Cash, cash equivalents and short-term investments (Detail) - USD ($) $ in Thousands	Dec. 28, 2018	Dec. 29, 2017
Fair Value Measurements [Line Items]
Amortized Cost	$ 208,693	$ 196,791
Unrealized Gains	91	0
Unrealized Losses	(230)	(393)
Estimated Fair Value	208,554	196,398
U.S. Treasury and agency securities
Fair Value Measurements [Line Items]
Amortized Cost	81,634	71,997
Unrealized Gains	91	0
Unrealized Losses	(230)	(393)
Estimated Fair Value	81,495	71,604
Cash
Fair Value Measurements [Line Items]
Amortized Cost	120,846	115,052
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	120,846	115,052
Money market securities
Fair Value Measurements [Line Items]
Amortized Cost	6,213	9,742
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	6,213	9,742
Total cash equivalents
Fair Value Measurements [Line Items]
Amortized Cost	6,213	9,742
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	6,213	9,742
Total cash and cash equivalents
Fair Value Measurements [Line Items]
Amortized Cost	127,059	124,794
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	127,059	124,794
Total short-term investments
Fair Value Measurements [Line Items]
Amortized Cost	81,634	71,997
Unrealized Gains	91	0
Unrealized Losses	(230)	(393)
Estimated Fair Value	$ 81,495	$ 71,604

Amount of accumulated gross unrealized gain on investments in available for sale securities.

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- Definition

Amount of accumulated gross unrealized loss on investments in available for sale securities.

+ References

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- Definition

This item represents the fair value of cash, cash equivalents and short-term investments.

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- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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