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Note 3 - Fair Value Measurements (Tables)
12 Months Ended
Dec. 31, 2018
Notes Tables  
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]
           
Fair Value Measurements at Reporting Date Using
 
   
 
 
 
 
Quoted Prices in
   
Significant
   
 
 
 
   
 
 
 
 
Active Markets
   
Other
   
Significant
 
   
Balance at
   
for Identical
   
Observable
   
Unobservable
 
   
December 31,
   
Assets
   
Inputs
   
Inputs
 
   
2018
   
(Level 1)
   
(Level 2)
   
(Level 3)
 
Money market funds
  $
13,874
    $
13,874
    $
    $
 
U.S. treasury bills and government agency obligations
   
59,741
     
59,741
     
     
 
U.S. corporate debt securities
   
59,087
     
     
59,087
     
 
Foreign corporate debt securities
   
5,046
     
     
5,046
     
 
U.S. commercial paper
   
61,885
     
     
61,885
     
 
Foreign commercial paper
   
123,861
     
     
123,861
     
 
Total
  $
323,494
    $
73,615
    $
249,879
    $
 
           
Fair Value Measurements at Reporting Date Using
 
   
 
 
 
 
Quoted Prices in
   
Significant
   
 
 
 
   
 
 
 
 
Active Markets
   
Other
   
Significant
 
   
Balance at
   
for Identical
   
Observable
   
Unobservable
 
   
December 31
,
   
Assets
   
Inputs
   
Inputs
 
   
2017
   
(Level 1)
   
(Level 2)
   
(Level 3)
 
Money market funds
  $
91,386
    $
91,386
    $
    $
 
U.S. corporate debt securities
   
17,520
     
     
17,520
     
 
U.S. commercial paper
   
39,863
     
     
39,863
     
 
Foreign commercial paper
   
19,854
     
     
19,854
     
 
Total
  $
168,623
    $
91,386
    $
77,237
    $