CONVERTIBLE DEBT - Components of Convertible Notes (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Oct. 01, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Carrying amount	$ 97,563	$ 129,507
2022 Notes
Debt Instrument [Line Items]
Principal amount	37,707	37,707
Less: Debt discount, net of amortization	(849)	(1,357)
Less: Debt issuance costs, net of amortization	(266)	(425)
Carrying amount	$ 36,592	$ 35,925
Remaining debt discount amortization period (years)	1 year 2 months 12 days	1 year 10 months 24 days
Effective interest rate on liability component (as a percent)	6.95%	6.95%
2024 Notes
Debt Instrument [Line Items]
Principal amount	$ 115,500	$ 115,500
Less: Debt discount, net of amortization	(15,789)	(19,294)
Less: Debt issuance costs, net of amortization	(2,148)	(2,624)
Carrying amount	$ 97,563	$ 93,582
Remaining debt discount amortization period (years)	2 years 10 months 24 days	3 years 8 months 12 days
Effective interest rate on liability component (as a percent)	7.95%	7.95%