Collaborative arrangements and strategic partnerships - Narrative (Detail) $ / shares in Units, shares in Millions		1 Months Ended					3 Months Ended		12 Months Ended
	Dec. 13, 2021 shares	Aug. 31, 2018 USD ($) target accounting_unit $ / shares shares	Sep. 30, 2017 USD ($)	Feb. 29, 2016 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2013 USD ($)	Jun. 30, 2022 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2022 USD ($) shares	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Oct. 27, 2022 USD ($)	Dec. 23, 2021 USD ($) performance_obligation	May 31, 2020 USD ($)	Mar. 31, 2018 USD ($)
Bristol-Myers Squibb
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability									$ 0	$ 25,762,000
Term of collaboration agreement						3 years
Additions							$ 10,000,000		10,000,000
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 | bb21217 research and development services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price									5,400,000
Bristol-Myers Squibb | BMS collaborative arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability						$ 75,000,000
Gross profit									3,100,000	9,400,000
Bristol-Myers Squibb | Amended collaborative agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability					$ 25,000,000
Term of collaboration agreement					3 years
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
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
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
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
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
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
Bristol-Myers Squibb | Amended Ide-cel co development, co-promote and profit share agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone and royalty obligation buy-out									184,029,000					$ 200,000,000
Bristol-Myers Squibb | bb21217 license agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, option fee received			$ 15,000,000
Development milestone payments receivable									40,971,000
Milestone and royalty obligation buy-out									15,971,000
Estimated variable consideration									0
Bristol-Myers Squibb | bb21217 license agreement | U.S.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable			85,000,000
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
Bristol-Myers Squibb | Ide-cel research and development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue									0	0	$ 0
Bristol-Myers Squibb | Ide-cel license and manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price									347,000,000
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									13,226,000	16,895,000	99,052,000
Bristol-Myers Squibb | bb21217 research and development services | Phase I, initial obligation
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue									0	0	0
Bristol-Myers Squibb | bb21217 research and development services | Phase I, additional obligation
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue									0	0	12,400,000
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									35,800,000
Revenue									35,762,000	0	0
Combined performance obligation, remaining									35,500,000
Remaining performance obligation not yet received									1,800,000
Bristol-Myers Squibb | B B21217 License Agreement | U.S.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional fee receivable if option to co-develop and co-promote is not exercised									10,000,000
Regeneron Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of initial collaboration targets | target		6
Research collaboration term		5 years
Number of accounting units | accounting_unit		2
Joint research activities remaining to be recognized									3,700,000	23,300,000
Regeneron Collaboration Agreement | Collaboration
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue balance recognized as gross revenues									19,600,000	7,500,000	7,400,000
Regeneron Collaboration Agreement | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable		$ 130,000,000
Regeneron Collaboration Agreement | Research and development services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price									100,000,000
Purchase price premium									$ 37,000,000
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]
Issuance of common stock to Regeneron (in shares) | shares	0.1
Common stock price per share (in dollars per share) | $ / shares		$ 238.10
Investment in common stock		$ 100,000,000
Purchase price premium		$ 37,000,000
Collaborative arrangement research initial funding obligation, percentage		50.00%
Regeneron Collaboration Agreement | Share purchase agreement | Bluebird Bio
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Issuance of common stock to Regeneron (in shares) | shares		0.4
Regeneron Collaboration Agreement | Share purchase agreement | Common stock
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Issuance of common stock to Regeneron (in shares) | shares									0.4
Investment in common stock		$ 63,000,000
Novo Nordisk A/S | Novo Collaboration and License Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability													$ 5,000,000
Revenue									$ 6,501,000	$ 0	$ 0
Number of performance obligations | performance_obligation													2
Estimated variable consideration									11,700,000
Estimated variable consideration, upfront payment									5,000,000
Estimated variable consideration, research reimbursement									$ 6,700,000
Novo Nordisk A/S | Novo Collaboration and License Agreement | License and manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable													$ 26,000,000
Novo Nordisk A/S | Novo Collaboration and License Agreement | Development and Commercialization Milestones
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable													72,000,000
Novo Nordisk A/S | Novo Collaboration and License Agreement | Scientific milestones
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable													15,000,000
Novo Nordisk A/S | Novo Collaboration and License Agreement | Extension of research plan without achieving scientific milestones
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable													$ 9,000,000
JW Therapeutics
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability												$ 3,000,000
Estimated variable consideration, reimbursement												4,300,000
Estimated variable consideration												$ 7,300,000