Fair Value of Financial Instruments - Available-for-sale investments (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 30, 2017
Cost
Due in one year or less, Cost	$ 313,832
Due after one year through ten years, Cost	131,655
Due after ten years, Cost	24,042
Total Cost	469,529
Fair Value
Due in one year or less, Fair Value	313,333
Due after one year through ten years, Fair Value	130,420
Due after ten years, Fair Value	23,681
Total Fair Value	467,434
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	280,445	$ 396,029
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	20,063	20,286
Total fair value of available-for-sale securities, continuous loss position	300,508	416,315
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(1,683)	(1,378)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(434)	(407)
Available-for-sale securities, total gross unrealized losses	(2,117)	(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	153,566	244,880
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	3,269	3,027
Total fair value of available-for-sale securities, continuous loss position	156,835	247,907
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(893)	(931)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(6)	(15)
Available-for-sale securities, total gross unrealized losses	(899)	(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	126,879	151,149
Fair value of available-for-sale securities, continuous loss position for twelve months or greater	11,152	11,578
Total fair value of available-for-sale securities, continuous loss position	138,031	162,727
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(790)	(447)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(70)	(73)
Available-for-sale securities, total gross unrealized losses	(860)	(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	5,642	5,681
Total fair value of available-for-sale securities, continuous loss position	5,642	5,681
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(358)	(319)
Available-for-sale securities, total gross unrealized losses	$ (358)	$ (319)