Cash, cash equivalents and short-term investments (Detail) - USD ($) $ in Thousands	Mar. 30, 2018	Dec. 29, 2017
Fair Value Measurements [Line Items]
Amortized Cost	$ 179,083	$ 196,791
Unrealized Gains	0	0
Unrealized Losses	(446)	(393)
Estimated Fair Value	178,637	196,398
U.S. agency securities
Fair Value Measurements [Line Items]
Amortized Cost	75,931	71,997
Unrealized Gains	0	0
Unrealized Losses	(446)	(393)
Estimated Fair Value	75,485	71,604
Cash
Fair Value Measurements [Line Items]
Amortized Cost	97,081	115,052
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	97,081	115,052
Money market securities
Fair Value Measurements [Line Items]
Amortized Cost	6,071	9,742
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	6,071	9,742
Total cash equivalents
Fair Value Measurements [Line Items]
Amortized Cost	6,071	9,742
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	6,071	9,742
Total cash and cash equivalents
Fair Value Measurements [Line Items]
Amortized Cost	103,152	124,794
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	103,152	124,794
Total short-term investments
Fair Value Measurements [Line Items]
Amortized Cost	75,931	71,997
Unrealized Gains	0	0
Unrealized Losses	(446)	(393)
Estimated Fair Value	$ 75,485	$ 71,604

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

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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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This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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