MEZZANINE EQUITY, COMMON STOCK AND STOCKHOLDERS' (DEFICIT) EQUITY - SCHEDULE OF STOCK OPTION ACTIVITY (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Number of Shares
Beginning balance (in shares)	21,543,828
Granted during the period (in shares)	0
Exercised during the period (in shares)	(5,819,965)
Cancelled and forfeited during the period (in shares)	(276,657)
Ending balance (in shares)	15,447,206	21,543,828
Options vested and expected to vest (in shares)	15,447,206
Options exercisable (in shares)	14,040,377
Weighted-Average Exercise Price
Beginning balance, Weighted-Average Exercise Price (in dollars per share)	$ 2.19
Granted during the period, Weighted-Average Exercise Price (in dollars per share)	0
Exercised during the period, Weighted-Average Exercise Price (in dollars per share)	2.02
Cancelled and forfeited during the period, Weighted-Average Exercise Price (in dollars per share)	4.51
Ending balance, Weighted-Average Exercise Price (in dollars per share)	2.21	$ 2.19
Options vested and expected to vest, Weighted-Average Exercise Price (in dollars per share)	2.21
Options exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 1.79
Weighted- Average Remaining Life
Weighted- Average Remaining Life	5 years 7 months 28 days	6 years 6 months 7 days
Options vested and expected to vest, Weighted-Average Remaining Life	5 years 7 months 28 days
Options exercisable, Weighted-Average Remaining Life	5 years 5 months 26 days
Total Intrinsic Value (in thousands)
Total Intrinsic Value (in thousands)	$ 615,827	$ 304,590
Options vested and expected to vest, Total Intrinsic Value	615,827
Options exercisable, Total Intrinsic Value	$ 565,754