Collaborative arrangements - Additional Information (Detail)	1 Months Ended						3 Months Ended	12 Months Ended
	May 31, 2020 USD ($)	Aug. 31, 2018 USD ($) target $ / shares shares	Sep. 30, 2017 USD ($)	Feb. 29, 2016 USD ($)	Jun. 30, 2015 USD ($) product	Mar. 31, 2013 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) shares	Mar. 31, 2018 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue								$ (9,817,000)	$ 16,674,000	$ 41,872,000
Investment in common stock	$ 541,500,000
Common shares
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock shares issued (in shares) | shares										420,000
Bristol-Myers Squibb
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability								26,582,000	18,265,000
Term of collaboration agreement						3 years
Deferred revenue								(8,200,000)
Bristol-Myers Squibb | Research and development services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price								40,900,000
Bristol-Myers Squibb | BMS collaborative arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability						$ 75,000,000.0
Bristol-Myers Squibb | Amended collaborative arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability					$ 25,000,000.0
Term of collaboration agreement					3 years
Products, number of candidates | product					2
Bristol-Myers Squibb | Ide-cel license and manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price								347,017,000
Remaining performance obligation revenue								1,082,000
Bristol-Myers Squibb | Ide-cel license and manufacturing services | License and manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability								800,000	8,500,000
Remaining performance obligation revenue								1,100,000
Bristol-Myers Squibb | Ide-cel license and manufacturing services | License and manufacturing services | Outside of U.S.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue								99,053,000	25,522,000	$ 35,900,000
Bristol-Myers Squibb | Ide-cel license agreement | First product candidates
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, option fee received				$ 10,000,000.0
Bristol-Myers Squibb | Ide-cel license agreement | First product candidates | U.S.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable				85,000,000.0
Bristol-Myers Squibb | Ide-cel license agreement | First product candidates | Outside of U.S.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable				$ 55,000,000.0
Bristol-Myers Squibb | Ide-cel co-development, co-promote and profit share agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development milestone payments receivable								387,929,000
Contract with customer development milestone payment achieved							$ 10,000,000.0
Bristol-Myers Squibb | Ide-cel co-development, co-promote and profit share agreement | Outside of U.S.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable											$ 60,000,000.0
Bristol-Myers Squibb | Ide-cel co-development, co-promote and profit share agreement | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development milestone payments receivable											$ 10,000,000.0
Bristol-Myers Squibb | bb21217 license agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, option fee received			$ 15,000,000.0					15,000,000
Contract with customer development milestone payment achieved								$ 10,000,000.0
Additional fee receivable if option to co-develop and co-promote is not exercised			10,000,000.0
Deferred revenue recognition period								2 years
Milestone and royalty obligation buy-out								$ 15,971,000
Bristol-Myers Squibb | bb21217 license agreement | U.S.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable			85,000,000.0
Bristol-Myers Squibb | bb21217 license agreement | Outside of U.S.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable			$ 55,000,000.0
Bristol-Myers Squibb | bb21217 license and manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price								27,330,000
Remaining performance obligation revenue								27,330,000
Bristol-Myers Squibb | bb21217 license and manufacturing services | License and manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability								25,800,000	9,800,000
Remaining performance obligation revenue								27,300,000
Deferred revenue balance recognized as gross revenues								0	0	0
Bristol-Myers Squibb | Amended Ide Cel Co Development, Co-Promote And Profit Share Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue								191,700,000
Catch-up adjustment to revenue								169,200,000
Milestone and royalty obligation buy-out	$ 200,000,000.0							184,029,000
Bristol-Myers Squibb | Ide-cel Research and Development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue								0	2,300,000	5,800,000
Bristol-Myers Squibb | bb21217 research and development services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price								5,444,000
Remaining performance obligation revenue								0
Bristol-Myers Squibb | bb21217 research and development services | Phase I, Initial Obligation [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue								0	2,200,000	2,900,000
Bristol-Myers Squibb | bb21217 research and development services | Phase I, Additional Obligation [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue								12,400,000	400,000	$ 0.0
Regeneron Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of initial collaboration targets | target		6
Research collaboration term		5 years
Joint research activities remaining to be recognized								30,800,000	38,200,000
Regeneron Collaboration Agreement | Service revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue balance recognized as gross revenues								7,400,000	$ 5,700,000
Regeneron Collaboration Agreement | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable		$ 130,000,000.0
Regeneron Collaboration Agreement | Research and development services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price								100,000,000.0
Purchase price premium								$ 37,000,000.0
Collaborative arrangement amortization period								5 years
Collaborative arrangement amount attributed to equity sold								$ 54,500,000
Collaborative arrangement amount attributed to joint research activities								$ 45,500,000
Regeneron Collaboration Agreement | Share purchase agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock shares issued (in shares) | shares		400,000
Common stock price per share (in dollars per share) | $ / shares		$ 238.10
Investment in common stock		$ 100,000,000.0
Purchase price premium		$ 37,000,000.0
Collaborative arrangement research initial funding obligation, percentage		50.00%
Regeneron Collaboration Agreement | Share purchase agreement | Common shares
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock shares issued (in shares) | shares								400,000
Investment in common stock		$ 63,000,000.0