SHARE-BASED COMPENSATION - Stock Option Activity (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2021 $ / shares shares	Dec. 31, 2020 $ / shares shares
Stock Option Awards
Beginning balance (in shares) | shares	5,621,364
Granted (in shares) | shares	587,001
Exercised (in shares) | shares	(172,728)
Forfeited (in shares) | shares	(399,483)
Ending balance (in shares) | shares	5,636,154	5,621,364
Vested or expect to vest (in shares) | shares	5,636,154
Vested or expect to vest (in shares) | shares	2,084,658
Weighted Average Exercise Price
Beginning balance (in usd per share) | $ / shares	$ 7.94
Granted (in usd per share) | $ / shares	13.35
Exercised (in usd per share) | $ / shares	7.86
Forfeited (in usd per share) | $ / shares	8.78
Ending balance (in usd per share) | $ / shares	7.68	$ 7.94
Vested or expect to vest at end of period (in usd per share) | $ / shares	7.68
Vested or expect to vest at end of period (in usd per share) | $ / shares	$ 7.80
Weighted Average Remaining Contractual Life (Years)
Weighted Average Remaining Contractual Life (Years)	8 years 5 months 26 days	9 years 2 months 12 days
Weighted average remaining contractual life vested or expect to vest	8 years 5 months 26 days
Weighted average remaining contractual life vested or expect to vest	8 years 4 months 9 days