CONVERTIBLE PREFERRED STOCK AND EQUITY - Common Stock Shares Reserved For Future Issuance (Details) - shares shares in Thousands	Sep. 30, 2021	Dec. 31, 2020
Class of Stock [Line Items]
Reserved shares of common stock for future issuance (in shares)	54,046	86,665
Conversion of outstanding redeemable convertible preferred stock
Class of Stock [Line Items]
Reserved shares of common stock for future issuance (in shares)		63,756
Stock options issued and outstanding
Class of Stock [Line Items]
Reserved shares of common stock for future issuance (in shares)	15,430	21,691
Warrants issued and outstanding
Class of Stock [Line Items]
Reserved shares of common stock for future issuance (in shares)		318
Remaining shares available for future issuance
Class of Stock [Line Items]
Reserved shares of common stock for future issuance (in shares)		900
Restricted stock units issued and outstanding
Class of Stock [Line Items]
Reserved shares of common stock for future issuance (in shares)	5,546
Public warrants outstanding
Class of Stock [Line Items]
Reserved shares of common stock for future issuance (in shares)	10,000
Private placement warrants outstanding
Class of Stock [Line Items]
Reserved shares of common stock for future issuance (in shares)	5,333
2021 Incentive Award Plan available shares
Class of Stock [Line Items]
Reserved shares of common stock for future issuance (in shares)	17,737