Annual report pursuant to Section 13 and 15(d)

CASH, CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS (Details)

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CASH, CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
USD ($)
item
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Cash, cash equivalent and short term investments        
Cash and cash equivalents $ 76,322 $ 38,290 $ 17,632 $ 43,456
Short-term Investments 52,215 77,461    
Cash, cash equivalents and short-term investments 128,537 115,751    
Available-For-Sale Securities Reconciliation        
Amortized Cost 116,830 112,456    
Gross Unrealized Gains 2 1    
Gross Unrealized Losses (27) (83)    
Fair Value $ 116,805 112,374    
Weighted-average time to maturity of cash equivalents and available-for-sale securities 72 days      
Number of investments in continuous unrealized loss position for more than twelve months | item 0      
Fair value and gross unrealized losses of investments in individual securities in unrealized loss position        
Number of individual securities in unrealized loss position for twelve months or less | item 31      
Fair Value $ 50,445      
Unrealized Losses (27)      
Money market funds        
Cash, cash equivalent and short term investments        
Cash, cash equivalents and short-term investments 9,106 2,795    
U.S. treasury bills        
Cash, cash equivalent and short term investments        
Cash, cash equivalents and short-term investments   6,726    
Available-For-Sale Securities Reconciliation        
Amortized Cost   6,733    
Gross Unrealized Losses   (7)    
Fair Value   6,726    
Government-sponsored enterprises securities        
Cash, cash equivalent and short term investments        
Cash, cash equivalents and short-term investments 7,872 7,826    
Available-For-Sale Securities Reconciliation        
Amortized Cost 7,873 7,835    
Gross Unrealized Losses (1) (9)    
Fair Value 7,872 7,826    
Fair value and gross unrealized losses of investments in individual securities in unrealized loss position        
Fair Value 2,473      
Unrealized Losses (1)      
Corporate bonds and commercial paper        
Cash, cash equivalent and short term investments        
Cash, cash equivalents and short-term investments 108,933 97,822    
Available-For-Sale Securities Reconciliation        
Amortized Cost 108,957 97,888    
Gross Unrealized Gains 2 1    
Gross Unrealized Losses (26) (67)    
Fair Value 108,933 97,822    
Fair value and gross unrealized losses of investments in individual securities in unrealized loss position        
Fair Value 47,972      
Unrealized Losses (26)      
Cash        
Cash, cash equivalent and short term investments        
Cash, cash equivalents and short-term investments $ 2,626 $ 582