Collaborative arrangements - Additional Information (Detail)					3 Months Ended	12 Months Ended
	Aug. 24, 2018 USD ($) target $ / shares shares	Sep. 28, 2017 USD ($)	Feb. 16, 2016 USD ($)	Mar. 19, 2013 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($)	Aug. 03, 2018 USD ($)	Mar. 28, 2018 USD ($)	Jun. 03, 2015 USD ($) product
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Investment in common stock						$ 0	$ 649,368,000	$ 1,006,570,000
Joint research activities remaining to be recognized						38,200,000	$ 44,000,000.0
Common shares
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock shares issued (in shares) | shares							420,000
Bristol-Myers Squibb
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability						18,265,000	$ 34,939,000
Term of collaboration agreement				3 years
Deferred revenue balance recognized as gross revenues						17,600,000
Bristol-Myers Squibb | BMS collaborative arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability				$ 75,000,000.0
Bristol-Myers Squibb | Amended collaborative arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability											$ 25,000,000.0
Term of collaboration agreement				3 years
Products, number of candidates | product											2
Estimated variable consideration						$ 87,200,000
Deferred revenue recognition period						3 years
Bristol-Myers Squibb | Amended collaborative arrangement | Research and development services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price						$ 40,900,000
Bristol-Myers Squibb | Ide-cel license and manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price						166,300,000
Bristol-Myers Squibb | Ide-cel license and manufacturing services | License and manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability						8,500,000	23,000,000.0
Remaining performance obligation revenue						17,800,000
Bristol-Myers Squibb | Ide-cel license agreement
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
Collaborative Arrangements and Non-collaborative Arrangement Transactions [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 Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development milestone payments receivable						10,000,000
Contract with customer development milestone payment achieved					$ 10,000,000.0
Contract with customer 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 Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development milestone payments receivable										$ 70,000,000.0
Bristol-Myers Squibb | bb21217 license agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
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 Arrangements and Non-collaborative Arrangement Transactions [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 Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price						5,400,000
Bristol-Myers Squibb | bb21217 license and manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [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 Arrangements and Non-collaborative Arrangement Transactions [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	0
Regeneron Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of initial collaboration targets | target	6
Research collaboration term	5 years
Investment in common stock						0	54,484,000	$ 0
Regeneron Collaboration Agreement | Collaboration revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue balance recognized as gross revenues						5,700,000	$ 1,600,000
Regeneron Collaboration Agreement | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable									$ 130,000,000.0
Regeneron Collaboration Agreement | Research and development services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [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 Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock shares issued (in shares) | shares	400,000
Common stock price per share (in dollars 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 Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock shares issued (in shares) | shares						400,000
Investment in common stock	$ 63,000,000.0