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Note 10 - Income Taxes (Details Textual) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Income Tax Expense (Benefit), Total $ 32 $ 52  
Operating Loss Carryforwards, Total     $ 260,598
Operating Loss Carryforwards, Subject to Expiration     $ 227,781
Operating Loss Carry Forwards Annual Limit 14,284    
Unrecognized Tax Benefits, Ending Balance 0    
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued, Total $ 0    
Rights [Member]      
Term of Extended Rights Plan (Year) 3 years    
Threshold Percentage of Beneficial Ownership for Significant Dilution in Ownership Interest 4.90%    
Class of Warrant or Right Number of Rights Per Common Stock Share 1    
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right (in shares) 0.001    
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) $ 4.25    
Class of Warrant or Right Current Beneficial Ownership Percentage That Will Not Trigger Preferred Share Purchase Rights 4.90%