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Note 7 - Marketable Securities (Tables)
 9 Months Ended
Sep. 30, 2018
Notes Tables  
Debt Securities, Held-to-maturity [Table Text Block]
   
September 30, 2018
  
(in thousands)
  
Cost
    
Unrealized
Gains
    
Unrealized
Losses
    
Fair Value
  
U.S. municipal securities
   $
20,607
     $
-
     $
(74
 )   $
20,533
  
Corporate debt securities
    
43,525
      
-
      
(153
 )    
43,372
  
U.S. government agency securities
    
22,197
      
-
      
(156
 )    
22,041
  
Certificates of deposit/time deposits
    
4,456
      
-
      
(29
 )    
4,427
  
Total marketable securities
   $
90,785
     $
-
     $
(412
 )   $
90,373
  
   
December 31, 2017
  
(in thousands)
  
Cost
    
Unrealized
Gains
    
Unrealized
Losses
    
Fair
Value
  
U.S. municipal securities
   $
28,364
     $
-
     $
(104
 )   $
28,260
  
Corporate debt securities
    
29,806
      
-
      
(128
 )    
29,678
  
U.S. government agency securities
    
32,320
      
-
      
(199
 )    
32,121
  
Certificates of deposit/time deposits
    
3,968
      
-
      
(32
 )    
3,936
  
Total marketable securities
   $
94,458
     $
-
     $
(463
 )   $
93,995
  
Investments Classified by Contractual Maturity Date [Table Text Block]
   
September 30,
  
(in thousands)
  
2018
  
Due in one year or less
   $
57,561
  
Due after one year through five years
    
33,224
  
Total marketable securities
   $
90,785