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, 2017
USD ($)
item
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Cash, cash equivalent and short term investments        
Cash and cash equivalents $ 38,290 $ 17,632 $ 43,456 $ 15,203
Short-term Investments 77,461 57,134    
Cash, cash equivalents and short-term investments 115,751 74,766    
Available-For-Sale Securities Reconciliation        
Amortized Cost 112,456 65,048    
Gross Unrealized Gains 1 5    
Gross Unrealized Losses (83) (23)    
Fair Value $ 112,374 65,030    
Weighted-average time to maturity of cash equivalents and available-for-sale securities 110 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 38      
Fair Value $ 60,743      
Unrealized Losses (83)      
Money market funds        
Cash, cash equivalent and short term investments        
Cash, cash equivalents and short-term investments 2,795 9,496    
U.S. treasury bills        
Cash, cash equivalent and short term investments        
Cash, cash equivalents and short-term investments 6,726 4,300    
Available-For-Sale Securities Reconciliation        
Amortized Cost 6,733 4,300    
Gross Unrealized Losses (7)      
Fair Value 6,726 4,300    
Fair value and gross unrealized losses of investments in individual securities in unrealized loss position        
Fair Value 6,726      
Unrealized Losses (7)      
Government-sponsored enterprises securities        
Cash, cash equivalent and short term investments        
Cash, cash equivalents and short-term investments 7,826 16,459    
Available-For-Sale Securities Reconciliation        
Amortized Cost 7,835 16,457    
Gross Unrealized Gains   3    
Gross Unrealized Losses (9) (1)    
Fair Value 7,826 16,459    
Fair value and gross unrealized losses of investments in individual securities in unrealized loss position        
Fair Value 7,826      
Unrealized Losses (9)      
Corporate bonds and commercial paper        
Cash, cash equivalent and short term investments        
Cash, cash equivalents and short-term investments 97,822 44,271    
Available-For-Sale Securities Reconciliation        
Amortized Cost 97,888 44,291    
Gross Unrealized Gains 1 2    
Gross Unrealized Losses (67) (22)    
Fair Value 97,822 44,271    
Fair value and gross unrealized losses of investments in individual securities in unrealized loss position        
Fair Value 46,191      
Unrealized Losses (67)      
Cash        
Cash, cash equivalent and short term investments        
Cash, cash equivalents and short-term investments $ 582 $ 240