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) - Celgene Corporation [Member] - USD ($) $ in Thousands		3 Months Ended
	Sep. 28, 2017	Mar. 31, 2019
Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front non-refundable payment		$ 75,000
Amended Celgene Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front non-refundable payment		25,000
Ide-cel License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, license fee		10,000
Vectors and Associated Payload [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated variable consideration		84,664
Ide-cel Transaction Price [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price		194,664
Transaction price unsatisfied		32,821
Ide-cel Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price		38,438
Ide-cel License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price		156,226
Transaction price unsatisfied		32,821
bb21217 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, license fee	$ 15,000	15,000
Estimated variable consideration		26,700
Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated variable consideration		26,687
bb21217 Transaction Price [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price		41,687
Transaction price unsatisfied		37,361
bb21217 Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price		5,444
Transaction price unsatisfied		1,118
bb21217 License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price		36,243
Transaction price unsatisfied		$ 36,243