Property and Equipment - Summary of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	$ 15,888	$ 12,345
Accumulated depreciation and amortization	(9,218)	(6,573)
Property and equipment, net	6,670	5,772
Machinery, Manufacturing and Demonstration Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	10,866	7,579
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	2,187	2,124
Furniture [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	598	614
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	$ 2,237	$ 2,028