Note 14 - Income Taxes (Details Textual) - USD ($) xbrli-pure in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Expense (Benefit), Total	$ 0
Effective Income Tax Rate Reconciliation, Percent, Total	0.00%	0.00%
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	$ 4,352,000	$ (16,506,000)
Deferred Tax Assets, Tax Credit Carryforwards, Research	631,000	527,000
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued, Total	0
Unrecognized Tax Benefits, Ending Balance	245,000	$ 223,000	$ 450,000
Domestic Tax Authority [Member]
Operating Loss Carryforwards, Total	95,395,000
State and Local Jurisdiction [Member]
Operating Loss Carryforwards, Total	20,239,000
State and Local Jurisdiction [Member] | California Franchise Tax Board [Member]
Deferred Tax Assets, Tax Credit Carryforwards, Research	$ 817,000