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Amortized Cost and Fair Value of Available for Sale Investments (Detail) - USD ($) $ in Thousands | Dec. 26, 2015 | Dec. 27, 2014
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 108,229 | $ 103,306
Gross Unrealized Gains | 196 | 506
Gross Unrealized Losses | 341 | 343
Fair Value | 108,084 | 103,469
Money market investments |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 7,594 | 1,729
Fair Value | 7,594 | 1,729
Asset-backed securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 4,523 | 5,106
Gross Unrealized Gains | 1 | 1
Gross Unrealized Losses | 58 | 50
Fair Value | 4,466 | 5,057
Corporate bonds and direct obligations of government agencies |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 76,839 | 76,964
Gross Unrealized Gains | 190 | 491
Gross Unrealized Losses | 270 | 284
Fair Value | 76,759 | 77,171
U.S. Treasury Obligations |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 19,273 | 19,507
Gross Unrealized Gains | 5 | 14
Gross Unrealized Losses | 13 | 9
Fair Value | $ 19,265 | $ 19,512

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