Leases - Additional Information (Details) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended
	Jul. 31, 2017	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Lessee Lease Description [Line Items]
Right-of-use assets		$ 13,544,000		$ 13,544,000		$ 14,453,000
Liabilities under lease		13,097,000		13,097,000
Operating lease liability, current		1,000,000		1,000,000		880,000
Operating lease liability, net of current		12,097,000		12,097,000		12,869,000
Right-of-use amortization				910,000	$ 850,000
Watertown Lease
Lessee Lease Description [Line Items]
Operating lease costs		637,000	$ 637,000	1,912,000	1,912,000
Costs for leasehold improvements				0	500,000
Right-of-use assets		13,500,000		13,500,000		14,500,000
Liabilities under lease		13,100,000		13,100,000		13,800,000
Operating lease liability, current		1,000,000		1,000,000		900,000
Operating lease liability, net of current		12,100,000		12,100,000		$ 12,900,000
Right-of-use amortization				900,000	900,000
Watertown Lease | Restricted Cash
Lessee Lease Description [Line Items]
Lease collateral amount		$ 2,600,000		$ 2,600,000
Watertown Lease | Office and Laboratory Space
Lessee Lease Description [Line Items]
Lease commencement date	2018-04
Lessee operating lease, description				In July 2017, the Company entered into a lease of office and laboratory space for its headquarters at 490 Arsenal Way in Watertown, Massachusetts (the “Watertown Lease”). The Watertown Lease commenced in April 2018 with rent commencing in May 2018
Rent commencing date	2018-05
Operating lease costs				$ 3,100,000	$ 3,000,000