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Note 4 - Investments in Debt Securities (Tables)
3 Months Ended
Mar. 31, 2021
Notes Tables  
Debt Securities, Available-for-sale [Table Text Block]
     
Amortized Cost
   
Fair Value
   
Unrealized gains
 
     
March 31,
   
December 31,
   
March 31,
   
December 31,
   
March 31,
   
December 31,
 
 
Longest
Maturity Date
 
2021
   
2020
   
2021
   
2020
   
2021
   
2020
 
                                                   
Corporate bonds
2024
  $
26,495
    $
26,222
    $
26,495
    $
26,338
    $
-
    $
116
 
Certificates of deposit
One-year
   
775
     
1,150
     
775
     
1,150
     
-
     
-
 
 
 
  $
27,270
    $
27,372
    $
27,270
    $
27,488
    $
-
    $
116