Debt - Interest expense (Details) - Convertible Senior Notes - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jul. 04, 2020	Jun. 29, 2019	Jul. 04, 2020	Jun. 29, 2019
Interest expense related to the Notes
Contractual interest expense	$ 1,374	$ 1,360	$ 2,841	$ 2,750
Amortization of debt discount	4,132	2,900	7,378	5,786
Amortization of debt issuance costs	491	437	981	873
Interest Expense, Total	$ 5,997	$ 4,697	$ 11,200	$ 9,409

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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