Fair Value of Financial Instruments - Available-for-sale investments (Details) - USD ($) $ in Thousands	Oct. 03, 2020	Dec. 28, 2019
Cost
Due in one year or less, Cost	$ 349,278
Due after one year through ten years, Cost	228,563
Due after ten years, Cost	38,350
Total Cost	616,191
Fair Value
Due in one year or less, Fair Value	350,447
Due after one year through ten years, Fair Value	230,349
Due after ten years, Fair Value	37,750
Total Fair Value	618,546
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	80,155	$ 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	85,555	92,913
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(68)	(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	(668)	(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	18,940	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	18,940	19,130
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(9)	(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	(9)	(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	61,215	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	61,215	68,136
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(59)	(81)
Available-for-sale securities, total gross unrealized losses	(59)	(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)