Fair Value of Financial Instruments - Available-for-sale investments (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 29, 2018	Dec. 30, 2017
Cost
Due in one year or less, Cost	$ 341,518
Due after one year through ten years, Cost	128,856
Due after ten years, Cost	37,446
Total Cost	507,820
Fair Value
Due in one year or less, Fair Value	340,710
Due after one year through ten years, Fair Value	128,062
Due after ten years, Fair Value	37,193
Total Fair Value	505,965
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	252,417	$ 396,029
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	78,075	20,286
Total fair value of available-for-sale securities, continuous loss position	330,492	416,315
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(1,030)	(1,378)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(837)	(407)
Available-for-sale securities, total gross unrealized losses	(1,867)	(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	123,102	244,880
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	38,681	3,027
Total fair value of available-for-sale securities, continuous loss position	161,783	247,907
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(593)	(931)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(369)	(15)
Available-for-sale securities, total gross unrealized losses	(962)	(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	129,315	151,149
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	33,644	11,578
Total fair value of available-for-sale securities, continuous loss position	162,959	162,727
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(437)	(447)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(218)	(73)
Available-for-sale securities, total gross unrealized losses	(655)	(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	5,750	5,681
Total fair value of available-for-sale securities, continuous loss position	5,750	5,681
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(250)	(319)
Available-for-sale securities, total gross unrealized losses	$ (250)	$ (319)