Debt - Interest expense (Details) - 1.375% Convertible Senior Notes - USD ($) $ in Thousands	3 Months Ended
Apr. 04, 2020	Mar. 30, 2019
Interest expense related to the Notes
Contractual interest expense	$ 1,467	$ 1,390
Amortization of debt discount	3,246	2,886
Amortization of debt issuance costs	490	435
Interest Expense, Total	$ 5,203	$ 4,711

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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