Fair Value of Financial Instruments - Available-for-sale investments (Details) - USD ($) $ in Thousands	3 Months Ended
	Apr. 01, 2017	Dec. 31, 2016
Cost
Due in one year or less, Cost	$ 291,314
Due after one year through ten years, Cost	149,197
Due after ten years, Cost	117,270
Total Cost	557,781
Fair Value
Due in one year or less, Fair Value	291,287
Due after one year through ten years, Fair Value	149,157
Due after ten years, Fair Value	116,527
Total Fair Value	556,971
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	134,804	$ 101,480
Fair value of available-for-sale securities, continuous loss position for twelve months or longer	5,257	5,196
Total fair value of available-for-sale securities, continuous loss position	140,061	106,676
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(235)	(288)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(742)	(804)
Available-for-sale securities, total gross unrealized losses	(977)	(1,092)
Other than temporary impairment losses
Other-than-temporary impairment losses	0
Corporate bonds
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	45,299	18,561
Total fair value of available-for-sale securities, continuous loss position	45,299	18,561
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(145)	(128)
Available-for-sale securities, total gross unrealized losses	(145)	(128)
Municipal bonds
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	38,781	69,379
Total fair value of available-for-sale securities, continuous loss position	38,781	69,379
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(42)	(140)
Available-for-sale securities, total gross unrealized losses	(42)	(140)
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	28,251	10,364
Total fair value of available-for-sale securities, continuous loss position	28,251	10,364
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(23)	(16)
Available-for-sale securities, total gross unrealized losses	(23)	(16)
Asset-backed securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	17,478	3,176
Total fair value of available-for-sale securities, continuous loss position	17,478	3,176
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(22)	(4)
Available-for-sale securities, total gross unrealized losses	(22)	(4)
Auction rate securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for twelve months or longer	5,257	5,196
Total fair value of available-for-sale securities, continuous loss position	5,257	5,196
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(742)	(804)
Available-for-sale securities, total gross unrealized losses	(742)	$ (804)
Agency bonds
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	3,498
Total fair value of available-for-sale securities, continuous loss position	3,498
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(1)
Available-for-sale securities, total gross unrealized losses	(1)
International government bonds
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	1,497
Total fair value of available-for-sale securities, continuous loss position	1,497
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)