Discontinued operations - Narrative (Details) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2022 USD ($) $ / shares	Dec. 31, 2021 USD ($) $ / shares	Nov. 04, 2021 $ / shares	Nov. 03, 2021 agreement	Sep. 30, 2021 USD ($)
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	$ 0.01
Number of transition services agreements | agreement				2
Initial term of transition services agreement				2 years
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
Selling, general and administrative expense related to transition services agreement	$ 3.1
Accounts receivable due from related party	3.0
Accounts payable to related party	5.3
2seventy bio | 60 Binney Street Lease
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Related party expense	$ 1.1