Fair Value of Financial Instruments - Available-for-sale investments (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 28, 2019	Dec. 29, 2018
Cost
Due in one year or less, Cost	$ 408,664
Due after one year through ten years, Cost	176,798
Due after ten years, Cost	10,300
Total Cost	595,762
Fair Value
Due in one year or less, Fair Value	409,445
Due after one year through ten years, Fair Value	177,497
Due after ten years, Fair Value	10,015
Total Fair Value	596,957
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	80,178	$ 125,977
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	35,192	170,765
Total fair value of available-for-sale securities, continuous loss position	115,370	296,742
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(130)	(283)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(322)	(1,272)
Available-for-sale securities, total gross unrealized losses	(452)	(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	23,804	13,278
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	20,922	88,696
Total fair value of available-for-sale securities, continuous loss position	44,726	101,974
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(30)	(10)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(35)	(583)
Available-for-sale securities, total gross unrealized losses	(65)	(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	56,374	112,699
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	8,555	76,310
Total fair value of available-for-sale securities, continuous loss position	64,929	189,009
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(100)	(273)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(2)	(448)
Available-for-sale securities, total gross unrealized losses	(102)	(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	5,715	5,759
Total fair value of available-for-sale securities, continuous loss position	5,715	5,759
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(285)	(241)
Available-for-sale securities, total gross unrealized losses	$ (285)	$ (241)