Leases - Additional Information (Detail)			1 Months Ended		3 Months Ended		6 Months Ended
	Jan. 01, 2019 USD ($)	Jan. 17, 2018 USD ($) ft² Option	Jul. 31, 2016 USD ($)	Aug. 31, 2015 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)
Lessee Lease Description [Line Items]
Annual lease payments							$ 632,000
Rent expense						$ 200,000		$ 400,000
Meiji License Agreement [Member]
Lessee Lease Description [Line Items]
Embedded finance leases fixed assets related payments					$ 1,600,000
Finance lease liability							$ 0
Office Space [Member]
Lessee Lease Description [Line Items]
Operating lease beginning date				2016-01
Operating lease expiration date				2020-12
Annual lease payments				$ 400,000
Term of lease				5 years
Operating lease, existence of option to extend				true
Lease renewal term				5 years
Security deposit				$ 200,000
Annual rent escalations and tenant incentives				700,000
Amount reimbursed				$ 300,000
Operating lease decrease in right of use assets	$ 200,000
Office Space [Member] | Lease Agreements [Member] | Cambridge, Massachusetts [Member] | U.S. REIF Central Plaza Massachusetts, LLC [Memebr]
Lessee Lease Description [Line Items]
Term of lease		7 years
Operating lease, existence of option to extend		true
Lease renewal term		5 years
Operating lease decrease in right of use assets	$ 400,000
Lease expiration date		Dec. 22, 2018
Area of office space | ft²		7,800
Number of renewal options | Option		2
Operating lease, option to extend							the Company has two consecutive options to extend the Lease Term for an additional period of five years (the “Option Terms”), subject to certain conditions, upon notice to the Landlord.
Operating leases annual base rent initial		$ 500,000
Operating leases annual base rent final		$ 600,000
Operating lease percentage of annual base rent		100.00%
Leasehold improvements receivable from the Landlord		$ 400,000
Laboratory Space [Member]
Lessee Lease Description [Line Items]
Annual lease payments			$ 300,000
Lease expiration date							Nov. 30, 2019