Leases - Additional Information (Detail) $ in Thousands	1 Months Ended	9 Months Ended
	Mar. 31, 2021 USD ($) USD_per_sqft	Sep. 30, 2021 USD ($) ft² USD_per_sqft	Dec. 31, 2020 USD ($)
Lessee Lease Description [Line Items]
Operating lease right-of-use assets		$ 81,788	$ 39,114
Albany Lease [Member]
Lessee Lease Description [Line Items]
Area of space leased | ft²		39,000
Operating lease, description		In March 2020, the Company entered into an agreement to lease approximately 39,000 square feet of office and laboratory space at 281 Albany Street in Cambridge, Massachusetts under an operating lease agreement (the “281 Albany Lease”). The Company’s obligation to pay rent will start on the date that is six months after the commencement date or the date on which the Company occupies the premises, whichever occurs earlier (the “Rent Commencement Date”).
Term of lease		10 years
Base rent per square foot for first year | USD_per_sqft	99.00
Base rent per square foot for last year | USD_per_sqft	128.87
Operating lease right-of-use assets	$ 40,400
Operating lease, liability	$ 34,800
Incremental borrowing rate	5.52%
Prepaid lease payments	$ 5,600
Amount receivable on cost of construction and tenant improvement	4,400
Operating lease, existence of option to extend		true
Operating lease, options to extend		The Company has the option to extend the 281 Albany Lease for two successive five-year terms.
Albany Lease [Member] | Other Assets [Member]
Lessee Lease Description [Line Items]
Letter of credit	$ 1,900
Tudor Lease [Member]
Lessee Lease Description [Line Items]
Area of space leased | ft²		13,662
Operating lease, description		In July 2021, the Company entered into an agreement to lease 13,662 square feet of office space at 17 Tudor Street in Cambridge, Massachusetts under an operating lease agreement (the “17 Tudor Lease”). The Company’s obligation to pay rent will start on November 1, 2021.
Term of lease		5 years
Base rent per square foot for first year | USD_per_sqft		74.00
Base rent per square foot for last year | USD_per_sqft		83.29
Operating lease right-of-use assets		$ 4,900
Operating lease, liability		$ 4,900
Incremental borrowing rate		4.15%
Operating lease, options to extend		The initial term of the 17 Tudor Lease is five years and the Company has an option to extend the 17 Tudor Lease for one three-year term
Tudor Lease [Member] | Other Assets [Member]
Lessee Lease Description [Line Items]
Letter of credit		$ 200
Waltham Lease [Member]
Lessee Lease Description [Line Items]
Operating lease right-of-use assets		$ 2,500