Collaborative arrangements and strategic partnerships - Narrative (Detail)		1 Months Ended						3 Months Ended	12 Months Ended
	Dec. 13, 2021 shares	Jan. 31, 2023 USD ($) $ / shares 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, 2019 USD ($)	Dec. 31, 2023 USD ($) shares	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Apr. 30, 2023 USD ($)	Oct. 27, 2022 USD ($)	Dec. 23, 2021 USD ($) performance_obligation	May 31, 2020 USD ($)	Mar. 31, 2018 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Investment in common stock									$ 9,859,000	$ 0	$ 0
Deferred revenue, current portion									15,403,000	3,000,000
Deferred revenue, net of current portion									3,918,000	5,000,000
Common stock									5,000	4,000
Bristol-Myers Squibb
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability									0	0
Term of collaboration agreement							3 years
Additions									0
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									50,000,000	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]
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															$ 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
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									14,751,000	13,226,000	16,895,000
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 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									0	35,762,000	0
Allocation of Transaction Price									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]
Contract with customer liability									4,600,000
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
Regeneron Collaboration Agreement | Collaboration
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue balance recognized as gross revenues									21,600,000	19,600,000	7,500,000
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]
Contract with customer liability										1,100,000
Collaboration agreement, transaction price									100,000,000
Purchase price premium									$ 37,000,000
Collaborative arrangement amortization period			5 years						5 years
Collaborative arrangement amount attributed to equity sold									$ 54,500,000
Collaborative arrangement amount attributed to joint research activities									$ 45,500,000
Collaborative Arrangement Amount Attributed To Joint Research Activities, Net Funding Advancement			$ 8,500,000
Regeneron Collaboration Agreement | Share purchase agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability		$ 10,100,000
Issuance of common stock to Regeneron (in shares) | shares	100,000	1,114,827
Common stock price per share (in dollars per share) | $ / shares		$ 17.94	$ 238.10
Investment in common stock		$ 20,000,000	$ 100,000,000
Purchase price premium			$ 37,000,000
Collaborative arrangement research initial funding obligation, percentage			50.00%
Common stock		$ 9,900,000
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			400,000
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									400,000
Investment in common stock			$ 63,000,000
Regeneron Collaboration Agreement | Regeneron Collaboration Agreement, Amendment One
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability									$ 8,300,000
Collaborative arrangement research initial funding obligation, percentage		50.00%
Deferred revenue, current portion									4,400,000
Deferred revenue, net of current portion									3,900,000
Regeneron Collaboration Agreement | Regeneron Collaboration Agreement, Amendment One | Pre-Clinical Costs To Study Combinations
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative arrangement research initial funding obligation, percentage		75.00%
Regeneron Collaboration Agreement | Regeneron Collaboration Agreement, Amendment One | Clinical Study Costs Involving Regeneron Agents
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative arrangement research initial funding obligation, percentage		100.00%
Regeneron Collaboration Agreement | Regeneron Collaboration Agreement, Amendment One | MUC-16 Development Candidate Nomination
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable		$ 2,000,000
Regeneron Collaboration Agreement | Regeneron Collaboration Agreement, Amendment One | MUC-16 IND Acceptance
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable		3,000,000
Regeneron Collaboration Agreement | Regeneron Collaboration Agreement, Amendment One | Last Patient Dosed Or Dosing Of The 10th Patient
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable		5,000,000
Regeneron Collaboration Agreement | Regeneron Collaboration Agreement, Amendment One | MUC-16 Development Candidate Nomination After Achievement Of IND Acceptance
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable		1,000,000
Regeneron Collaboration Agreement | Regeneron Collaboration Agreement, Amendment One | MUC-16
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable		$ 4,000,000
Regeneron Collaboration Agreement | Regeneron Collaboration Agreement, Original And Amendment Combined
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability									11,200,000
Allocation of Transaction Price									11,259,000
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			$ 5,000,000
Revenue									5,765,000	$ 6,501,000	$ 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		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
Novo Nordisk A/S | Novo Collaboration and License Agreement | Milestone Achievement, Option To In-License Technology From Third Party
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability												6,000,000
Milestone payments receivable												$ 9,000,000
Novo Nordisk A/S | Novo Collaboration and License Agreement | Milestone Achievement, Option To In-License Technology From Third Party And Upfront Payment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability									11,000,000
JW Therapeutics
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability									$ 600,000				$ 3,000,000
Estimated variable consideration, reimbursement													4,300,000
Estimated variable consideration													$ 7,300,000