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Note 3 - Fair Value Measurements (Tables)
12 Months Ended
Dec. 31, 2016
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
 
 
 
2016
 
 
(Level 1)
 
 
(Level 2)
 
 
(Level 3)
 
Money 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
    $
 
           
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
5
 
 
(Level 1)
 
 
(Level 2)
 
 
(Level 3)
 
Money market funds
  $
67,089
    $
67,089
    $
    $
 
United States corporate debt securities
   
28,715
     
     
28,715
     
 
Foreign corporate debt securities
   
4,922
     
     
4,922
     
 
United States commercial paper
   
7,770
     
     
7,770
     
 
Foreign commercial paper
   
15,579
     
     
15,579
     
 
Total
  $
124,075
    $
67,089
    $
56,986
    $