Leases - Narrative (Details) $ in Thousands	1 Months Ended
	Aug. 31, 2024 ft²	Jul. 31, 2021 tradingDay	Dec. 31, 2024 USD ($)	Sep. 30, 2024 ft²	Dec. 31, 2023 USD ($)	Nov. 30, 2023	Nov. 30, 2021
Lessee, Lease, Description [Line Items]
Termination notice period						7 months
Restricted cash			$ 14,062		$ 13,496
Operating lease right-of-use assets			47,156		13,941
Total operating lease liability			40,794
Prepaid expenses			(6,777)		(19,057)
Total payments under dispute			10,800
38 Sidney Street, Cambridge, Massachusetts Lease
Lessee, Lease, Description [Line Items]
Number of renewal options | tradingDay		1
Operating lease, extension (in years)		6 months
64 Sidney Street, Cambridge, Massachusetts Lease
Lessee, Lease, Description [Line Items]
Number of renewal options | tradingDay		1
Operating lease, extension (in years)		6 months
60 First Street, Cambridge, Massachusetts Lease
Lessee, Lease, Description [Line Items]
Operating lease, extension (in years)							10 years
Term of contract, lease not yet commenced (in years)							10 years
Restricted cash			13,100		$ 13,100
Operating lease right-of-use assets			44,900
Total operating lease liability			33,600
60 First Street, Cambridge, Massachusetts Lease | Revision of Prior Period, Reclassification, Adjustment
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets			11,300
Prepaid expenses			$ 11,300
480 Arsenal Amendment
Lessee, Lease, Description [Line Items]
Area of operating lease | ft²	16,000
500 Arsenal Lease
Lessee, Lease, Description [Line Items]
Term of contract, lease not yet commenced (in years)			11 months
Restricted cash			$ 600
Area of operating lease | ft²				48,500
Tenant improvements			2,400
Expansion Space
Lessee, Lease, Description [Line Items]
Area of operating lease | ft²	9,400
Expansion Space, Termination Period One
Lessee, Lease, Description [Line Items]
Termination period	150 days
Expansion Space, Termination Period Two
Lessee, Lease, Description [Line Items]
Termination period	30 days
500 Arsenal Lease, Additional Tenant Improvement Allowance
Lessee, Lease, Description [Line Items]
Tenant improvements			$ 1,200