Income Taxes - Temporary Differences (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Accrued expenses and other reserves	$ 1,505	$ 1,964	$ 1,749
Right-of-use-asset	(378)
Lease liabilities	461
Tax credits, deferred R&D, and other	44	65	197
Net operating loss	12,758	10,793	8,610
Valuation allowance	(14,390)	(12,822)	(10,556)
Net deferred tax assets	$ 0	$ 0	$ 0