Fair Value of Financial Instruments - Available-for-sale investments (Details) - USD ($) $ in Thousands	Dec. 30, 2017	Dec. 31, 2016
Cost
Due in one year or less, Cost	$ 393,600
Due after one year through ten years, Cost	198,981
Due after ten years, Cost	81,884
Total Cost	674,465
Fair Value
Due in one year or less, Fair Value	393,201
Due after one year through ten years, Fair Value	197,922
Due after ten years, Fair Value	81,561
Total Fair Value	672,684
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	396,029	$ 101,480
Fair value of available-for-sale securities, continuous loss position for twelve months or longer	20,286	5,196
Total fair value of available-for-sale securities, continuous loss position	416,315	106,676
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(1,378)	(288)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(407)	(804)
Available-for-sale securities, total gross unrealized losses	(1,785)	(1,092)
Government debt securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	244,880	79,743
Fair value of available-for-sale securities, continuous loss position for twelve months or longer	3,027
Total fair value of available-for-sale securities, continuous loss position	247,907	79,743
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(931)	(156)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(15)
Available-for-sale securities, total gross unrealized losses	(946)	(156)
Corporate debt securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	151,149	21,737
Fair value of available-for-sale securities, continuous loss position for twelve months or longer	11,578
Total fair value of available-for-sale securities, continuous loss position	162,727	21,737
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(447)	(132)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(73)
Available-for-sale securities, total gross unrealized losses	(520)	(132)
Auction rate securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for twelve months or longer	5,681	5,196
Total fair value of available-for-sale securities, continuous loss position	5,681	5,196
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(319)	(804)
Available-for-sale securities, total gross unrealized losses	$ (319)	$ (804)