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Note 12 - Selected Statements of Income Data (Tables)
 12 Months Ended
Dec. 31, 2018
Notes Tables  
Interest and Other Income [Table Text Block]
   
Year ended December 31,
  
   
2016
    
2017
    
2018
  
                         
Interest income
   $
3,300
     $
4,233
     $
4,499
  
Loss on available-for-sale marketable securities, net
    
(9
 )    
—
      
(67
 )
Amortization of premium on available-for-sale marketable securities, net
    
(1,064
 )    
(1,179
 )    
(773
 )
Foreign exchange loss, net
    
(188
 )    
(28
 )    
(241
 )
Total
   $
2,039
     $
3,026
     $
3,418
  
Equity Securities without Readily Determinable Fair Value [Table Text Block]
Initial cost basis
   $
1,806
  
Downward adjustments
    
(870
 )
Total carrying value at the end of the period
   $
936