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, 2020
USD ($)
item
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Cash, cash equivalent and short term investments        
Cash and cash equivalents $ 30,373,000 $ 22,521,000 $ 76,322,000 $ 38,290,000
Short-term Investments 26,954,000 75,557,000    
Cash, cash equivalents and short-term investments 57,327,000 98,078,000    
Available-For-Sale Securities Reconciliation        
Amortized Cost 35,856,000 87,227,000    
Fair Value $ 35,852,000 87,250,000    
Weighted-average time to maturity of cash equivalents and available-for-sale securities 78 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 16      
Fair Value $ 23,686,000      
Unrealized Losses (4,000)      
Money market funds        
Cash, cash equivalent and short term investments        
Cash, cash equivalents and short-term investments 19,487,000 7,457,000    
U.S. treasury bills        
Cash, cash equivalent and short term investments        
Cash, cash equivalents and short-term investments 10,034,000 12,539,000    
Available-For-Sale Securities Reconciliation        
Amortized Cost 10,036,000 12,532,000    
Fair Value 10,034,000 12,539,000    
Fair value and gross unrealized losses of investments in individual securities in unrealized loss position        
Fair Value 9,034,000      
Unrealized Losses (2,000)      
Government-sponsored enterprises securities        
Cash, cash equivalent and short term investments        
Cash, cash equivalents and short-term investments 4,920,000 19,017,000    
Available-For-Sale Securities Reconciliation        
Amortized Cost 4,920,000 19,010,000    
Fair Value 4,920,000 19,017,000    
Corporate bonds and commercial paper        
Cash, cash equivalent and short term investments        
Cash, cash equivalents and short-term investments 20,898,000 55,694,000    
Available-For-Sale Securities Reconciliation        
Amortized Cost 20,900,000 55,685,000    
Fair Value 20,898,000 55,694,000    
Fair value and gross unrealized losses of investments in individual securities in unrealized loss position        
Fair Value 14,652,000      
Unrealized Losses (2,000)      
Gross Unrealized Gains        
Available-For-Sale Securities Reconciliation        
Gross Unrealized Gains (Losses)   30,000    
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    
Gross Unrealized Losses        
Available-For-Sale Securities Reconciliation        
Gross Unrealized Gains (Losses) (4,000) (7,000)    
Gross Unrealized Losses | U.S. treasury bills        
Available-For-Sale Securities Reconciliation        
Gross Unrealized Gains (Losses) (2,000) (1,000)    
Gross Unrealized Losses | Government-sponsored enterprises securities        
Available-For-Sale Securities Reconciliation        
Gross Unrealized Gains (Losses)   (1,000)    
Gross Unrealized Losses | Corporate bonds and commercial paper        
Available-For-Sale Securities Reconciliation        
Gross Unrealized Gains (Losses) (2,000) (5,000)    
Cash        
Cash, cash equivalent and short term investments        
Cash, cash equivalents and short-term investments $ 1,988,000 $ 3,371,000