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Note 6 - Marketable Securities (Tables)
 9 Months Ended
Sep. 30, 2019
Notes Tables  
Schedule of Available-for-sale Securities Reconciliation [Table Text Block]
   
September 30, 2019 (Unaudited)
  
   
Amortized

cost
    
Gross
unrealized

gains
    
Gross
unrealized

losses
    
Fair

value
  
Available-for-sale - matures within one year:
    
 
 
 
  
 
 
 
  
 
 
 
Corporate bonds
   $
17,245
     $
13
     $
(30
 )   $
17,228
  
     
17,245
      
13
      
(30
 )    
17,228
  
Available-for-sale - matures after one year through five years:
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
 
Government bonds
    
501
      
—
      
(1
 )    
500
  
Corporate bonds
    
52,770
      
162
      
(198
 )    
52,734
  
     
53,271
      
162
      
(199
 )    
53,234
  
                                 
Total
   $
70,516
     $
175
     $
(229
 )   $
70,462
  
   
December 31, 2018
 
   
Amortized

cost
    
Gross
unrealized

gains
    
Gross
unrealized

losses
    
Fair

value
  
Available-for-sale - matures within one year:
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
 
Corporate bonds
   $
6,094
     $
—
     $
(32
 )   $
6,062
  
     
6,094
      
—
      
(32
 )    
6,062
  
Available-for-sale - matures after one year through five years:
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
 
Certificate of deposits
    
747
      
—
      
—
      
747
  
Government bonds
    
501
      
—
      
(5
 )    
496
  
Corporate bonds
    
71,350
      
134
      
(1,320
 )    
70,164
  
     
72,598
      
134
      
(1,325
 )    
71,407
  
                                 
Total
   $
78,692
     $
134
     $
(1,357
 )   $
77,469
  
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Table Text Block]
   
Less than 12 months
    
12 months or greater
  
   
Fair value
    
Gross
unrealized loss
    
Fair value
    
Gross
unrealized loss
  
As of September 30, 2019 (unaudited)
   $
12,974
     $
(110
 )   $
30,835
     $
(119
 )
As of December 31, 2018
   $
16,580
     $
(192
 )   $
52,590
     $
(1,165
 )
Schedule of Realized Gain (Loss) [Table Text Block]
   
Nine months ended

September 30,
    
Three months ended

September 30,
  
   
2019
(unaudited)
    
2018
(unaudited)
    
2019
(unaudited)
    
2018
(unaudited)
  
                                 
Gross realized gains from sale of available-for-sale marketable securities
   $
1
     $
4
     $
1
     $
—
  
Gross realized losses from sale of available-for-sale marketable securities
   $
(39
 )   $
(34
 )   $
(4
 )   $
(32
 )