Note 11 - Income Taxes - Summary of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Tax Assets, Operating Loss Carryforwards	$ 2,651	$ 42,156	$ 34,488
Federal and state tax credits	2,244	21,223	16,404
Accrued expenses	399	544	27
Patents	191	360	443
Stock-based compensation	1,262	1,353	596
Other	202	320	276
	6,949	65,956	52,234
Valuation allowance	(6,949)	(65,956)	(52,234)
Net deferred tax asset	$ 0	$ 0	$ 0