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Note 5 - Fair Value Measurements (Tables)
 6 Months Ended
Jun. 30, 2018
Notes Tables  
Fair Value, Assets Measured on Recurring Basis [Table Text Block]
        Fair Value Measurements at Reporting
Date Using
    June 30, 2018   Quoted Prices in
Active Markets
for Identical Assets
(Level 1)		   Significant Other
Observable Inputs
(Level 2)		   Significant
Unobservable Inputs
(Level 3)
Cash equivalents:                
Money market funds   $
17,366
     $
17,366
     $
-
     $
-
  
U.S. Treasury Bills    
43,596
      
43,596
      
-
      
-
  
Total cash equivalents   $
60,962
     $
60,962
     $
-
     $
-
  
                                 
Investments:                                
Bank certificates of deposit   $
13,250
     $
-
     $
13,250
     $
-
  
        Fair Value Measurements at Reporting
Date Using
    December 31, 2017   Quoted Prices in
Active Markets
for Identical Assets
(Level 1)		   Significant Other
Observable Inputs
(Level 2)		   Significant
Unobservable Inputs
(Level 3)
Cash equivalents:                
Money market funds   $
5,893
     $
5,893
     $
-
     $
-
  
Bank certificates of deposit    
500
      
-
      
500
      
-
  
Total cash equivalents   $
6,393
     $
5,893
     $
500
     $
-
  
                                 
Investments:                                
Bank certificates of deposit   $
24,000
     $
-
     $
24,000
     $
-