Income Taxes - Temporary Differences (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Allowance for doubtful accounts	$ 0	$ 18	$ 18
Accrued expenses and other reserves	1,749	3,138	2,244
Tax credits, deferred R&D, and other	197	142	122
Net operating loss	8,610	9,239	5,384
Valuation allowance	(10,556)	(12,537)	(7,768)
Net deferred tax assets	$ 0	$ 0	$ 0