Balance Sheet Components - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 15,889	$ 12,736
Less: accumulated depreciation and amortization	(10,593)	(9,805)
Property and equipment, net	5,296	2,931
Prototype
Property Plant And Equipment [Line Items]
Property and equipment, gross	6,471	6,342
Machine and Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	5,605	4,214
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,274	1,270
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	347	263
Software
Property Plant And Equipment [Line Items]
Property and equipment, gross	790	$ 647
Construction in Progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 1,402