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Amortized Cost and Fair Value of Available for Sale Investments (Detail) - USD ($) $ in Thousands	Jun. 27, 2015	Dec. 27, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 106,384	$ 103,306
Gross Unrealized Gains	406	506
Gross Unrealized Losses	144	343
Fair Value	106,646	103,469
Money market investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,486	1,729
Fair Value	5,486	1,729
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,039	5,106
Gross Unrealized Gains		1
Gross Unrealized Losses	29	50
Fair Value	5,010	5,057
Corporate bonds and direct obligations of government agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	76,231	76,964
Gross Unrealized Gains	393	491
Gross Unrealized Losses	113	284
Fair Value	76,511	77,171
U.S. Treasury Obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	19,628	19,507
Gross Unrealized Gains	13	14
Gross Unrealized Losses	2	9
Fair Value	$ 19,639	$ 19,512

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