Component of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	$ 4,842	$ 4,105
Less: accumulated depreciation	(3,631)	(3,262)
Property and equipment, net	1,211	843
Office and computer equipment
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	1,215	1,162
Software
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	913	907
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	1,553	1,158
Research and development equipment
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	$ 1,161	$ 878