Fair Value of Financial Instruments - Available-for-sale investments (Details) - USD ($) $ in Thousands	9 Months Ended
	Oct. 01, 2016	Jan. 02, 2016
Cost
Due in one year or less, Cost	$ 99,623
Due after one year through ten years, Cost	58,238
Due after ten years, Cost	20,725
Total Cost	178,586
Fair Value
Due in one year or less, Fair Value	99,620
Due after one year through ten years, Fair Value	58,251
Due after ten years, Fair Value	19,705
Total Fair Value	177,576
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	57,328	$ 32,487
Fair value of available-for-sale securities, continuous loss position for twelve months or longer	6,980	8,324
Total fair value of available-for-sale securities, continuous loss position	64,308	40,811
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(57)	(40)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(1,020)	(876)
Available-for-sale securities, total gross unrealized losses	(1,077)	(916)
Other than temporary impairment losses
Other-than-temporary impairment losses	0
Municipal bonds
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	42,653	29,271
Fair value of available-for-sale securities, continuous loss position for twelve months or longer		1,198
Total fair value of available-for-sale securities, continuous loss position	42,653	30,469
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(39)	(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	(39)	(32)
Corporate bonds
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	9,368	996
Total fair value of available-for-sale securities, continuous loss position	9,368	996
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(16)	(3)
Available-for-sale securities, total gross unrealized losses	(16)	(3)
Auction rate securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for twelve months or longer	6,980	7,126
Total fair value of available-for-sale securities, continuous loss position	6,980	7,126
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(1,020)	(874)
Available-for-sale securities, total gross unrealized losses	(1,020)	(874)
U.S. government bonds
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	5,307
Total fair value of available-for-sale securities, continuous loss position	5,307
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(2)
Available-for-sale securities, total gross unrealized losses	$ (2)
International government bonds
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve 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)