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Note 2 - Investments (Tables)
6 Months Ended
Jun. 30, 2018
Notes Tables  
Available-for-sale Securities [Table Text Block]
   
June 30, 2018
   
December 31, 2017
 
   
Amortized
cost
   
Unrealized
loss
   
Fair value
   
Amortized
cost
   
Unrealized
loss
   
Fair value
 
Corporate bonds
  $
5,559
    $
(3
)
  $
5,556
    $
33,778
    $
(20
)
  $
33,758