Balance Sheet Components - Schedule of Accrued Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Accrued liabilities
Accrued payroll-related expenses	$ 1,011	$ 1,558
Accrued joint development agreement expenses	750	661
Accrued inventory	1,821	1,678
Deferred rent		390
Other	842	714
Total accrued liabilities	$ 4,424	$ 5,001