Property and Equipment - Summary of Property and Equipment (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 17,787	$ 17,499
Accumulated depreciation and amortization	(12,005)	(11,162)
Property and equipment, net	5,782	6,337
Machinery, manufacturing and demonstration equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	12,565	12,320
Computer equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,297	2,260
Furniture
Property, Plant and Equipment [Line Items]
Property and equipment, gross	638	639
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 2,287	$ 2,280