Fair Value of Financial Instruments - Available-for-sale investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 28, 2019	Dec. 29, 2018
Cost
Due in one year or less, Cost	$ 381,583
Due after one year through ten years, Cost	196,702
Due after ten years, Cost	18,900
Total Cost	597,185
Fair Value
Due in one year or less, Fair Value	382,259
Due after one year through ten years, Fair Value	197,370
Due after ten years, Fair Value	18,547
Total Fair Value	598,176
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	80,063	$ 125,977
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	12,850	170,765
Total fair value of available-for-sale securities, continuous loss position	92,913	296,742
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(100)	(283)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(360)	(1,272)
Available-for-sale securities, total gross unrealized losses	(460)	(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	11,947	13,278
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	7,183	88,696
Total fair value of available-for-sale securities, continuous loss position	19,130	101,974
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(19)	(10)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(7)	(583)
Available-for-sale securities, total gross unrealized losses	(26)	(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	68,116	112,699
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	20	76,310
Total fair value of available-for-sale securities, continuous loss position	68,136	189,009
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(81)	(273)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses		(448)
Available-for-sale securities, total gross unrealized losses	(81)	(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	5,647	5,759
Total fair value of available-for-sale securities, continuous loss position	5,647	5,759
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(353)	(241)
Available-for-sale securities, total gross unrealized losses	$ (353)	$ (241)