Leases - Additional Information (Details) - USD ($)		1 Months Ended					12 Months Ended
	Jan. 31, 2022	Sep. 30, 2021	Apr. 30, 2021	Feb. 28, 2021	Sep. 30, 2019	Jul. 31, 2019	Dec. 31, 2021	Dec. 31, 2020	Mar. 31, 2019
Leases [Line Items]
Lease expiration month and year							2022-09
Short-term lease expenses							$ 400,000	$ 400,000
Non-lease costs							37,500	58,200
ROU asset							29,743,000	8,692,000
Lease liability							30,066,000
Operating lease costs							6,200,000	4,000,000.0
Cash paid for operating lease							5,200,000	3,600,000
Variable non-lease costs							$ 14,500,000	4,500,000
Operating lease, weighted average remaining lease term							7 years 2 months 1 day
Operating lease, weighted average discount rate, percent							4.96%
Subsequent Event
Leases [Line Items]
Lease termination date	Jan. 31, 2022
DMS Agreement
Leases [Line Items]
Lease expiration month and year						2021-07
Lease effective date			2021-04			2019-07
Lease termination description							On July 30, 2021, the Company provided a notification to reduce capacity to the third-party supplier which terminates this lease agreement in the first quarter of 2022.
Lease expense recognized							$ 500,000	$ 0
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 December 31, 2021, the Company has no reasonable certainty that this option to extend will be exercised.
Lease commencement month and year		2021-09
Additional one time lease period		5 years
Tenant improvement allowance to be reimbursed		$ 3,100,000
Tenant improvement allowance, reimbursement period		2 years
Tenant improvement allowance							$ 0
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