Fair Value of Financial Instruments - Available-for-sale investments (Details) - USD ($) $ in Thousands	Jul. 04, 2020	Dec. 28, 2019
Cost
Due in one year or less, Cost	$ 438,505
Due after one year through ten years, Cost	207,194
Due after ten years, Cost	6,000
Total Cost	651,699
Fair Value
Due in one year or less, Fair Value	439,825
Due after one year through ten years, Fair Value	209,840
Due after ten years, Fair Value	5,400
Total Fair Value	655,065
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	65,760	$ 80,063
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	5,400	12,850
Total fair value of available-for-sale securities, continuous loss position	71,160	92,913
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(66)	(100)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(600)	(360)
Available-for-sale securities, total gross unrealized losses	(666)	(460)
Government debt securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	8,186	11,947
Fair value of available-for-sale securities, continuous loss position for twelve months or greater		7,183
Total fair value of available-for-sale securities, continuous loss position	8,186	19,130
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(1)	(19)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses		(7)
Available-for-sale securities, total gross unrealized losses	(1)	(26)
Corporate debt securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	57,574	68,116
Fair value of available-for-sale securities, continuous loss position for twelve months or greater		20
Total fair value of available-for-sale securities, continuous loss position	57,574	68,136
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(65)	(81)
Available-for-sale securities, total gross unrealized losses	(65)	(81)
Auction rate securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	5,400	5,647
Total fair value of available-for-sale securities, continuous loss position	5,400	5,647
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(600)	(353)
Available-for-sale securities, total gross unrealized losses	$ (600)	$ (353)