Property and Equipment - Summary of Property and Equipment (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	$ 16,231	$ 15,888
Accumulated depreciation and amortization	(9,825)	(9,218)
Property and equipment, net	6,406	6,670
Machinery, Manufacturing and Demonstration Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	11,189	10,866
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	2,191	2,187
Furniture [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	603	598
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	$ 2,248	$ 2,237