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		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and development expense	$ (140,431)	$ (151,412)	$ (450,862)	$ (420,592)
Bristol-Myers Squibb | U.S. | Ide Cel Revenue Services | License and Manufacturing Services | Accounting Standards Update 2014-09
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue	0	0	108,196	0
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	(16,084)	(14,945)	(21,164)	(19,254)
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	$ 6,913	$ 3,512	$ 94,733	$ 20,475