401(k) Savings plan	12 Months Ended
	Dec. 31, 2021
Retirement Benefits [Abstract]
401(k) Savings plan	401(k) Savings planPrior to the spin-off, all 2seventy employees were covered by bluebird bio’s defined-contribution savings plan under Section 401(k) of the Internal Revenue Code (“bluebird 401(k) Plan”), which was established in 1997. Upon spin-off, 2seventy bio established a defined-contribution savings plan under Section 401(k) of the Internal Revenue Code (“2seventy 401(k) Plan”), which covers all employees who meet defined minimum age and service requirements, including those who became employees of the Company, and allows participants to defer a portion of their annual compensation on a pretax basis. Expense related to the bluebird and 2seventy 401(k) Plans for 2seventy employees totaled $2.4 million, $2.2 million, $2.0 million for the years ended December 31, 2021, 2020, and 2019, respectively.