Leases - Additional Information (Details) - USD ($)		1 Months Ended	12 Months Ended
	Mar. 26, 2019	May 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Mar. 31, 2019
Leases [Line Items]
Lease expiration month and year			2021-03
Short-term lease expenses			$ 400,000	$ 300,000
Non-lease costs			100,000	43,000
ROU asset			8,692,000	11,759,000
Lease liability			8,692,000
Operating lease costs			4,000,000	2,100,000
Cash paid for operating lease			3,600,000
Variable non-lease costs			$ 4,500,000	$ 1,800,000
Operating lease, weighted average remaining lease term			2 years 6 months 29 days
Operating lease, weighted average discount rate, percent			5.14%
DMS Agreement
Leases [Line Items]
Term of agreement	2 years
Expiration date	Jul. 19, 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						$ 6,900,000
Lease liability						$ 6,900,000
2020 DMS Agreement
Leases [Line Items]
Reservation fee		$ 3,000,000
Lease not yet commenced, lease term including renewal option		2 years
Lease not yet commenced, term of contract		1 year
Lease not yet commenced, renewal term		1 year
Prepaid reservation fee			$ 600,000