Schedule of Unrealized Loss on Available for Sale Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 26, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments with unrealized losses, Less than 12 months, Fair Value	$ 42,906	$ 58,791
Available-for-sale investments with unrealized losses, Less than 12 months, Unrealized Loss	207	288
Available-for-sale investments with unrealized losses, 12 months or longer, Fair Value	3,681	562
Available-for-sale investments with unrealized losses, 12 months or longer, Unrealized Loss	57	53
Available-for-sale investments with unrealized losses, Fair Value, Total	46,587	59,353
Available-for-sale investments with unrealized losses, Unrealized Loss, Total	264	341
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments with unrealized losses, Less than 12 months, Fair Value	1,363	4,422
Available-for-sale investments with unrealized losses, Less than 12 months, Unrealized Loss	6	58
Available-for-sale investments with unrealized losses, 12 months or longer, Fair Value	2,314
Available-for-sale investments with unrealized losses, 12 months or longer, Unrealized Loss	13
Available-for-sale investments with unrealized losses, Fair Value, Total	3,677	4,422
Available-for-sale investments with unrealized losses, Unrealized Loss, Total	19	58
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	28,809	39,276
Available-for-sale investments with unrealized losses, Less than 12 months, Unrealized Loss	195	217
Available-for-sale investments with unrealized losses, 12 months or longer, Fair Value	1,367	562
Available-for-sale investments with unrealized losses, 12 months or longer, Unrealized Loss	44	53
Available-for-sale investments with unrealized losses, Fair Value, Total	30,176	39,838
Available-for-sale investments with unrealized losses, Unrealized Loss, Total	239	270
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	12,734	15,093
Available-for-sale investments with unrealized losses, Less than 12 months, Unrealized Loss	6	13
Available-for-sale investments with unrealized losses, Fair Value, Total	12,734	15,093
Available-for-sale investments with unrealized losses, Unrealized Loss, Total	$ 6	$ 13