Note 8 - Warrants (Tables)
 3 Months Ended
Mar. 31, 2020
Notes Tables  
Schedule of Stockholders' Equity Note, Warrants or Rights [Table Text Block]
   
Shares
    
Wtd. Avg.
Exercise
Price
  
Outstanding at December 31, 2019
    
3,959,005
     $
4.33
  
Exercised
    
(2,000
)
    
4.75
  
Outstanding at March 31, 2020
    
3,957,005
     $
4.33