Collaborative arrangements - Additional Information (Detail)					3 Months Ended		6 Months Ended
	Aug. 24, 2018 USD ($) Target $ / shares shares	Sep. 28, 2017 USD ($)	Feb. 16, 2016 USD ($)	Mar. 19, 2013 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) shares	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Aug. 03, 2018 USD ($)	Mar. 28, 2018 USD ($)	Jun. 03, 2015 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue released							$ 21,700,000
Investment in common stock								$ 48,702,000
Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue balance recognized as gross revenues							11,700,000
Research and Development Services [Member] | Collaboration [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue balance recognized as gross revenues							10,000,000.0
Celgene Corporation [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract with customer liability					$ 23,223,000		23,223,000		$ 34,939,000
Term of collaboration agreement				3 years
Deferred revenue balance recognized as gross revenues							21,716,000
Celgene Corporation [Member] | Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract with customer liability				$ 75,000,000.0
Celgene Corporation [Member] | Amended Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract with customer liability												$ 25,000,000.0
Estimated variable consideration					87,200,000		$ 87,200,000
Deferred revenue recognition period							3 years
Celgene Corporation [Member] | Amended Collaborative Arrangement [Member] | Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price					40,900,000		$ 40,900,000
Celgene Corporation [Member] | Ide-cel License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, option fee received							10,000,000
Celgene Corporation [Member] | Ide-cel License Agreement [Member] | First Product Candidates [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, option fee received			$ 10,000,000.0
Celgene Corporation [Member] | Ide-cel Co-Development, Co-Promote and Profit Share Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development milestone payments achieved					10,000,000.0
Remaining potential development milestone payment receivable					60,000,000.0		60,000,000.0
Celgene Corporation [Member] | Ide-cel Co-Development, Co-Promote and Profit Share Agreement [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development milestone payments receivable											$ 70,000,000.0
Celgene Corporation [Member] | bb21217 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, option fee received		$ 15,000,000.0					15,000,000
Additional fee receivable if option to co-develop and co-promote is not exercised		$ 10,000,000.0
Estimated variable consideration					26,700,000		$ 26,700,000
Deferred revenue recognition period							2 years
Celgene Corporation [Member] | bb21217 License Agreement [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Clinical milestone payments receivable					10,000,000.0		$ 10,000,000.0
Regulatory milestone payments receivable					117,000,000.0		117,000,000.0
Commercial milestone payments receivable					78,000,000.0		78,000,000.0
Celgene Corporation [Member] | bb21217 License Agreement [Member] | Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract with customer liability					700,000		700,000		1,800,000
Collaboration agreement, transaction price					5,400,000		5,400,000
Remaining performance obligation revenue					700,000		700,000		1,800,000
Celgene Corporation [Member] | bb21217 License Agreement [Member] | License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract with customer liability					9,800,000		9,800,000		9,800,000
Remaining performance obligation revenue					36,200,000		36,200,000
Deferred revenue balance recognized as gross revenues					0	$ 0	0	$ 0
Celgene Corporation [Member] | bb21217 License Agreement, Co-promotion and Development [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development milestone payments receivable					70,000,000.0		70,000,000.0
Celgene Corporation [Member] | Ide-cel License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price					166,277,000		166,277,000
Remaining performance obligation revenue					26,723,000		26,723,000
Celgene Corporation [Member] | Ide-cel License and Manufacturing Services [Member] | License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract with customer liability					12,700,000		12,700,000		23,000,000.0
Remaining performance obligation revenue					$ 26,700,000		$ 26,700,000
Remaining performance obligation expected to be recognize as revenue, year					2020		2020
Regeneron Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of initial collaboration targets | Target	6
Research collaboration term	5 years
Collaboration agreements costs and profits share description								Regeneron chooses to opt-in, the parties will share equally in the costs of development and commercialization, and will share equally in any profits or losses therefrom in applicable opt-in territories.
Purchase price premium	$ 37,000,000.0
Collaboration research advancement remaining to be recognized					$ 41,500,000		$ 41,500,000		$ 44,000,000.0
Regeneron Collaboration Agreement [Member] | Collaboration [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue balance recognized as gross revenues					500,000	$ 0	2,500,000	$ 0
Regeneron Collaboration Agreement [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments receivable										$ 130,000,000.0
Regeneron Collaboration Agreement [Member] | Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price					100,000,000.0		100,000,000.0
Purchase price premium							37,000,000.0
Collaborative arrangement amount attributed to equity sold					54,500,000		54,500,000
Collaborative arrangement amount attributed to joint research activities					$ 45,500,000		$ 45,500,000
Collaborative arrangement amortization period							5 years
Regeneron Collaboration Agreement [Member] | Share Purchase Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock shares issued | shares	400,000						0.4
Investment in common stock	$ 100,000,000.0
Common stock price per share | $ / shares	$ 238.10
Purchase price premium	$ 37,000,000.0
Collaborative arrangement research initial funding obligation, percentage	50.00%
Regeneron Collaboration Agreement [Member] | Share Purchase Agreement [Member] | Common Shares [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Investment in common stock	$ 63,000,000.0