Debt - Carrying amount and interest expense of notes (Details) - 1.375% Convertible Senior Notes - USD ($) $ in Thousands	3 Months Ended
	Mar. 30, 2019	Mar. 31, 2018	Dec. 29, 2018
Liability component
Principal	$ 400,000		$ 400,000
Unamortized debt discount	(36,412)		(39,298)
Unamortized debt issuance costs	(5,495)		(5,931)
Net carrying amount	358,093		354,771
Equity component
Net carrying amount	$ 57,735		$ 57,735
Effective interest rate	4.75%		4.75%
Amortization period of debt discount and debt issuance costs	2 years 10 months 24 days
Interest expense related to the Notes
Contractual interest expense	$ 1,390	$ 1,390
Amortization of debt discount	2,886	2,753
Amortization of debt issuance costs	435	416
Interest Expense, Total	$ 4,711	$ 4,559