Fair Value of Financial Instruments - Available-for-sale investments (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 29, 2019	Dec. 29, 2018
Cost
Due in one year or less, Cost	$ 394,836
Due after one year through ten years, Cost	144,592
Due after ten years, Cost	13,300
Total Cost	552,728
Fair Value
Due in one year or less, Fair Value	395,419
Due after one year through ten years, Fair Value	145,639
Due after ten years, Fair Value	12,947
Total Fair Value	554,005
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	13,701	$ 125,977
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	97,431	170,765
Total fair value of available-for-sale securities, continuous loss position	111,132	296,742
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(7)	(283)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(496)	(1,272)
Available-for-sale securities, total gross unrealized losses	(503)	(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	12,126	13,278
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	58,758	88,696
Total fair value of available-for-sale securities, continuous loss position	70,884	101,974
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(6)	(10)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(98)	(583)
Available-for-sale securities, total gross unrealized losses	(104)	(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	1,575	112,699
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	33,026	76,310
Total fair value of available-for-sale securities, continuous loss position	34,601	189,009
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(1)	(273)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(45)	(448)
Available-for-sale securities, total gross unrealized losses	(46)	(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)