Discontinued operations - Narrative (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) $ / shares	Nov. 03, 2021 agreement	Oct. 19, 2021	Dec. 31, 2020 $ / shares
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Conversion ratio			0.3333
Common stock, par value (in dollars per share) | $ / shares	$ 0.01			$ 0.01
Number of transition services agreements | agreement		2
Initial term of transition services agreement		2 years
Selling, general and administrative expense related to transition services agreement	$ 0.7
Lentiviral Vector Manufacturing Facility
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net assets transferred to 2seventy bio	111.2
Loss on fixed assets disposal	$ 2.0
2seventy bio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Common stock, par value (in dollars per share) | $ / shares	$ 0.0001