CONVERTIBLE DEBT - Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Oct. 01, 2021	Sep. 25, 2020	Oct. 01, 2021	Sep. 25, 2020
2022 Notes
Debt Instrument [Line Items]
Contractual interest expense	$ 412	$ 412	$ 1,236	$ 540
Amortization of debt discount	171	161	508	210
Amortization of debt issuance costs	54	50	159	65
Total interest expense recognized	637	623	1,903	815
2024 Notes
Debt Instrument [Line Items]
Contractual interest expense	578	578	1,734	1,734
Amortization of debt discount	1,193	1,102	3,505	3,237
Amortization of debt issuance costs	162	153	476	443
Total interest expense recognized	$ 1,933	$ 1,833	$ 5,715	$ 5,414