Debt - Carrying amount of notes and interest expense (Details) - 1.375% Convertible Senior Notes - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jul. 01, 2017	Jul. 01, 2017	Mar. 06, 2017
Debt
Total Debt issuance costs			$ 10,600
Liability component
Principal	$ 400,000	$ 400,000	$ 400,000
Unamortized debt discount	(55,921)	(55,921)
Unamortized debt issuance costs	(8,440)	(8,440)
Net carrying amount	335,639	335,639
Equity component
Net carrying amount	$ 57,735	$ 57,735
Effective interest rate	4.75%	4.75%
Amortization of debt discount and debt issuance costs		4 years 8 months 12 days
Interest expense related to the Notes
Contractual interest expense	$ 1,375	$ 1,757
Amortization of debt discount	2,640	3,395
Amortization of debt issuance costs	398	512
Interest Expense, Total	$ 4,413	$ 5,664