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STOCK-BASED COMPENSATION - Summary of Stock Options Activity (Details)
$ / shares in Units, $ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2021
USD ($)
$ / shares
shares
Dec. 31, 2020
USD ($)
$ / shares
shares
Number of Shares    
Balance at beginning of period (in shares) | shares 7,073,584  
Granted (in shares) | shares 0  
Exercised (in shares) | shares (832,329)  
Canceled (in shares) | shares (277,212)  
Balance at end of period (in shares) | shares 5,964,043 7,073,584
Weighted-Average Exercise Price    
Balance at beginning of period (in dollars per share) | $ / shares $ 0.85  
Granted (in dollars per share) | $ / shares 0  
Exercised (in dollars per share) | $ / shares 0.39  
Canceled (in dollars per share) | $ / shares 0.71  
Balance at end of period (in dollars per share) | $ / shares $ 0.92 $ 0.85
Stock Options Additional Disclosures    
Number of Shares, Options vested and exercisable (in shares) | shares 3,312,726  
Weighted-Average Exercise Price, Options vested and exercisable (in dollars per share) | $ / shares $ 0.49  
Weighted-Average Remaining Contractual Term, Balance 7 years 4 months 13 days 7 years 10 months 24 days
Weighted-Average Remaining Contractual Term, Options vested and exercisable 6 years 5 months 1 day  
Aggregate Intrinsic Value, Balance | $ $ 43,989 $ 13,348
Aggregate Intrinsic Value, Options vested and exercisable | $ $ 25,871