Collaborative arrangements - Additional Information (Detail)					3 Months Ended
	Aug. 24, 2018 USD ($) target $ / shares shares	Sep. 28, 2017 USD ($)	Feb. 16, 2016 USD ($)	Mar. 19, 2013 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Mar. 28, 2018 USD ($)	Jun. 03, 2015 USD ($) product
Bristol-Myers Squibb
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer liability					$ 16,147,000	$ 18,265,000
Term of collaboration agreement				3 years
Deferred revenue balance recognized as gross revenues					$ 2,100,000
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
Deferred revenue recognition period					3 years
Number of product candidates | product									2
Estimated variable consideration					$ 87,200,000
Bristol-Myers Squibb | Amended Collaborative Arrangement | Research and Development Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price					40,900,000
Bristol-Myers Squibb | Ide-cel License Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, option fee received					10,000,000
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 Co-Development, Co-Promote and Profit Share Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Development milestone payments receivable					10,000,000
Development milestone payments achieved						10,000,000.0
Remaining potential development milestone payment receivable					$ 60,000,000.0
Bristol-Myers Squibb | Ide-cel Co-Development, Co-Promote and Profit Share Agreement | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Development milestone payments receivable								$ 70,000,000.0
Bristol-Myers Squibb | bb21217 License Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue recognition period					2 years
Collaboration agreement, option fee received		$ 15,000,000.0			$ 15,000,000
Development milestone payments receivable					70,000,000.0
Additional fee receivable if option to co-develop and co-promote is not exercised		$ 10,000,000.0
Estimated variable consideration					26,700,000
Bristol-Myers Squibb | bb21217 License Agreement | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Clinical milestone payments receivable					10,000,000.0
Regulatory milestone payments receivable					117,000,000.0
Commercial milestone payments receivable					78,000,000.0
Bristol-Myers Squibb | bb21217 License Agreement | Research and Development Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price					5,400,000
Bristol-Myers Squibb | Ide-cel License and Manufacturing Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price					166,300,000
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					6,400,000	8,500,000
Remaining performance obligation revenue					13,400,000
Bristol-Myers Squibb | Ide-cel Research And Development
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue					0.0
Bristol-Myers Squibb | Ide-cel Research And Development | Restatement Adjustment
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue							$ 200,000
Bristol-Myers Squibb | bb21217 Research and Development Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price					5,444,000
Remaining performance obligation revenue					0
Bristol-Myers Squibb | bb21217 Research and Development Services | Phase I, Initial Obligation
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue					0.0		700,000
Bristol-Myers Squibb | bb21217 Research and Development Services | Phase I, Additional Obligation
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue					2,400,000
Bristol-Myers Squibb | bb21217 license and manufacturing services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price					36,243,000
Remaining performance obligation revenue					36,243,000
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					9,800,000	9,800,000
Remaining performance obligation revenue					36,200,000
Deferred revenue balance recognized as gross revenues					0		0
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					35,900,000	$ 38,200,000
Regeneron Collaboration Agreement | Collaboration
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue balance recognized as gross revenues					$ 2,300,000		$ 2,000,000.0
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