DEBT - Narrative (Details) - USD ($) $ in Thousands			1 Months Ended	9 Months Ended	12 Months Ended
	Oct. 30, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2021	Dec. 31, 2018	Oct. 29, 2020	Dec. 30, 2019
SVB Term Loan
Debt Instrument [Line Items]
Total principal amount	$ 58,000		$ 56,000	$ 58,000	$ 56,000	$ 56,000
Debt instrument interest rate (as a percent)			6.30%		6.30%	6.30%
Final payment percentage of original principal amount (as a percent)	3.70%		3.15%		3.15%	3.15%
Length of revenue (in months)	12 months	12 months	12 months
Number of monthly payments (in months)	36 months		36 months
Number of monthly payments if company elects (in months)			30 months
Proceeds from term loan					$ 55,400
Legal and consulting fees					600
Revenue cushion covenant (as a percent)		25.00%
Annual growth covenant (as a percent)		10.00%
Debt instrument minimum liquidity ratio financial covenant	1.70%	1.75%				1.75%	1.50%
Debt instrument prepayment premium (as a percent)	3.00%	2.00%				2.00%	1.00%
Debt covenant term	30 months
Debt instrument final payment	$ 800
SVB Term Loan | Prime Rate
Debt Instrument [Line Items]
Debt instrument floating rate (as a percent)	2.40%
CRG Term Loan
Debt Instrument [Line Items]
Debt extinguishment loss					2,400
Write-offs of unamortized debt discount and debt issuance costs				$ 300	$ 300