401(k) Savings plan - Additional Information (Detail) - 401 (k) - USD ($) $ in Millions	1 Months Ended		12 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Defined Contribution Plan Disclosure [Line Items]
Contributions to the 401(k) plan		$ 6.0
Expenses related to 401(k) plan			$ 2.8	$ 3.0	$ 2.9
Forecast
Defined Contribution Plan Disclosure [Line Items]
Contributions to the 401(k) plan	$ 2.8
Forecast | 2seventy bio
Defined Contribution Plan Disclosure [Line Items]
Contributions to the 401(k) plan	$ 2.0