Note 13 - Income Taxes (Details Textual) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Expense (Benefit), Total	$ 0
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	(16,506,000)	$ 11,667,000
Deferred Tax Assets, Tax Credit Carryforwards, Research	527,000	951,000
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued, Total	0
Unrecognized Tax Benefits, Ending Balance	223,000	$ 450,000	$ 397,000
Domestic Tax Authority [Member]
Operating Loss Carryforwards, Total	77,333,000
State and Local Jurisdiction [Member]
Operating Loss Carryforwards, Total	6,171,000
State and Local Jurisdiction [Member] | California Franchise Tax Board [Member]
Deferred Tax Assets, Tax Credit Carryforwards, Research	$ 742,000