Debt - Carrying amount of notes and interest expense (Details) - 1.375% Convertible Senior Notes - USD ($) $ in Thousands	3 Months Ended
	Apr. 01, 2017	Mar. 06, 2017
Debt
Total Debt issuance costs		$ 10,600
Liability component
Principal	$ 400,000	$ 400,000
Unamortized debt discount	(58,561)
Unamortized debt issuance costs	(8,937)
Net carrying amount	332,502
Equity component
Net carrying amount	$ 57,718
Effective interest rate	4.75%
Amortization of debt discount and debt issuance costs	4 years 10 months 24 days
Interest expense related to the Notes
Contractual interest expense	$ 382
Amortization of debt discount	754
Amortization of debt issuance costs	115
Interest Expense, Total	$ 1,251