Balance Sheet Components - Property and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 6,928	$ 6,928
Less: accumulated depreciation and amortization	(6,384)	(6,324)
Property and equipment, net	544	604
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	3,145	3,145
Research Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	3,219	3,219
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	350	350
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 214	$ 214

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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