Collaborative arrangements - Summary of Revenue Recognized or Expense Incurred Related to Combined Unit of Accounting for its License and Vector Manufacturing of Ide-cel (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	$ 12,471	$ 15,957
Celgene Corporation [Member] | U.S. [Member] | Ide-cel License and Manufacturing Services [Member] | ASU 2014-09 [Member] | License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue		3,761
Celgene Corporation [Member] | U.S. [Member] | Ide-cel Research and Development Services [Member] | ASU 2014-09 [Member] | License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	(3,244)
Celgene Corporation [Member] | Outside of U.S. [Member] | ASU 2014-09 [Member] | License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License and manufacturing revenue (expense)	$ 9,064	$ 8,942