Marketable Securities Available-for-sale Securities - Continuous Loss Position (Details) - USD ($)	3 Months Ended
	Apr. 01, 2018	Dec. 31, 2017
Long-term AFS securities
Other-than-temporary impairment loss	$ 0
Total short-term AFS securities
Short-term AFS securities
Fair value, less than 12 months	226,764,000	$ 230,832,000
Gross unrealized losses, less than 12 months	(474,000)	(283,000)
Fair value, greater than 12 months	49,030,000	35,866,000
Gross unrealized losses, greater than 12 months	(335,000)	(157,000)
Fair value, total	275,794,000	266,698,000
Gross unrealized losses, total	(809,000)	(440,000)
Total long-term AFS securities
Long-term AFS securities
Fair value, less than 12 months	167,423,000	157,744,000
Gross unrealized losses, less than 12 months	(2,158,000)	(1,107,000)
Fair value, greater than 12 months	63,347,000	83,667,000
Gross unrealized losses, greater than 12 months	(951,000)	(998,000)
Fair value, total	230,770,000	241,411,000
Gross unrealized losses, total	(3,109,000)	(2,105,000)
Corporate Debt Securities
Short-term AFS securities
Fair value, less than 12 months	137,958,000	140,111,000
Gross unrealized losses, less than 12 months	(373,000)	(199,000)
Fair value, greater than 12 months	8,874,000	9,996,000
Gross unrealized losses, greater than 12 months	(87,000)	(28,000)
Fair value, total	146,832,000	150,107,000
Gross unrealized losses, total	(460,000)	(227,000)
Long-term AFS securities
Fair value, less than 12 months	91,661,000	81,118,000
Gross unrealized losses, less than 12 months	(1,244,000)	(579,000)
Fair value, greater than 12 months	4,943,000	10,886,000
Gross unrealized losses, greater than 12 months	(82,000)	(104,000)
Fair value, total	96,604,000	92,004,000
Gross unrealized losses, total	(1,326,000)	(683,000)
US Treasury Securities
Short-term AFS securities
Fair value, less than 12 months	69,009,000	70,951,000
Gross unrealized losses, less than 12 months	(62,000)	(34,000)
Fair value, greater than 12 months	0	0
Gross unrealized losses, greater than 12 months	0	0
Fair value, total	69,009,000	70,951,000
Gross unrealized losses, total	(62,000)	(34,000)
Long-term AFS securities
Fair value, less than 12 months	23,910,000	23,998,000
Gross unrealized losses, less than 12 months	(244,000)	(125,000)
Fair value, greater than 12 months	58,404,000	72,781,000
Gross unrealized losses, greater than 12 months	(869,000)	(894,000)
Fair value, total	82,314,000	96,779,000
Gross unrealized losses, total	(1,113,000)	(1,019,000)
U.S. governmental agency securities
Short-term AFS securities
Fair value, less than 12 months	19,797,000	19,770,000
Gross unrealized losses, less than 12 months	(39,000)	(50,000)
Fair value, greater than 12 months	40,156,000	25,870,000
Gross unrealized losses, greater than 12 months	(248,000)	(129,000)
Fair value, total	59,953,000	45,640,000
Gross unrealized losses, total	(287,000)	(179,000)
Long-term AFS securities
Fair value, less than 12 months	51,852,000	52,628,000
Gross unrealized losses, less than 12 months	(670,000)	(403,000)
Fair value, greater than 12 months	0	0
Gross unrealized losses, greater than 12 months	0	0
Fair value, total	51,852,000	52,628,000
Gross unrealized losses, total	$ (670,000)	$ (403,000)