Marketable Securities - Continuous Loss Position (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 30, 2018
Long-term AFS securities
Other than temporary impairment, loss	$ 0
Corporate Debt Securities [Member]
Short-term AFS securities
Fair value, less than 12 months, short-term AFS securities	140,111,000	$ 76,886,000
Gross unrealized losses, less than 12 months, short-term AFS securities	(199,000)	(115,000)
Fair value, 12 months or greater, short-term AFS securities	9,996,000	61,459,000
Gross unrealized losses, 12 months or greater, short-term AFS securities	(28,000)	(349,000)
Fair value, short-term AFS securities	150,107,000	138,345,000
Gross unrealized losses, short-term AFS securities	(227,000)	(464,000)
Long-term AFS securities
Fair value, less than 12 months, long-term AFS securities	81,118,000	81,655,000
Gross unrealized losses, less than 12 months, long-term AFS securities	(579,000)	(570,000)
Fair value, 12 months or greater, long-term AFS securities	10,886,000	27,265,000
Gross unrealized losses, 12 months or greater, long-term AFS securities	(104,000)	(462,000)
Fair value, long-term AFS securities	92,004,000	108,920,000
Gross unrealized losses, long-term AFS securities	(683,000)	(1,032,000)
US Treasury Securities [Member]
Short-term AFS securities
Fair value, less than 12 months, short-term AFS securities	70,951,000	70,830,000
Gross unrealized losses, less than 12 months, short-term AFS securities	(34,000)	(31,000)
Fair value, 12 months or greater, short-term AFS securities	0	28,207,000
Gross unrealized losses, 12 months or greater, short-term AFS securities	0	(201,000)
Fair value, short-term AFS securities	70,951,000	99,037,000
Gross unrealized losses, short-term AFS securities	(34,000)	(232,000)
Long-term AFS securities
Fair value, less than 12 months, long-term AFS securities	23,998,000	20,479,000
Gross unrealized losses, less than 12 months, long-term AFS securities	(125,000)	(29,000)
Fair value, 12 months or greater, long-term AFS securities	72,781,000	23,762,000
Gross unrealized losses, 12 months or greater, long-term AFS securities	(894,000)	(318,000)
Fair value, long-term AFS securities	96,779,000	44,241,000
Gross unrealized losses, long-term AFS securities	(1,019,000)	(347,000)
US Government Agency Securities [Member]
Short-term AFS securities
Fair value, less than 12 months, short-term AFS securities	19,770,000	11,664,000
Gross unrealized losses, less than 12 months, short-term AFS securities	(50,000)	(4,000)
Fair value, 12 months or greater, short-term AFS securities	25,870,000	80,311,000
Gross unrealized losses, 12 months or greater, short-term AFS securities	(129,000)	(650,000)
Fair value, short-term AFS securities	45,640,000	91,975,000
Gross unrealized losses, short-term AFS securities	(179,000)	(654,000)
Long-term AFS securities
Fair value, less than 12 months, long-term AFS securities	52,628,000	21,579,000
Gross unrealized losses, less than 12 months, long-term AFS securities	(403,000)	(36,000)
Fair value, 12 months or greater, long-term AFS securities	0	11,868,000
Gross unrealized losses, 12 months or greater, long-term AFS securities	0	(132,000)
Fair value, long-term AFS securities	52,628,000	33,447,000
Gross unrealized losses, long-term AFS securities	(403,000)	(168,000)
Certificates of Deposit [Member]
Short-term AFS securities
Fair value, less than 12 months, short-term AFS securities		1,599,000
Gross unrealized losses, less than 12 months, short-term AFS securities		0
Fair value, 12 months or greater, short-term AFS securities		0
Gross unrealized losses, 12 months or greater, short-term AFS securities		0
Fair value, short-term AFS securities		1,599,000
Gross unrealized losses, short-term AFS securities		0
Short-term Available-for Sale Securities [Member]
Short-term AFS securities
Fair value, less than 12 months, short-term AFS securities	230,832,000	160,979,000
Gross unrealized losses, less than 12 months, short-term AFS securities	(283,000)	(150,000)
Fair value, 12 months or greater, short-term AFS securities	35,866,000	169,977,000
Gross unrealized losses, 12 months or greater, short-term AFS securities	(157,000)	(1,200,000)
Fair value, short-term AFS securities	266,698,000	330,956,000
Gross unrealized losses, short-term AFS securities	(440,000)	(1,350,000)
Long-term AFS securities [Member]
Long-term AFS securities
Fair value, less than 12 months, long-term AFS securities	157,744,000	123,713,000
Gross unrealized losses, less than 12 months, long-term AFS securities	(1,107,000)	(635,000)
Fair value, 12 months or greater, long-term AFS securities	83,667,000	62,895,000
Gross unrealized losses, 12 months or greater, long-term AFS securities	(998,000)	(912,000)
Fair value, long-term AFS securities	241,411,000	186,608,000
Gross unrealized losses, long-term AFS securities	$ (2,105,000)	$ (1,547,000)