Collaborative arrangements - Summary of Revenue Recognized or Expense Incurred for Joint Ide-cel Development Efforts Related to Combined Unit of Accounting for its License and Vector Manufacturing of Ide-cel (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaborative arrangement revenue	$ 1,670	$ 109,674	$ 3,190	$ 111,976
Research and development expense	(144,315)	(156,308)	(298,793)	(310,431)
Revenue catch-up adjustment	7,472	198,890	20,267	220,753
Bristol-Myers Squibb | U.S. | Ide Cel Research And Development Services | License and Manufacturing Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and development expense	(9,193)	0	(26,018)	(5,080)
Bristol-Myers Squibb | U.S. | Ide Cel Revenue Services | License and Manufacturing Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaborative arrangement revenue	0	108,196	0	108,196
Revenue catch-up adjustment		169,200
Bristol-Myers Squibb | Outside of U.S. | Ide Cel License And Manufacturing Services | License and Manufacturing Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue	$ 4,280	73,850	$ 9,384	$ 87,820
Revenue catch-up adjustment		$ 169,200