Debt - Interest expense (Details) - Convertible Senior Notes - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Oct. 03, 2020	Sep. 28, 2019	Oct. 03, 2020	Sep. 28, 2019
Interest expense related to the Notes
Contractual interest expense	$ 1,365	$ 1,360	$ 4,206	$ 4,110
Amortization of debt discount	6,041	2,930	13,419	8,716
Amortization of debt issuance costs	546	442	1,527	1,315
Interest Expense, Total	$ 7,952	$ 4,732	$ 19,152	$ 14,141

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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