Balance Sheet Accounts and Supplemental Disclosures - Property and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 5,159	$ 4,643
Less: accumulated depreciation and amortization	(4,073)	(3,978)
Total property and equipment, net	1,086	665
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	4,110	3,687
Office furniture and equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	657	605
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 392	$ 351

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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