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Note 4 - Investments (Tables)
 12 Months Ended
Dec. 31, 2018
Notes Tables  
Available-for-sale Securities [Table Text Block]
   
December 31, 201
8
  
   
Cost
    
Gross
Unrealized
Gain
    
Gross
Unrealized
Loss
    
Fair Value
  
Available-for-sale securities:
                                
Municipal bonds
   $
6,751
     $
1
     $
(19
 )   $
6,733
  
Corporate notes/bonds
    
146,466
      
14
      
(354
 )    
146,126
  
Variable rate demand notes
    
8,900
      
—
      
—
      
8,900
  
Asset-backed securities
    
32,986
      
—
      
(63
 )    
32,923
  
Commercial paper
    
39,707
      
—
      
(6
 )    
39,701
  
Certificate of deposit
    
956
      
—
      
—
      
956
  
Total investments
   $
235,766
     $
15
     $
(442
 )   $
235,339
  
   
December 31, 201
7
  
   
Cost
    
Gross
Unrealized
Gain
    
Gross
Unrealized
Loss
    
Fair Value
  
Available-for-sale securities:
                                
Municipal bonds
   $
27,725
     $
—
     $
(68
 )   $
27,657
  
Corporate notes/bonds
    
146,549
      
49
      
(197
 )    
146,401
  
Variable rate demand notes
    
3,500
      
—
      
—
      
3,500
  
Asset-backed securities
    
7,197
      
—
      
(12
 )    
7,185
  
Commercial paper
    
57,006
      
1
      
(13
 )    
56,994
  
Total investments
   $
241,977
     $
50
     $
(290
 )   $
241,737
  
Investments Classified by Contractual Maturity Date [Table Text Block]
   
Cost
    
Fair Value
  
Due in one year or less
   $
147,908
     $
147,712
  
Due between one and five years
    
75,458
      
75,236
  
Due after five years
    
12,400
      
12,391
  
    $
235,766
     $
235,339