Fair Value of Financial Instruments - Available-for-sale investments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 30, 2019	Dec. 29, 2018
Cost
Due in one year or less, Cost	$ 363,143
Due after one year through ten years, Cost	159,416
Due after ten years, Cost	6,000
Total Cost	528,559
Fair Value
Due in one year or less, Fair Value	362,990
Due after one year through ten years, Fair Value	159,877
Due after ten years, Fair Value	5,761
Total Fair Value	528,628
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	42,490	$ 125,977
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	122,000	170,765
Total fair value of available-for-sale securities, continuous loss position	164,490	296,742
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(36)	(283)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(743)	(1,272)
Available-for-sale securities, total gross unrealized losses	(779)	(1,555)
Other than temporary impairment losses
Other-than-temporary impairment losses	0
Government debt securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	11,437	13,278
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	72,515	88,696
Total fair value of available-for-sale securities, continuous loss position	83,952	101,974
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(3)	(10)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(325)	(583)
Available-for-sale securities, total gross unrealized losses	(328)	(593)
Corporate debt securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	31,053	112,699
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	43,724	76,310
Total fair value of available-for-sale securities, continuous loss position	74,777	189,009
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(33)	(273)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(179)	(448)
Available-for-sale securities, total gross unrealized losses	(212)	(721)
Auction rate securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	5,761	5,759
Total fair value of available-for-sale securities, continuous loss position	5,761	5,759
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(239)	(241)
Available-for-sale securities, total gross unrealized losses	$ (239)	$ (241)