Property and Equipment - Summary of Property and Equipment (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	$ 17,629	$ 17,499
Accumulated depreciation and amortization	(11,706)	(11,162)
Property and equipment, net	5,923	6,337
Machinery, Manufacturing and Demonstration Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	12,425	12,320
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	2,297	2,260
Furniture [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	635	639
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	$ 2,272	$ 2,280