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Note 6 - Marketable Securities (Tables)
 3 Months Ended
Mar. 31, 2019
Notes Tables  
Debt Securities, Held-to-maturity [Table Text Block]
                                 
   
March 31, 2019
  
(in thousands)
  
Cost
    
Unrealized
Gains
    
Unrealized
Losses
    
Fair Value
  
U.S. municipal securities
   $
10,825
     $
7
     $
(6
 )   $
10,826
  
Corporate debt securities
    
24,513
      
52
      
(27
 )    
24,538
  
U.S. government agency securities
    
14,505
      
-
      
(40
 )    
14,465
  
Certificates of deposit/time deposits
    
3,464
      
10
      
(10
 )    
3,464
  
Total marketable securities
   $
53,307
     $
69
     $
(83
 )   $
53,293
  
                                 
                                 
   
December 31, 2018
  
(in thousands)
  
Cost
    
Unrealized
Gains
    
Unrealized
Losses
    
Fair Value
  
U.S. municipal securities
   $
17,509
     $
1
     $
(33
 )   $
17,477
  
Corporate debt securities
    
31,769
      
-
      
(96
 )    
31,673
  
U.S. government agency securities
    
16,843
      
-
      
(88
 )    
16,755
  
Certificates of deposit/time deposits
    
4,208
      
-
      
(25
 )    
4,183
  
Total marketable securities
   $
70,329
     $
1
     $
(242
 )   $
70,088
  
                                 
Investments Classified by Contractual Maturity Date [Table Text Block]
         
   
March 31,
  
(in thousands)
  
2019
  
Due in one year or less
   $
35,045
  
Due after one year through five years
    
18,262
  
Total marketable securities
   $
53,307