CONVERTIBLE PREFERRED STOCK WARRANT LIABILITY - Narrative (Details) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Feb. 28, 2021 USD ($) scenario $ / shares	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jul. 23, 2021 $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares	Sep. 30, 2017 $ / shares shares
Class of Warrant or Right [Line Items]
Number of warrants (in shares) | shares						85,750,000		2
Number of shares called by warrants (in shares) | shares							1,546,799	1,546,799
Warrant exercise price (in dollars per share) | $ / shares	$ 1.94					$ 11.50		$ 1.94
Convertible preferred stock warrant liability		$ 0		$ 0			$ 2,960
Warrant fair value per share (in dollars per share) | $ / shares							$ 1.94
Proceeds from the exercise of public warrants	$ 3,000	173,300		173,273	$ 0
Issuance of Series D convertible preferred stock upon exercise of warrants	$ 12,900
Change in fair value of preferred stock warrant liability			$ 100		200
Probability-weighted expected return method, number of scenarios considered | scenario	3
Probability-weighted expected return method, likelihood percentage of OPM scenario	20.00%
Probability-weighted expected return method, likelihood percentage of as-converted SPAC scenario	70.00%
Probability-weighted expected return method, likelihood percentage of as-converted IPO scenario	10.00%
Convertible Preferred Stock Warrant Liability
Class of Warrant or Right [Line Items]
Change in fair value of preferred stock warrant liability		$ 0	$ 57	$ 6,976	$ 171