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Note 6 - Marketable Securities (Tables)
 6 Months Ended
Jun. 30, 2017
Notes Tables  
Held-to-maturity Securities [Table Text Block]
       
   
June 30, 2017
  
(in thousands)
  
Cost
    
Unrealized
Gains
    
Unrealized
Losses
    
Fair Value
  
U.S. municipal securities
   $
38,522
     $
22
     $
(59
 )   $
38,485
  
Corporate debt securities
    
33,899
      
2
      
(98
 )    
33,803
  
U.S. government agency securities
    
38,349
      
-
      
(136
 )    
38,212
  
Certificates of deposit/time deposits
    
6,937
      
3
      
(26
 )    
6,914
  
Total marketable securities
   $
117,707
     $
27
     $
(319
 )   $
117,415
  
                                 
       
   
December 31, 2016
  
(in thousands)
  
Cost
    
Unrealized
Gains
    
Unrealized
Losses
    
Fair Value
  
U.S. municipal securities
   $
46,578
     $
1
     $
(187
 )   $
46,392
  
Corporate debt securities
    
38,490
      
2
      
(147
 )    
38,345
  
U.S. government agency securities
    
31,706
      
1
      
(141
 )    
31,566
  
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]
       
   
June 30,
  
(in thousands)
  
2017
  
Due in one year or less
   $
48,272
  
Due after one year through five years
    
69,435
  
Total marketable securities
   $
117,707