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Note 14 - Income Taxes (Details Textual) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
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 3,000,000  
Unrecognized Tax Benefits that Would Impact Effective Tax Rate $ 636,000 $ 511,000  
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent 21.00% 35.00%  
Domestic Tax Authority [Member] | Internal Revenue Service (IRS) [Member]      
Operating Loss Carryforwards, Total $ 69,200,000    
Domestic Tax Authority [Member] | Internal Revenue Service (IRS) [Member] | Research Tax Credit Carryforward [Member]      
Tax Credit Carryforward, Amount 2,000,000    
State and Local Jurisdiction [Member]      
Operating Loss Carryforwards, Total 54,400,000    
State and Local Jurisdiction [Member] | Research Tax Credit Carryforward [Member]      
Tax Credit Carryforward, Amount $ 1,500,000