Collaborative arrangements and strategic partnerships - Narrative (Detail) $ / shares in Units, $ in Thousands, shares in Millions		1 Months Ended		3 Months Ended		9 Months Ended
	Aug. 24, 2018 USD ($) $ / shares shares	Jul. 31, 2021 USD ($) agreement batch	Aug. 31, 2018 target	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Nov. 04, 2021 company	Dec. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaborative arrangement revenue				$ 14,831	$ 2,422	$ 18,020	$ 114,398
Share of collaboration loss				0	0	(10,071)	0
Deferred revenue						210	8,558
Investment in common stock						0	541,536
Proceeds from divestiture of business						110,300	0
Goodwill				12,056		12,056			$ 13,128
Lentiviral Vector Manufacturing Facility
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of manufacturing agreements | agreement		2
Proceeds from divestiture of business		$ 110,300
Net operating losses, percentage of reimbursement		50.00%
Net operating losses, reimbursement cap		$ 15,000
Goodwill		$ 1,100
Gain (loss) on disposal				2,000		2,000
Subsequent Event
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of independent publicly traded companies | company								2
Maximum | Lentiviral Vector Manufacturing Facility
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of lentiviral vector batches | batch		8
Bristol-Myers Squibb
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price				31,000		31,000
Estimated variable consideration				1,800		1,800
Deferred revenue						800
Bristol-Myers Squibb | Phase I, Additional Obligation
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Estimated variable consideration										$ 27,300
Bristol-Myers Squibb | Research and Development Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Estimated variable consideration										$ 5,400
Bristol-Myers Squibb | Ide-cel License Agreement | U.S.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaborative arrangement revenue				13,000		13,000
Share of collaboration loss				0		10,100
Bristol-Myers Squibb | bb21217 License Agreement | U.S.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Additional fee receivable if option to co-develop and co-promote is not exercised				10,000		10,000
Milestone payments receivable				85,000		85,000
Bristol-Myers Squibb | bb21217 Research And Development Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payment achieved						10,000
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				25,900		25,900			$ 30,800
Regeneron Collaboration Agreement | Collaboration
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Liability, revenue recognized				$ 1,700	$ 2,400	$ 4,900	$ 6,200
Regeneron Collaboration Agreement | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payments receivable	$ 130,000
Regeneron Collaboration Agreement | Research and Development Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price	100,000
Purchase price premium	$ 37,000
Collaborative arrangement amortization period	5 years
Collaborative arrangement amount attributed to equity sold	$ 54,500
Collaborative arrangement amount attributed to joint research activities	$ 45,500
Regeneron Collaboration Agreement | Share Purchase Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Common stock shares issued (in shares) | shares	0.4
Common stock price per share (in dollars per share) | $ / shares	$ 238.10
Investment in common stock	$ 100,000
Purchase price premium	$ 37,000
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	0.4
Investment in common stock	$ 63,000