Note 4 - Marketable Securities (Tables)
 6 Months Ended
Jun. 30, 2019
Notes Tables  
Fair Value, Marketable Securities Measured on a Recurring Basis [Table Text Block]
   
 
 
 
  
Fair Value Measurements as of
June 30
, 2019
Using Inputs Considered as
  
   
Fair Value
    
Level 1
    
Level 2
    
Level 3
  
Commercial paper and corporate bonds
   $
2,994
     $
—
     $
2,994
     $
—
  
Government securities
    
4,998
      
—
      
4,998
      
—
  
Total marketable securities
   $
7,992
     $
—
     $
7,992
     $
—