Interest Expense on Convertible Senior Notes (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Oct. 01, 2017	Sep. 30, 2018	Oct. 01, 2017
Debt Instrument
Contractual interest expense on the coupon	$ 1,438	$ 1,422	$ 4,313	$ 4,297
Amortization of the discount component and debt issue fees recognized as interest expense	3,520	3,350	10,431	9,926
Total interest expense on the convertible debt	$ 4,958	$ 4,772	$ 14,744	$ 14,223

Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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