Leases - Additional Information (Details) - USD ($)		1 Months Ended			3 Months Ended		6 Months Ended
	Jul. 30, 2021	Apr. 30, 2021	Feb. 28, 2021	Jul. 31, 2019	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Sep. 30, 2019
Leases [Line Items]
Lease expiration month and year							2022-03
Short-term lease expenses					$ 100,000	$ 100,000	$ 200,000	$ 200,000
Non-lease costs					16,000	13,000	33,000	28,000
ROU asset					9,559,000		9,559,000		$ 8,692,000
Lease liability					9,164,000		9,164,000
Operating lease costs					1,300,000	900,000	2,500,000	1,800,000
Cash paid for operating lease					1,100,000	900,000	2,300,000	1,800,000
Variable non-lease costs					$ 2,200,000	$ 1,600,000	$ 3,800,000	$ 2,700,000
Operating lease, weighted average remaining lease term					1 year 11 months 12 days		1 year 11 months 12 days
Operating lease, weighted average discount rate, percent					4.76%		4.76%
DMS Agreement
Leases [Line Items]
Lease expiration month and year				2021-07
Lease expense recognized					$ 0		$ 0
Lease effective date		2021-04		2019-07
Term of agreement				2 years
Expiration date	Feb. 05, 2022
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						2 years
Estimated lease term				4 years 3 months
Estimated lease expiring month and year				2023-07
ROU asset				$ 6,900,000
Lease liability				$ 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