Schedule of Unrealized Loss on Available for Sale Investments (Detail) - USD ($) $ in Thousands	Sep. 26, 2015	Dec. 27, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments with unrealized losses, Less than 12 months, Fair Value	$ 23,577	$ 28,352
Available-for-sale investments with unrealized losses, Less than 12 months, Unrealized Loss	72	149
Available-for-sale investments with unrealized losses, 12 months or longer, Fair Value	4,093	14,580
Available-for-sale investments with unrealized losses, 12 months or longer, Unrealized Loss	39	194
Available-for-sale investments with unrealized losses, Fair Value, Total	27,670	42,932
Available-for-sale investments with unrealized losses, Unrealized Loss, Total	111	343
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments with unrealized losses, Less than 12 months, Fair Value	3,744	2,006
Available-for-sale investments with unrealized losses, Less than 12 months, Unrealized Loss	17	13
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	3,744	4,453
Available-for-sale investments with unrealized losses, Unrealized Loss, Total	17	50
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	15,759	19,354
Available-for-sale investments with unrealized losses, Less than 12 months, Unrealized Loss	54	135
Available-for-sale investments with unrealized losses, 12 months or longer, Fair Value	4,093	11,373
Available-for-sale investments with unrealized losses, 12 months or longer, Unrealized Loss	39	149
Available-for-sale investments with unrealized losses, Fair Value, Total	19,852	30,727
Available-for-sale investments with unrealized losses, Unrealized Loss, Total	93	284
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	4,074	6,992
Available-for-sale investments with unrealized losses, Less than 12 months, Unrealized Loss	1	1
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	4,074	7,752
Available-for-sale investments with unrealized losses, Unrealized Loss, Total	$ 1	$ 9