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Debt - Carrying amount and interest expense of notes (Details) - 1.375% Convertible Senior Notes - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 29, 2019
Jun. 30, 2018
Jun. 29, 2019
Jun. 30, 2018
Dec. 29, 2018
Liability component          
Principal $ 400,000   $ 400,000   $ 400,000
Unamortized debt discount (33,512)   (33,512)   (39,298)
Unamortized debt issuance costs (5,058)   (5,058)   (5,931)
Net carrying amount 361,430   361,430   354,771
Equity component          
Net carrying amount $ 57,735   $ 57,735   $ 57,735
Effective interest rate 4.75%   4.75%   4.75%
Amortization period of debt discount and debt issuance costs     2 years 8 months 12 days    
Interest expense related to the Notes          
Contractual interest expense $ 1,360 $ 1,375 $ 2,750 $ 2,765  
Amortization of debt discount 2,900 2,767 5,786 5,520  
Amortization of debt issuance costs 437 418 873 834  
Interest Expense, Total $ 4,697 $ 4,560 $ 9,409 $ 9,119