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Note 8 - Marketable Securities (Tables)
 12 Months Ended
Dec. 31, 2018
Notes Tables  
Debt Securities, Held-to-maturity [Table Text Block]
                                 
   
December 31, 2018
  
(in thousands)
  
Amortized
Cost
    
Unrealized Gains
    
Unrealized Losses
    
Fair Value
  
                                 
U.S. government agency securities
   $
16,843
     $
-
     $
(88
 )   $
16,755
  
Corporate debt securities
    
31,769
      
-
      
(96
 )    
31,673
  
U.S. municipal securities
    
17,509
      
1
      
(33
 )    
17,477
  
Certificates of deposit/time deposits
    
4,208
      
-
      
(25
 )    
4,183
  
Total marketable securities
   $
70,329
     $
1
     $
(242
 )   $
70,088
  
                                 
                                 
   
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
  
                                 
Investments Classified by Contractual Maturity Date [Table Text Block]
         
   
December 31,
  
(in thousands)
  
2018
  
         
Due in one year or less
   $
46,750
  
Due after one year through five years
    
23,579
  
Total marketable securities
   $
70,329