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Note 4 - Fair Value Measurements (Tables)
9 Months Ended
Sep. 30, 2017
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
 
   
September
30
,
   
Assets
   
Inputs
   
Inputs
 
   
201
7
   
(Level 1)
   
(Level 2)
   
(Level 3)
 
M
oney market funds
  $
62,512
    $
62,512
    $
    $
 
United States
commercial paper
   
4,493
     
     
4,493
     
 
Foreign
commercial paper
   
3,999
     
     
3,999
     
 
Total
  $
71,004
    $
62,512
    $
8,492
    $
 
           
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
 
   
201
6
   
(Level 1)
   
(Level 2)
   
(Level 3)
 
M
oney market funds
  $
11,493
    $
11,493
    $
    $
 
United States corporate debt securities
   
12,039
     
     
12,039
     
 
Foreign corporate debt securities
   
14,057
     
     
14,057
     
 
United States commercial paper
   
8,635
     
     
8,635
     
 
Foreign commercial paper
   
2,993
     
     
2,993
     
 
Total
  $
49,217
    $
11,493
    $
37,724
    $