FAIR VALUE MEASUREMENTS - Assets and Liabilities at Fair Value, Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Liabilities:
Convertible preferred stock warrant liability	$ 0	$ 2,960
Fair Value, Recurring
Assets:
Restricted cash	35,686	26,006
Total assets	36,191	26,511
Liabilities:
Convertible preferred stock warrant liability	836,835	2,960
Total liabilities	836,835	2,960
Fair Value, Recurring | Certificates of deposit
Assets:
Certificates of deposit	505	505
Level 1 | Fair Value, Recurring
Assets:
Restricted cash	35,686	26,006
Total assets	35,686	26,006
Liabilities:
Convertible preferred stock warrant liability	0	0
Total liabilities	0	0
Level 1 | Fair Value, Recurring | Certificates of deposit
Assets:
Certificates of deposit	0	0
Level 2 | Fair Value, Recurring
Assets:
Restricted cash	0	0
Total assets	505	505
Liabilities:
Convertible preferred stock warrant liability	0	0
Total liabilities	0	0
Level 2 | Fair Value, Recurring | Certificates of deposit
Assets:
Certificates of deposit	505	505
Level 3 | Fair Value, Recurring
Assets:
Restricted cash	0	0
Total assets	0	0
Liabilities:
Convertible preferred stock warrant liability	836,835	2,960
Total liabilities	836,835	2,960
Level 3 | Fair Value, Recurring | Certificates of deposit
Assets:
Certificates of deposit	$ 0	$ 0