Debt - Interest expense (Details) - 1.375% Convertible Senior Notes - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 28, 2019	Sep. 29, 2018	Sep. 28, 2019	Sep. 29, 2018
Interest expense related to the Notes
Contractual interest expense	$ 1,360	$ 1,360	$ 4,110	$ 4,125
Amortization of debt discount	2,930	2,801	8,716	8,322
Amortization of debt issuance costs	442	423	1,315	1,256
Interest Expense, Total	$ 4,732	$ 4,584	$ 14,141	$ 13,703

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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