Balance Sheet Account Detail - Schedule of Property and Equipment Net of Accumulated Depreciation (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, at cost	$ 6,865	$ 6,780
(Less): Accumulated depreciation	(6,342)	(6,191)
Property and equipment, net of accumulated depreciation	523	589
Computer hardware and software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, at cost	3,080	2,994
Laboratory equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, at cost	635	630
Office furniture [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, at cost	212	218
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, at cost	$ 2,938	$ 2,938