Debt - Carrying amount and interest expense of notes (Details) - 1.375% Convertible Senior Notes - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017	Dec. 30, 2017
Liability component
Principal	$ 400,000		$ 400,000		$ 400,000
Unamortized debt discount	(42,178)		(42,178)		(50,499)
Unamortized debt issuance costs	(6,365)		(6,365)		(7,622)
Net carrying amount	351,457		351,457		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 4 months 24 days
Interest expense related to the Notes
Contractual interest expense	$ 1,360	$ 1,360	$ 4,125	$ 3,117
Amortization of debt discount	2,801	2,674	8,322	6,068
Amortization of debt issuance costs	423	404	1,256	916
Interest Expense, Total	$ 4,584	$ 4,438	$ 13,703	$ 10,101