Debt - Interest expense (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jul. 03, 2021	Jul. 04, 2020	Jul. 03, 2021	Jul. 04, 2020
Interest expense related to the notes
Interest Expense, Total	$ 6,486	$ 11,778	$ 17,810	$ 17,319
Convertible Senior Notes
Interest expense related to the notes
Contractual interest expense	836	1,374	1,999	2,841
Amortization of debt discount	4,992	4,132	10,930	7,378
Amortization of debt issuance costs	374	491	892	981
Interest Expense, Total	$ 6,202	$ 5,997	$ 13,821	$ 11,200

Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

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Amount of amortization expense attributable to debt issuance costs.

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Amount of the cost of borrowed funds accounted for as interest expense.

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Represents the portion of interest incurred in the period on debt arrangements that was charged against earnings, excluding amortization of debt discount (premium) and financing costs.

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