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Schedule of Unrealized Loss on Available-for-Sale Investments (Detail) - USD ($)
$ in Thousands
Dec. 26, 2020
Dec. 28, 2019
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale investments with unrealized losses, Less than 12 months, Fair Value $ 3,222 $ 10,525
Available-for-sale investments with unrealized losses, Less than 12 months, Unrealized Loss 98 47
Available-for-sale investments with unrealized losses, 12 months or longer, Fair Value   4,260
Available-for-sale investments with unrealized losses, 12 months or longer, Unrealized Loss   3
Available-for-sale investments with unrealized losses, Fair Value, Total 3,222 14,785
Available-for-sale investments with unrealized losses, Unrealized Loss, Total 98 50
Asset-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale investments with unrealized losses, Less than 12 months, Fair Value 541 571
Available-for-sale investments with unrealized losses, Less than 12 months, Unrealized Loss 26 1
Available-for-sale investments with unrealized losses, Fair Value, Total 541 571
Available-for-sale investments with unrealized losses, Unrealized Loss, Total 26 1
Corporate bonds and direct obligations of government agencies    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale investments with unrealized losses, Less than 12 months, Fair Value 2,681 8,728
Available-for-sale investments with unrealized losses, Less than 12 months, Unrealized Loss 72 41
Available-for-sale investments with unrealized losses, 12 months or longer, Fair Value   4,260
Available-for-sale investments with unrealized losses, 12 months or longer, Unrealized Loss   3
Available-for-sale investments with unrealized losses, Fair Value, Total 2,681 12,988
Available-for-sale investments with unrealized losses, Unrealized Loss, Total $ 72 44
U.S. Treasury obligations    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale investments with unrealized losses, Less than 12 months, Fair Value   1,226
Available-for-sale investments with unrealized losses, Less than 12 months, Unrealized Loss   5
Available-for-sale investments with unrealized losses, Fair Value, Total   1,226
Available-for-sale investments with unrealized losses, Unrealized Loss, Total   $ 5