Leases - Additional Information (Detail) ft² in Thousands, $ in Thousands		1 Months Ended	9 Months Ended
	Mar. 13, 2019 USD ($) m² ft²	Aug. 31, 2017 USD ($) ft²	Sep. 30, 2019 USD ($)	Jan. 01, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Right-of-use assets			$ 2,110
Operating lease liabilities			3,025
Lease liability, net of current portion			2,147
Current portion of lease liability			$ 878
Period for written notice for cancellable	3 months
Cambridge Lease Agreement
Lessee, Lease, Description [Line Items]
Current office space under lease agreement | ft²		13,000
Option to extend the lease agreement			The Company has the option to extend the lease for one period of five additional years.
Lease, renewal term of contract		5 years
Base rent		$ 825
Waltham Lease
Lessee, Lease, Description [Line Items]
Current office space under lease agreement | ft²	6,000
Base rent	$ 262
Operating lease expiration	5 years
Vienna Lease
Lessee, Lease, Description [Line Items]
Current office space under lease agreement | m²	400
Base rent	$ 155
Lease, term of contract	2 years
Period for written notice for cancellable	3 months 19 days
Accounting Standards Update 2016-02
Lessee, Lease, Description [Line Items]
Right-of-use assets				$ 2,000
Operating lease liabilities				2,500
Lease liability, net of current portion				1,900
Current portion of lease liability				613
Net deferred, accrued and prepaid rent				$ 512