Balance Sheet Components - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 30,268	$ 25,898
Less: accumulated depreciation and amortization	(16,816)	(14,334)
Property and equipment, net	13,452	11,564
Prototype
Property Plant And Equipment [Line Items]
Property and equipment, gross	12,425	11,929
Machinery and Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	11,684	7,831
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	4,327	4,438
Software
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,244	1,142
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 588	$ 558

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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