Leases - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended					3 Months Ended		9 Months Ended
	Feb. 28, 2022	Sep. 30, 2021	Feb. 28, 2021	Sep. 30, 2019	Jul. 31, 2019	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021	Mar. 31, 2019
Leases [Line Items]
ROU asset						$ 19,127		$ 19,127		$ 29,743
Lease liability						21,671		21,671
Operating lease costs						1,200	$ 2,000	4,100	$ 4,400
Cash paid for operating lease						1,100	1,300	3,200	3,600
Variable lease costs						$ 1,500	5,300	$ 3,600	9,100
Operating lease, weighted average remaining lease term						7 years 9 months 29 days		7 years 9 months 29 days
Operating lease, weighted average discount rate, percent						5.00%		5.00%
DMS Agreement
Leases [Line Items]
Lease expiration month and year					2021-07
Term of agreement					2 years
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. The DMS Agreement (or any individual SOW) may be terminated earlier by AlloVir at any time by providing 190 days’ notice.
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							$ 6,900
Lease liability				$ 6,300							$ 6,900
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
Lease liability			2,500
Utilization fee			$ 3,200
Lessee operating lease, lease term including renewal option			2 years
Prepaid utilization fee			$ 600
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. At September 30, 2022, the Company has no reasonable certainty that this option to extend will be exercised.
Tenant improvement allowance to be reimbursed		$ 3,100					3,100		3,100
Tenant improvement allowance, reimbursement period		2 years
Tenant improvement allowance						$ 700		$ 700
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					6,000		6,000
Lease liability		6,000					6,000		6,000
Sublease with AMAG Pharmaceuticals
Leases [Line Items]
ROU asset		17,300					17,300		17,300
Lease liability		$ 17,300					$ 17,300		$ 17,300
Original SOW Amendment under DMS Agreement | Lease Modification under ASC 842
Leases [Line Items]
ROU asset	$ (1,400)
Lease liability	$ (1,400)
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)
Lease liability	(2,600)
Change Order to Modify Terms of SOW under 2020 DMS Agreement | Lease Modification under ASC 842
Leases [Line Items]
ROU asset	(1,500)
Lease liability	$ (1,300)