SHARE-BASED COMPENSATION - Stock Options (Details) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares
Number of Shares under Option (in thousands)
Outstanding at beginning of period (in shares) | shares	10,147
Granted (in shares) | shares	8,026
Exercised (in shares) | shares	(34)
Canceled or forfeited (in shares) | shares	(2,628)
Outstanding at end of period (in shares) | shares	15,511	10,147
Exercisable at end of period (in shares) | shares	6,706
Weighted-average Exercise Price per Option
Outstanding at beginning of period (in dollars per share) | $ / shares	$ 1.26
Granted (in dollars per share) | $ / shares	3.40
Exercised (in dollars per share) | $ / shares	1.23
Canceled or forfeited (in dollars per share) | $ / shares	3.75
Outstanding at end of period (in dollars per share) | $ / shares	1.95	$ 1.26
Exercisable (in dollars per share) | $ / shares	$ 1.61
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average Remaining Contractual Life (in years), Outstanding	8 years 3 months 18 days	8 years 6 months
Weighted-average Remaining Contractual Life (in years), Exercisable	7 years 7 months 6 days
Aggregate Intrinsic Value, Outstanding | $	$ 63	$ 3,160
Aggregate Intrinsic Value, Exercisable | $	$ 49