Collaborative arrangements - Summary of Total Transaction Price, Allocation of Total Transaction Price to Identified Performance Obligations Under Arrangement and Amount of Transaction Price Unsatisfied (Detail) - Bristol-Myers Squibb - USD ($) $ in Thousands		3 Months Ended
	Sep. 28, 2017	Mar. 31, 2020
Collaborative Arrangement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Up-front non-refundable payment		$ 75,000
Amended BMS Collaboration Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Up-front non-refundable payment		25,000
Ide-cel License Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, license fee		10,000
Ide-cel Manufacturing Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price		166,277
Transaction price unsatisfied		13,362
Vectors and Associated Payload
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Estimated variable consideration		87,189
Ide-cel Transaction Price
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price		207,189
Transaction price unsatisfied		13,362
Ide-cel Research and Development Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price		40,912
Transaction price unsatisfied		0
bb21217 License Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, license fee	$ 15,000	15,000
Estimated variable consideration		26,700
Manufacturing Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Estimated variable consideration		26,687
bb21217 Transaction Price
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price		41,687
Transaction price unsatisfied		36,243
bb21217 Research and Development Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price		5,444
Transaction price unsatisfied		0
bb21217 license and manufacturing services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price		36,243
Transaction price unsatisfied		$ 36,243