Fair Value of Financial Instruments - Available-for-sale investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 29, 2018	Dec. 30, 2017
Cost
Due in one year or less, Cost	$ 338,623
Due after one year through ten years, Cost	169,058
Due after ten years, Cost	19,360
Total Cost	527,041
Fair Value
Due in one year or less, Fair Value	337,910
Due after one year through ten years, Fair Value	168,657
Due after ten years, Fair Value	19,119
Total Fair Value	525,686
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	125,977	$ 396,029
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	170,765	20,286
Total fair value of available-for-sale securities, continuous loss position	296,742	416,315
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(283)	(1,378)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(1,272)	(407)
Available-for-sale securities, total gross unrealized losses	(1,555)	(1,785)
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	13,278	244,880
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	88,696	3,027
Total fair value of available-for-sale securities, continuous loss position	101,974	247,907
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(10)	(931)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(583)	(15)
Available-for-sale securities, total gross unrealized losses	(593)	(946)
Corporate debt securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	112,699	151,149
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	76,310	11,578
Total fair value of available-for-sale securities, continuous loss position	189,009	162,727
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(273)	(447)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(448)	(73)
Available-for-sale securities, total gross unrealized losses	(721)	(520)
Auction rate securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	5,759	5,681
Total fair value of available-for-sale securities, continuous loss position	5,759	5,681
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(241)	(319)
Available-for-sale securities, total gross unrealized losses	$ (241)	$ (319)