Property and Equipment, Net - Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,941	$ 9,699
Less accumulated depreciation and amortization	764	8,085
Property and equipment, net	1,177	1,614
Computer and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	158	149
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	628	795
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	27	173
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,128	$ 8,582