Schedule of Unrealized Loss on Available for Sale Investments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 27, 2014	Dec. 28, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments with unrealized losses, Less than 12 months, Fair Value	$ 28,352	$ 26,355
Available-for-sale investments with unrealized losses, Less than 12 months, Unrealized Loss	149	455
Available-for-sale investments with unrealized losses, 12 months or longer, Fair Value	14,580
Available-for-sale investments with unrealized losses, 12 months or longer, Unrealized Loss	194
Available-for-sale investments with unrealized losses, Fair Value, Total	42,932	26,355
Available-for-sale investments with unrealized losses, Unrealized Loss, Total	343	455
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments with unrealized losses, Less than 12 months, Fair Value	2,006	3,429
Available-for-sale investments with unrealized losses, Less than 12 months, Unrealized Loss	13	72
Available-for-sale investments with unrealized losses, 12 months or longer, Fair Value	2,447
Available-for-sale investments with unrealized losses, 12 months or longer, Unrealized Loss	37
Available-for-sale investments with unrealized losses, Fair Value, Total	4,453	3,429
Available-for-sale investments with unrealized losses, Unrealized Loss, Total	50	72
Corporate bonds and direct obligations of government agencies
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments with unrealized losses, Less than 12 months, Fair Value	19,354	22,169
Available-for-sale investments with unrealized losses, Less than 12 months, Unrealized Loss	135	370
Available-for-sale investments with unrealized losses, 12 months or longer, Fair Value	11,373
Available-for-sale investments with unrealized losses, 12 months or longer, Unrealized Loss	149
Available-for-sale investments with unrealized losses, Fair Value, Total	30,727	22,169
Available-for-sale investments with unrealized losses, Unrealized Loss, Total	284	370
U.S. Treasury Obligations
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments with unrealized losses, Less than 12 months, Fair Value	6,992	757
Available-for-sale investments with unrealized losses, Less than 12 months, Unrealized Loss	1	13
Available-for-sale investments with unrealized losses, 12 months or longer, Fair Value	760
Available-for-sale investments with unrealized losses, 12 months or longer, Unrealized Loss	8
Available-for-sale investments with unrealized losses, Fair Value, Total	7,752	757
Available-for-sale investments with unrealized losses, Unrealized Loss, Total	$ 9	$ 13

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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