Fair Value of Financial Instruments - Available-for-sale investments (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Jan. 02, 2016
Cost
Due in one year or less, Cost	$ 159,670
Due after one year through ten years, Cost	59,628
Due after ten years, Cost	21,400
Total Cost	240,698
Fair Value
Due in one year or less, Fair Value	159,624
Due after one year through ten years, Fair Value	59,426
Due after ten years, Fair Value	20,596
Total Fair Value	239,646
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than 12 months	101,480	$ 32,487
Fair value of available-for-sale securities, continuous loss position for 12 months or greater	5,196	8,324
Total fair value of available-for-sale securities, continuous loss position	106,676	40,811
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(288)	(40)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(804)	(876)
Available-for-sale securities, total gross unrealized losses	(1,092)	(916)
Municipal bonds
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than 12 months	69,379	29,271
Fair value of available-for-sale securities, continuous loss position for 12 months or greater		1,198
Total fair value of available-for-sale securities, continuous loss position	69,379	30,469
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(140)	(30)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses		(2)
Available-for-sale securities, total gross unrealized losses	(140)	(32)
Corporate bonds
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than 12 months	18,561	996
Total fair value of available-for-sale securities, continuous loss position	18,561	996
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(128)	(3)
Available-for-sale securities, total gross unrealized losses	(128)	(3)
U.S. government bonds
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than 12 months	10,364
Total fair value of available-for-sale securities, continuous loss position	10,364
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(16)
Available-for-sale securities, total gross unrealized losses	(16)
Auction rate securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for 12 months or greater	5,196	7,126
Total fair value of available-for-sale securities, continuous loss position	5,196	7,126
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(804)	(874)
Available-for-sale securities, total gross unrealized losses	(804)	(874)
Asset-backed securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than 12 months	3,176
Total fair value of available-for-sale securities, continuous loss position	3,176
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(4)
Available-for-sale securities, total gross unrealized losses	$ (4)
International government bonds
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than 12 months		2,220
Total fair value of available-for-sale securities, continuous loss position		2,220
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses		(7)
Available-for-sale securities, total gross unrealized losses		$ (7)