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Note 7 - Marketable Securities (Tables)
 3 Months Ended
Mar. 31, 2018
Notes Tables  
Debt Securities, Held-to-maturity [Table Text Block]
       
   
March 31, 2018
  
(in thousands)
  
Cost
    
Unrealized
Gains
    
Unrealized
Losses
    
Fair Value
  
U.S. municipal securities
   $
23,417
     $
-
     $
(96
 )   $
23,321
  
Corporate debt securities
    
24,377
      
-
      
(194
 )    
24,183
  
U.S. government agency securities
    
32,372
      
-
      
(207
 )    
32,165
  
Certificates of deposit/time deposits
    
3,968
      
-
      
(39
 )    
3,929
  
Total marketable securities
   $
84,134
     $
-
     $
(536
 )   $
83,598
  
                                 
       
   
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]
       
   
March 31,
  
(in thousands)
  
2018
  
Due in one year or less
   $
53,467
  
Due after one year through five years
    
30,667
  
Total marketable securities
   $
84,134