Leases - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended
	May 31, 2020	Mar. 31, 2019	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Sep. 30, 2019
Leases [Line Items]
Lease expiration month and year			2021-09
Short-term lease expenses			$ 100	$ 100
Non-lease costs			17	15
ROU asset			10,748		$ 8,692
Lease liability			10,197
Operating lease costs			1,300	1,000
Cash paid for operating lease			1,100	1,200
Variable non-lease costs			$ 900	$ 800
Operating lease, weighted average remaining lease term			2 years 2 months 19 days
Operating lease, weighted average discount rate, percent			4.76%
DMS Agreement
Leases [Line Items]
Term of agreement		2 years
Expiration date		Jul. 31, 2021
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
Lease liability		$ 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