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Note 6 - Marketable Securities (Tables)
 9 Months Ended
Sep. 30, 2017
Notes Tables  
Held-to-maturity Securities [Table Text Block]
       
September 30, 2017
  
(in thousands)
    
Cost
    
Unrealized
Gains
    
Unrealized
Losses
    
Fair Value
  
U.S. municipal securities
   $
35,388
     $
23
     $
(43
 )   $
35,368
  
Corporate debt securities
    
29,841
      
3
      
(68
 )    
29,776
  
U.S. government agency securities
    
37,866
      
-
      
(125
 )    
37,741
  
Certificates of deposit/time deposits
    
5,456
      
1
      
(25
 )    
5,432
  
Total marketable securities
   $
108,552
     $
27
     $
(261
 )   $
108,318
  
       
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]
       
September 30,
  
(in thousands)
    
2017
  
Due in one year or less
   $
53,201
  
Due after one year through five years
    
55,351
  
Total marketable securities
   $
108,552