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Note 4 - Marketable Securities (Tables)
 3 Months Ended
Mar. 31, 2019
Notes Tables  
Schedule of Available-for-sale Securities Reconciliation [Table Text Block]
   
Amortized
cost
    
Gross unrealized
holding
gains
    
Gross unrealized
holding
losses
    
Estimated fair value
  
March 31, 2019
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
 
Available-for-sale securities:
                                
Corporate bonds and notes
   $
5,465
     $
37
     $
(12
)
   $
5,490
  
Municipal bonds
    
3,634
      
6
      
(7
)
    
3,633
  
Total available-for-sale securities
   $
9,099
     $
43
     $
(19
)
   $
9,123
  
                                 
December 31, 2018
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
 
Available-for-sale securities:
                                
Corporate bonds and notes
   $
2,911
     $
1
     $
(31
)
   $
2,881
  
Municipal bonds
    
1,849
      
—
      
(15
)
    
1,834
  
Total available-for-sale securities
   $
4,760
     $
1
     $
(46
)
   $
4,715
  
Available-for-sale Securities, Debt Maturities [Table Text Block]
   
Amortized cost
    
Estimated fair value
  
Due within one year
   $
2,888
     $
2,888
  
Due after one year through five years
    
6,059
      
6,080
  
Due after five years
    
152
      
155
  
Total available-for-sale securities
   $
9,099
     $
9,123
  
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Table Text Block]
   
Less than 12 months
    
More than 12 months
    
Total
  
   
Estimated fair value
    
Gross unrealized holding losses
    
Estimated fair value
    
Gross unrealized holding losses
    
Estimated fair value
    
Gross unrealized holding losses
  
As of March 31, 201
9
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
 
Corporate bonds and notes
   $
104
     $
(1
)
   $
5,386
     $
(11
)
   $
5,490
     $
(12
)
Municipal bonds
    
—
      
—
      
3,633
      
(7
)
    
3,633
      
(7
)
Total
   $
104
     $
(1
)
   $
9,019
     $
(18
)
   $
9,123
     $
(19
)