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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and Development Expense	$ (156,308)	$ (146,540)	$ (310,431)	$ (269,180)
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	0	(1,065)	(5,080)	(4,309)
Bristol-Myers Squibb | U.S. | Ide Cel Revenue Services [Member] | License and Manufacturing Services | Accounting Standards Update 2014-09 [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue	108,196	0	108,196	0
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	$ 73,850	$ 7,899	$ 87,820	$ 16,963