Income Taxes - Deferred Income Taxes Components (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforward	$ 41,086	$ 42,013
Property & equipment	524	690
Inventory	58	1,735
Stock options	8,195	7,208
Intangible assets, definite-lived	42	220
General business credit	6,651	3,984
Deferred revenue	253	370
Other	45	121
Valuation allowance	(56,854)	(56,341)
Deferred tax assets	$ 0	$ 0