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Note 4 - Investments in Debt Securities (Tables)
 12 Months Ended
Dec. 31, 2019
Notes Tables  
Debt Securities, Available-for-sale [Table Text Block]
       
Amortized Cost
    
Fair Value
  
       
 
 
 
  
December 31,
    
 
 
 
   
Longest
Maturity Date
  
2019
    
2018
    
2019
    
2018
  
                                     
U.S. government and agency securities
 
2022
   $
1,612
     $
-
     $
1,612
     $
-
  
Corporate bonds
  
2022
    
7,475
      
-
      
7,477
      
-
  
Certificates of deposit
  
One-year
    
2,400
      
-
      
2,400
      
-
  
   
 
   $
11,487
     $
-
     $
11,489
     $
-