Leases - Additional Information (Details) - USD ($)	1 Months Ended					3 Months Ended
	Feb. 28, 2022	Sep. 30, 2021	Feb. 28, 2021	Sep. 30, 2019	Jul. 31, 2019	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021	Mar. 31, 2019
Leases [Line Items]
ROU asset						$ 21,735,000		$ 29,743,000
Lease liability						23,132,000
Operating lease costs						1,600,000	$ 1,300,000
Cash paid for operating lease						1,200,000	1,100,000
Variable lease costs						$ 1,200,000	$ 900,000
Operating lease, weighted average remaining lease term						8 years 25 days
Operating lease, weighted average discount rate, percent						5.02%
DMS Agreement
Leases [Line Items]
Lease expiration month and year					2021-07
Term of agreement					2 years
Operating lease, existence of option to extend					true
Lease, option to extend						The term may be extended by agreement of the parties for additional two-year periods upon written notice to the supplier at least 30 days prior to expiration of the then-current term.
Operating lease, existence of option to terminate					true
Lease, option to terminate						The DMS Agreement (or any individual SOW) may be terminated earlier by AlloVir at any time by providing 190 days’ notice.
Lease renewal term				2 years
Estimated lease term				3 years 9 months	4 years 3 months
Estimated lease expiring month and year				2023-07	2023-07
ROU asset				$ 6,300,000					$ 6,900,000
Lease liability				$ 6,300,000					$ 6,900,000
2020 DMS Agreement
Leases [Line Items]
Term of agreement			1 year
Lease renewal term			1 year
Estimated lease term			2 years
Estimated lease expiring month and year			2023-01
ROU asset			$ 3,100,000
Lease liability			2,500,000
Utilization fee			$ 3,200,000
Lessee operating lease, lease term including renewal option			2 years
Prepaid utilization fee			$ 600,000
Waltham Leases
Leases [Line Items]
Expiration date		Jul. 30, 2030
Operating lease, existence of option to extend		true
Lease, option to extend						The Company has the option to renew the leased space for an additional one time period of five years with written notice from the Company. As of March 31, 2022, the Company has no reasonable certainty that this option to extend will be exercised.
Tenant improvement allowance to be reimbursed		$ 3,100,000
Tenant improvement allowance, reimbursement period		2 years
Tenant improvement allowance						$ 0
Lease commencement month and year		2021-09
Additional one time lease period		5 years
New Lease Agreement with BP Bay Colony LLC
Leases [Line Items]
ROU asset		$ 6,000,000.0
Lease liability		6,000,000.0
Sublease with AMAG Pharmaceuticals
Leases [Line Items]
ROU asset		17,300,000
Lease liability		$ 17,300,000
Original SOW Amendment under DMS Agreement | Lease Modification under ASC 842
Leases [Line Items]
ROU asset	$ (1,400,000)
Lease liability	$ (1,400,000)
Second SOW Amendment under DMS Agreement
Leases [Line Items]
Lease expiration month and year	2022-02
Second SOW Amendment under DMS Agreement | Lease Modification under ASC 842
Leases [Line Items]
ROU asset	$ (2,600,000)
Lease liability	(2,600,000)
Change Order to Modify Terms of SOW under 2020 DMS Agreement | Lease Modification under ASC 842
Leases [Line Items]
ROU asset	(1,500,000)
Lease liability	$ (1,300,000)