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Note 5 - Short-term Investments
 6 Months Ended
Jun. 30, 2018
Notes to Financial Statements  
Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block]
5.
Short-Term Investments
 
The following is a summary of our short-term investments (in thousands):
 
 
   
June 30, 2018
  
   
 
 
 
  
Gross
    
Gross
    
 
 
 
   
Amortized
    
Unrealized
    
Unrealized
    
Estimated
  
   
Cost
    
Gains
    
Losses
    
Fair Value
  
U.S. treasury bills
   $
39,768
     $
3
     $
—
     $
39,771
  
U.S. corporate debt securities
    
19,176
      
—
      
(23
)
    
19,153
  
U.S. commercial paper
    
52,794
      
—
      
—
      
52,794
  
Foreign commercial paper
    
52,317
      
—
      
(5
)
    
52,312
  
Total
   $
164,055
     $
3
     $
(28
)
   $
164,030
  
 
   
D
ecember 31, 2017
  
   
 
 
 
  
Gross
    
Gross
    
 
 
 
   
Amortized
    
Unrealized
    
Unrealized
    
Estimated
  
   
Cost
    
Gains
    
Losses
    
Fair Value
  
U.S. corporate debt securities
   $
13,003
     $
—
     $
(10
)
   $
12,993
  
U.S. commercial paper
    
4,929
      
—
      
—
      
4,929
  
Foreign commercial paper
    
9,874
      
—
      
—
      
9,874
  
Total
   $
27,806
     $
—
     $
(10
)
   $
27,796
  
 
The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity. We regularly monitor and evaluate the realizable value of our marketable securities. We did
not
recognize any impairment losses during the
three
and
six
months ended
June 30, 2018
and
2017.
 
Realized gains and losses associated with our investments, if any, are reported in the statements of operations and comprehensive loss. We did
not
recognize any realized gains or losses during the
three
and
six
months ended
June 30, 2018
and
2017.