Description of the business - Narrative (Detail) $ / shares in Units, $ in Thousands				1 Months Ended		12 Months Ended
	Mar. 13, 2023 USD ($) $ / shares shares	Jan. 06, 2023 USD ($) shares	Dec. 13, 2021 shares	Nov. 30, 2022 USD ($)	Aug. 31, 2018 USD ($) $ / shares	Dec. 31, 2022 USD ($) employee	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2021
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Conversion ratio						0.3333			0.3333
Net loss						$ 254,153	$ 292,213	$ 120,114
Net cash used in operating activities						237,134	$ 207,034	$ 67,793
Cash, cash equivalents, and marketable securities						$ 267,700
ATM Facility | Cowen and Company, LLC
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Aggregate gross proceeds (up to)				$ 150,000
Regeneron Collaboration Agreement | Share purchase agreement
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Issuance of common stock to Regeneron (in shares) | shares			100,000
Gross proceeds from private placement offering					$ 100,000
Price per shares (in dollars per share) | $ / shares					$ 238.10
Subsequent Event | Underwritten Public Offering
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Shares issued in underwritten public offering (in shares) | shares	10,900,000
Price per shares (in dollars per share) | $ / shares	$ 11.50
Gross proceeds from public offering	$ 125,000
Additional shares available for purchase (in shares) | shares	1,630,434
Subsequent Event | Regeneron Collaboration Agreement | Share purchase agreement
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Issuance of common stock to Regeneron (in shares) | shares		1,114,827
Gross proceeds from private placement offering		$ 20,000
2seventy bio Securities Corporation
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of employees | employee						0