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DEBT - Scheduled Future Payments on Term Loan (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Oct. 30, 2020	Oct. 29, 2020	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term portion	$ 57,177	$ 56,940
SVB Term Loan
Debt Instrument [Line Items]
The remainder of 2021	0
2022	3,222
2023	19,333
2024	19,333
2024	16,112
Total future principal payments	58,000		$ 58,000	$ 56,000	$ 56,000
Less: unamortized debt discount	(823)
Carrying value of long-term debt	57,177
Less: current portion	0
Long-term portion	$ 57,177