401(k) Savings plan	12 Months Ended
	Dec. 31, 2021
Retirement Benefits [Abstract]
401(k) Savings plan	401(k) Savings planIn 1997, the Company established a defined-contribution savings plan under Section 401(k) of the Internal Revenue Code (the "401(k) Plan"). The 401(k) Plan covers all employees who meet defined minimum age and service requirements, and allows participants to defer a portion of their annual compensation on a pretax basis. In March 2022, the Company expects to make matching contributions of approximately $2.8 million related to employee contributions made during 2021. At the same time, the Company anticipates making matching contributions to 2seventy bio employees of $2.0 million for their contributions under the 401(k) Plan during 2021 through the Separation. In March 2021, the Company made $6.0 million of matching contributions related to employee contributions made during 2020. The match contribution is included in accrued expenses and other current liabilities as of December 31, 2021 and 2020.  Expense related to the 401(k) Plan from continuing operations totaled $2.8 million, $3.0 million, $2.9 million for the years ended December 31, 2021, 2020, and 2019, respectively.