Collaborative arrangements - Additional Information (Detail)		1 Months Ended					3 Months Ended			9 Months Ended
	Aug. 24, 2018 USD ($) target $ / shares shares	May 31, 2020 USD ($)	Sep. 30, 2017 USD ($)	Feb. 29, 2016 USD ($)	Jun. 30, 2015 USD ($) product	Mar. 31, 2013 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue										$ (8,558,000)	$ 14,556,000
Investment in common stock		$ 541,500,000								541,536,000	0
Bristol-Myers Squibb
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer liability							$ 26,823,000			26,823,000		$ 18,265,000
Term of collaboration agreement						3 years
Deferred revenue										(8,200,000)
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		$ 200,000,000.0					184,029,000			184,029,000
Deferred revenue										191,400,000
Catch-up adjustment to revenue										169,200,000
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			$ 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 | License and Manufacturing Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer liability							1,100,000			$ 1,100,000		8,500,000
Remaining performance obligation revenue							1,400,000			1,400,000
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							6,913,000	$ 3,512,000		94,733,000	20,475,000
Bristol-Myers Squibb | Ide-cel Research and Development
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue							0	0.0		0	2,300,000
Bristol-Myers Squibb | bb21217 Research and Development Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price							5,444,000			5,444,000
Remaining performance obligation revenue							0			0
Bristol-Myers Squibb | bb21217 Research and Development Services | Phase I, Initial Obligation
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue							0	700,000		0	2,100,000
Bristol-Myers Squibb | bb21217 Research and Development Services | Phase I, Additional Obligation
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue							5,600,000			12,000,000.0
Bristol-Myers Squibb | bb21217 license and manufacturing services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price							27,330,000			27,330,000
Remaining performance obligation revenue							27,330,000			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		9,800,000
Liability, revenue recognized							0	0		0	0
Remaining performance obligation revenue							27,300,000			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
Joint research activities remaining to be recognized							32,000,000.0			32,000,000.0		$ 38,200,000
Regeneron Collaboration Agreement | Collaboration
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Liability, revenue recognized							$ 2,400,000	$ 2,000,000.0		$ 6,200,000	$ 4,500,000
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