Summary of significant accounting policies and basis of presentation - Narrative (Detail) $ in Thousands	1 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2023 USD ($) segment shares	Dec. 31, 2022 USD ($) shares	Dec. 31, 2021 USD ($)	Nov. 04, 2021 USD ($)	Nov. 03, 2021 shares
Restricted Cash and Cash Equivalents Items [Line Items]
Transfers from bluebird bio		$ 0	$ 0	$ 354,889
Net assets					$ 384,500
Number of operating segments | segment		1
Accumulated goodwill impairment loss		$ 12,100
Interest income		$ 12,413	$ 2,932	88
Common stock, shares outstanding (in shares) | shares		50,632,000	37,928,000			0
Net cash provided by (used in) investing activities		$ 43,861	$ 3,753	(17,586)
Net cash used in operating activities		(166,858)	(228,140)	(206,855)
Restricted cash and cash equivalents included in restricted investments and other non-current assets		1,725	1,258	34
Adjustment
Restricted Cash and Cash Equivalents Items [Line Items]
Net cash provided by (used in) investing activities		8,000	(7,944)	(32,997)
Net cash used in operating activities		8,000	8,994	179
Restricted cash and cash equivalents included in restricted investments and other non-current assets			(31,768)	$ (32,818)
August 2023 Reduction
Restricted Cash and Cash Equivalents Items [Line Items]
Workforce reduction percentage	40.00%
Letter of credit
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and cash equivalents included in restricted investments and other non-current assets		$ 32,800	$ 30,000