Collaborative arrangements and strategic partnerships - Collaborative Arrangement Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative arrangement revenue													$ 71,601	$ 32,358	$ 26,921
Share of collaboration loss													0	(9,642)	(10,071)
ABECMA Commercial Activities
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative arrangement revenue	$ 1,970	$ 536	$ 24,543	$ 22,961	$ 8,717	$ 4,064	$ 0	$ 0	$ 8,818	$ 10,607	$ 0	$ 0	50,010	12,781	19,425
Share of collaboration loss	0	0	0	0	0	0	(4,290)	(5,352)	0	0	(10,071)	0	0	(9,642)	(10,071)
ABECMA Commercial Activities | Bristol-Myers Squibb
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement of costs	604	1,118	1,271	1,380	1,431	1,215	1,641	1,357	917	845	1,695	0	4,373	5,644	3,457
Share of collaboration loss	$ 1,366	$ (582)	$ 23,272	$ 21,581	$ 7,286	$ 2,849	$ (5,931)	$ (6,709)	$ 7,901	$ 9,762	$ (11,766)	$ 0	$ 45,637	$ (2,505)	$ 5,897