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Note 3 - Marketable Securities (Tables)
 12 Months Ended
Dec. 31, 2018
Notes Tables  
Schedule of Available-for-sale Securities Reconciliation [Table Text Block]
   
As at 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
  
   
As at December 31,
2017
  
   
Amortized

cost
    
Gross
unrealized

gains
    
Gross
unrealized

losses
    
Fair

value
  
Available-for-sale - matures within one year:
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
 
Corporate bonds
   $
11,803
     $
3
     $
(12
 )   $
11,794
  
     
11,803
      
3
      
(12
 )    
11,794
  
Available-for-sale - matures after one year through
five
years:
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
 
Certificate of deposits
    
747
      
—
      
—
      
747
  
Government bonds
    
501
      
—
      
(6
 )    
495
  
Corporate bonds
    
70,291
      
14
      
(677
 )    
69,628
  
     
71,539
      
14
      
(683
 )    
70,870
  
                                 
Total
   $
83,342
     $
17
     $
(695
 )   $
82,664
  
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 December 31, 2018
   $
16,580
     $
(192
 )   $
52,590
     $
(1,165
 )
As of December 31, 2017
   $
49,921
     $
(411
 )   $
22,960
     $
(284
 )
Schedule of Realized Gain (Loss) [Table Text Block]
   
Year ended December 31,
  
   
2016
    
2017
    
2018
  
                         
Gross realized gains from sale of available-for-sale marketable securities
   $
24
     $
47
     $
4
  
Gross realized losses from sale of available-for-sale marketable securities
   $
(33
 )   $
(47
 )   $
(71
 )