Debt - Carrying amount and interest expense of notes (Details) - 1.375% Convertible Senior Notes - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017	Dec. 30, 2017
Liability component
Principal	$ 400,000		$ 400,000		$ 400,000
Unamortized debt discount	(44,979)		(44,979)		(50,499)
Unamortized debt issuance costs	(6,788)		(6,788)		(7,622)
Net carrying amount	348,233		348,233		341,879
Equity component
Net carrying amount	$ 57,735		$ 57,735		$ 57,735
Effective interest rate	4.75%		4.75%		4.75%
Amortization of debt discount and debt issuance costs			3 years 8 months 12 days
Interest expense related to the Notes
Contractual interest expense	$ 1,375	$ 1,375	$ 2,765	$ 1,757
Amortization of debt discount	2,767	2,640	5,520	3,395
Amortization of debt issuance costs	418	398	834	512
Interest Expense, Total	$ 4,560	$ 4,413	$ 9,119	$ 5,664