.0
Accounts receivable		300,000			300,000		300,000		300,000
Deferred revenue		0			0		0		0		0
Accounts receivable											$ 0
NewCo [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue additions							62,500
Collaboration revenue					125,000.0		125,000.0
Deferred revenue		62,700,000			62,700,000		62,700,000		62,700,000
NewCo [Member] | New CAR T-Cell Therapy [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Equity Method Investment, Ownership Percentage										33.33%
NewCo [Member] | Second CoDevelopment and CoPromotion Agreement [Member] | New CAR T-Cell Therapy [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration Payment			$ 30,000,000.0
NewCo [Member] | NewCo Co/Co Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue					200,000		200,000
Deferred revenue		$ 62,500			$ 62,500		$ 62,500		$ 62,500
Equity Method Investment, Ownership Percentage		33.33%			33.33%		33.33%		33.33%
Collaboration transaction price							$ 62,900,000

[1] Including the following revenue from related party (see Notes 7 and 8):