Balance Sheet Components - Schedule of Accrued Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accrued liabilities
Accrued payroll-related expenses	$ 1,367	$ 1,558
Accrued joint development agreement expenses	675	661
Accrued inventory	632	1,678
Deferred rent		390
Other	493	714
Total accrued liabilities	$ 3,167	$ 5,001