Leases - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended				3 Months Ended		9 Months Ended
	Sep. 30, 2021	Apr. 30, 2021	Feb. 28, 2021	Jul. 31, 2019	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Sep. 30, 2019
Leases [Line Items]
Lease expiration month and year							2022-03
Short-term lease expenses					$ 400	$ 100	$ 600	$ 300
Non-lease costs					16	17	49	45
ROU asset	$ 31,487				31,487		31,487		$ 8,692
Lease liability	$ 31,350				31,350		31,350
Operating lease costs					2,000	900	4,400	2,700
Cash paid for operating lease					1,300	900	3,600	2,700
Variable lease costs					$ 5,300	$ 1,200	$ 9,100	$ 3,700
Operating lease, weighted average remaining lease term	7 years 3 months 3 days				7 years 3 months 3 days		7 years 3 months 3 days
Operating lease, weighted average discount rate, percent	4.95%				4.95%		4.95%
DMS Agreement
Leases [Line Items]
Lease expiration month and year				2021-07
Lease expense recognized					$ 500		$ 700
Lease effective date		2021-04		2019-07
Term of agreement				2 years
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.
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				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
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 September 30, 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
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