Fair Value of Financial Instruments - Available-for-sale investments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 30, 2017
Cost
Due in one year or less, Cost	$ 338,056
Due after one year through ten years, Cost	135,923
Due after ten years, Cost	98,269
Total Cost	572,248
Fair Value
Due in one year or less, Fair Value	337,433
Due after one year through ten years, Fair Value	134,453
Due after ten years, Fair Value	97,872
Total Fair Value	569,758
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	302,623	$ 396,029
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	21,368	20,286
Total fair value of available-for-sale securities, continuous loss position	323,991	416,315
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(1,988)	(1,378)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(501)	(407)
Available-for-sale securities, total gross unrealized losses	(2,489)	(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	182,320	244,880
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	4,432	3,027
Total fair value of available-for-sale securities, continuous loss position	186,752	247,907
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(1,059)	(931)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(10)	(15)
Available-for-sale securities, total gross unrealized losses	(1,069)	(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	120,303	151,149
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	11,327	11,578
Total fair value of available-for-sale securities, continuous loss position	131,630	162,727
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(929)	(447)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(100)	(73)
Available-for-sale securities, total gross unrealized losses	(1,029)	(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	5,609	5,681
Total fair value of available-for-sale securities, continuous loss position	5,609	5,681
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(391)	(319)
Available-for-sale securities, total gross unrealized losses	$ (391)	$ (319)