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	$ 0	$ 0
Deferred Tax Liabilities, Net, Total	0	0
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	3,200,000	3,200,000
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	753,000	$ 636,000
Domestic Tax Authority [Member] | Internal Revenue Service (IRS) [Member]
Operating Loss Carryforwards, Total	80,500,000
Domestic Tax Authority [Member] | Internal Revenue Service (IRS) [Member] | Research Tax Credit Carryforward [Member]
Tax Credit Carryforward, Amount	2,400,000
State and Local Jurisdiction [Member]
Operating Loss Carryforwards, Total	58,600,000
State and Local Jurisdiction [Member] | Research Tax Credit Carryforward [Member]
Tax Credit Carryforward, Amount	$ 1,800,000