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Note 7 - Marketable Securities (Tables)
 12 Months Ended
Dec. 31, 2017
Notes Tables  
Held-to-maturity Securities [Table Text Block]
       
   
December 31, 2017
  
(in thousands)
  
Amortized
Cost
    
Unrealized
Gains
    
Unrealized
Losses
    
Fair Value
  
                                 
U.S. government agency securities
   $
32,320
     $
-
     $
(199
 )   $
32,121
  
Corporate debt securities
    
29,806
      
-
      
(128
 )    
29,678
  
U.S. municipal securities
    
28,364
      
-
      
(104
 )    
28,260
  
Certificates of deposit/time deposits
    
3,968
      
-
      
(32
 )    
3,936
  
Total marketable securities
   $
94,458
     $
-
     $
(463
 )   $
93,995
  
                                 
       
   
December 31, 2016
  
(in thousands)
  
Amortized
Cost
    
Unrealized
Gains
    
Unrealized
Losses
    
Fair Value
  
                                 
U.S. government agency securities
   $
31,706
     $
1
     $
(141
 )   $
31,566
  
Corporate debt securities
    
38,490
      
2
      
(147
 )    
38,345
  
U.S. municipal securities
    
46,578
      
1
      
(187
 )    
46,392
  
Certificates of deposit/time deposits
    
7,182
      
12
      
(22
 )    
7,172
  
Total marketable securities
   $
123,956
     $
16
     $
(497
 )   $
123,475
  
                                 
Investments Classified by Contractual Maturity Date [Table Text Block]
       
   
December 31,
  
(in thousands)
  
2017
  
         
Due in one year or less
   $
57,424
  
Due after one year through five years
    
37,034
  
Total marketable securities
   $
94,458