Collaborative arrangements - Summary of Revenue Recognized or Expense Incurred (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development	$ 587,956	$ 582,413	$ 448,589
Bristol-Myers Squibb | U.S. | Ide Cel Revenue Services | License and manufacturing services | ASU 2014-09
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue	108,196	0	6,255
Bristol-Myers Squibb | U.S. | Ide-cel Research And Development Services | License and manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development	$ 41,599	$ 32,415	$ 8,689