Amortized Cost and Fair Value of Available-for-Sale Investments (Detail) - USD ($) $ in Thousands	Sep. 26, 2020	Dec. 28, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 118,452	$ 116,181
Gross Unrealized Gains	3,677	1,477
Gross Unrealized Losses	327	50
Fair Value	121,802	117,608
Money market investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	14,456	15,691
Fair Value	14,456	15,691
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	569	572
Gross Unrealized Losses	38	1
Fair Value	531	571
Corporate bonds and direct obligations of government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	101,090	97,583
Gross Unrealized Gains	3,540	1,465
Gross Unrealized Losses	289	44
Fair Value	104,341	99,004
U.S. Treasury obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,337	2,335
Gross Unrealized Gains	137	12
Gross Unrealized Losses		5
Fair Value	$ 2,474	$ 2,342

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

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=121645371&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount 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 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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