STOCK-BASED COMPENSATION - Summary of Company's Stock Option Activity and Related Information (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 Stock Options Outstanding
Outstanding at beginning of period (in shares) | shares	8,142,348
Option granted (in shares) | shares	0
Options exercised (in shares) | shares	(127,913)
Options cancelled or forfeited (in shares) | shares	(714,490)
Outstanding at end of period (in shares) | shares	7,299,945	8,142,348
Options exercisable (in shares) | shares	5,683,458
Options vested and expected to vest (in shares) | shares	7,183,948
Weighted- Average Exercise Price
Options outstanding at beginning of period (in USD per share) | $ / shares	$ 7.14
Options granted (in USD per share) | $ / shares	0
Options exercised (in USD per share) | $ / shares	4.79
Options cancelled or forfeited (in USD per share) | $ / shares	9.15
Options outstanding at end of period (in USD per share) | $ / shares	6.99	$ 7.14
Options exercisable (in USD per share) | $ / shares	7.01
Options vested and expected to vest (in USD per share) | $ / shares	$ 7.01
Weighted- Average Remaining Contractual Life (Years)
Options outstanding (in years)	6 years 4 months 24 days	7 years 3 months 18 days
Options exercisable (in years)	6 years 1 month 6 days
Options vested and expected to vest (in years)	6 years 4 months 24 days
Aggregate Intrinsic Value
Options outstanding | $	$ 9,418	$ 2,638
Options exercisable | $	7,024
Options vested and expected to vest | $	$ 9,116