Collaborative arrangements - Narrative (Detail)		1 Months Ended					3 Months Ended
	Aug. 24, 2018 USD ($) $ / shares shares	Aug. 31, 2018 target	Sep. 30, 2017 USD ($)	Feb. 29, 2016 USD ($)	Jun. 30, 2015 USD ($) product	Mar. 31, 2013 USD ($)	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2020 USD ($)	May 31, 2020 USD ($)	Mar. 31, 2018 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue							$ 990,000	$ 2,118,000
Bristol-Myers Squibb
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer liability							25,762,000			$ 26,582,000
Term of collaboration agreement						3 years
Deferred revenue							(800,000)
Bristol-Myers Squibb | Collaborative Arrangement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer liability						$ 75,000,000.0
Bristol-Myers Squibb | Amended Collaborative Arrangement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer liability					$ 25,000,000.0
Term of collaboration agreement					3 years
Number of product candidates | product					2
Bristol-Myers Squibb | Ide-cel License Agreement | First Product Candidates
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, option fee received				$ 10,000,000.0
Bristol-Myers Squibb | Ide-cel License Agreement | First Product Candidates | U.S.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payments receivable				85,000,000.0
Bristol-Myers Squibb | Ide-cel License Agreement | First Product Candidates | Outside of U.S.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payments receivable				$ 55,000,000.0
Bristol-Myers Squibb | Ide-cel Co-Development, Co-Promote and Profit Share Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payment achieved									$ 10,000,000.0
Bristol-Myers Squibb | Ide-cel Co-Development, Co-Promote and Profit Share Agreement | U.S.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payments receivable												$ 10,000,000.0
Bristol-Myers Squibb | Ide-cel Co-Development, Co-Promote and Profit Share Agreement | Outside of U.S.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payments receivable												$ 60,000,000.0
Bristol-Myers Squibb | Amended Ide Cel Co Development, Co-Promote And Profit Share Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone and royalty obligation buy-out							184,029,000				$ 200,000,000.0
Bristol-Myers Squibb | bb21217 License Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, option fee received			$ 15,000,000.0				15,000,000
Milestone payment achieved							10,000,000.0
Milestone and royalty obligation buy-out							$ 15,971,000
Additional fee receivable if option to co-develop and co-promote is not exercised			10,000,000.0
Deferred revenue recognition period							2 years
Bristol-Myers Squibb | bb21217 License Agreement | U.S.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payments receivable			85,000,000.0
Bristol-Myers Squibb | bb21217 License Agreement | Outside of U.S.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payments receivable			$ 55,000,000.0
Bristol-Myers Squibb | Ide-cel License and Manufacturing Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price							$ 347,017,000
Remaining performance obligation revenue							0
Bristol-Myers Squibb | Ide-cel License and Manufacturing Services | License and Manufacturing Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer liability							0.0			800,000
Remaining performance obligation revenue							0.0
Bristol-Myers Squibb | Ide-cel License and Manufacturing Services | License and Manufacturing Services | Outside of U.S.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue							5,104,000	13,970,000
Bristol-Myers Squibb | Ide-cel Research and Development
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue							0	0
Bristol-Myers Squibb | bb21217 Research and Development Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price							5,444,000
Remaining performance obligation revenue							0
Bristol-Myers Squibb | bb21217 Research and Development Services | Phase I, Additional Obligation
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue							0.0	2,400,000
Bristol-Myers Squibb | bb21217 license and manufacturing services
Collaborative Arrangement and Arrangement Other than Collaborative [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 Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer liability							25,800,000			25,800,000
Liability, revenue recognized							0	0
Remaining performance obligation revenue							27,300,000
Regeneron Collaboration Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of initial collaboration targets | target		6
Research collaboration term	5 years	5 years
Joint research activities remaining to be recognized							29,300,000			$ 30,800,000
Regeneron Collaboration Agreement | Collaboration
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Liability, revenue recognized							$ 1,500,000	$ 2,300,000
Regeneron Collaboration Agreement | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payments receivable	$ 130,000,000.0
Regeneron Collaboration Agreement | Research and Development Services
Collaborative Arrangement and Arrangement Other than Collaborative [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 Arrangement and Arrangement Other than Collaborative [Line Items]
Common stock shares issued (in shares) | shares	400,000
Common stock price 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 Arrangement and Arrangement Other than Collaborative [Line Items]
Common stock shares issued (in shares) | shares	400,000
Investment in common stock	$ 63,000,000.0