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)
 12 Months Ended
Dec. 31, 2019
USD ($)
item
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Cash, cash equivalent and short term investments        
Cash and cash equivalents $ 22,521,000 $ 76,322,000 $ 38,290,000 $ 17,632,000
Short-term Investments 75,557,000 52,215,000    
Cash, cash equivalents and short-term investments 98,078,000 128,537,000    
Available-For-Sale Securities Reconciliation        
Amortized Cost 87,227,000 116,830,000    
Fair Value $ 87,250,000 116,805,000    
Weighted-average time to maturity of cash equivalents and available-for-sale securities 117 days      
Number of investments in continuous unrealized loss position for more than 12 months 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 12 months or less | item 19      
Fair Value $ 31,252,000      
Unrealized Losses (7,000)      
Money market funds        
Cash, cash equivalent and short term investments        
Cash, cash equivalents and short-term investments 7,457,000 9,106,000    
U.S. treasury bills        
Cash, cash equivalent and short term investments        
Cash, cash equivalents and short-term investments 12,539,000      
Available-For-Sale Securities Reconciliation        
Amortized Cost 12,532,000      
Fair Value 12,539,000      
Fair value and gross unrealized losses of investments in individual securities in unrealized loss position        
Fair Value 4,512,000      
Unrealized Losses (1,000)      
Government-sponsored enterprises securities        
Cash, cash equivalent and short term investments        
Cash, cash equivalents and short-term investments 19,017,000 7,872,000    
Available-For-Sale Securities Reconciliation        
Amortized Cost 19,010,000 7,873,000    
Fair Value 19,017,000 7,872,000    
Fair value and gross unrealized losses of investments in individual securities in unrealized loss position        
Fair Value 8,009,000      
Unrealized Losses (1,000)      
Corporate bonds and commercial paper        
Cash, cash equivalent and short term investments        
Cash, cash equivalents and short-term investments 55,694,000 108,933,000    
Available-For-Sale Securities Reconciliation        
Amortized Cost 55,685,000 108,957,000    
Fair Value 55,694,000 108,933,000    
Fair value and gross unrealized losses of investments in individual securities in unrealized loss position        
Fair Value 18,731,000      
Unrealized Losses (5,000)      
Gross Unrealized Gains        
Available-For-Sale Securities Reconciliation        
Gross Unrealized Gains (Losses) 30,000 2,000    
Gross Unrealized Gains | U.S. treasury bills        
Available-For-Sale Securities Reconciliation        
Gross Unrealized Gains (Losses) 8,000      
Gross Unrealized Gains | Government-sponsored enterprises securities        
Available-For-Sale Securities Reconciliation        
Gross Unrealized Gains (Losses) 8,000      
Gross Unrealized Gains | Corporate bonds and commercial paper        
Available-For-Sale Securities Reconciliation        
Gross Unrealized Gains (Losses) 14,000 2,000    
Gross Unrealized Losses        
Available-For-Sale Securities Reconciliation        
Gross Unrealized Gains (Losses) (7,000) (27,000)    
Gross Unrealized Losses | U.S. treasury bills        
Available-For-Sale Securities Reconciliation        
Gross Unrealized Gains (Losses) (1,000)      
Gross Unrealized Losses | Government-sponsored enterprises securities        
Available-For-Sale Securities Reconciliation        
Gross Unrealized Gains (Losses) (1,000) (1,000)    
Gross Unrealized Losses | Corporate bonds and commercial paper        
Available-For-Sale Securities Reconciliation        
Gross Unrealized Gains (Losses) (5,000) (26,000)    
Cash        
Cash, cash equivalent and short term investments        
Cash, cash equivalents and short-term investments $ 3,371,000 $ 2,626,000