Leases - Additional Information (Detail)			3 Months Ended
	Sep. 30, 2016 ft²	Dec. 31, 2015 ft²	Mar. 31, 2019 USD ($)	Jan. 01, 2019 USD ($)
Lessee Lease Description [Line Items]
Right-of-use asset, net			$ 1,208,000	$ 1,300,000
lease liability			$ 1,400,000	$ 1,300,000
Lease option to extend			true
Lease option to termination			true
Operating lease discount rate				14.00%
Operating lease expense			$ 113,000
Lease payments			$ 140,000
Waltham, Massachusetts [Member]
Lessee Lease Description [Line Items]
Operating lease term of contract	5 years
Area of leased property | ft²	12,207
Lease Commencement Date	Mar. 01, 2017
Remaining lease term			35 months
New York [Member]
Lessee Lease Description [Line Items]
Operating lease term of contract		62 months
Area of leased property | ft²		4,039
Lease Commencement Date		Jan. 01, 2016
Remaining lease term			23 months