Income Taxes - Temporary Differences (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Allowance for doubtful accounts	$ 0	$ 0	$ 18
Accrued expenses and other reserves	1,964	1,749	3,138
Tax credits, deferred R&D, and other	65	197	142
Net operating loss	10,793	8,610	9,239
Valuation allowance	(12,822)	(10,556)	(12,537)
Net deferred tax assets	$ 0	$ 0	$ 0