Collaborative arrangements - Summary of Revenue Recognized Related to Research and Development Services (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue	$ 9,997	$ 8,910	$ 13,296	$ 12,471	$ 19,243	$ 11,528	$ 7,851	$ 15,957	$ 44,674	$ 54,579	$ 35,427
Ide-cel Research and Development | Balances Without Adoption of Topic 606
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue											6,208
Bristol-Myers Squibb | Ide-cel Research and Development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue									2,264	5,751
Bristol-Myers Squibb | bb21217 research and development services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue									$ 2,163	$ 2,884
Bristol-Myers Squibb | bb21217 research and development services | Balances Without Adoption of Topic 606
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue											$ 721