Cash, Cash Equivalents and Investments (Details) (USD $)	9 Months Ended
	Sep. 27, 2014	Dec. 28, 2013
Cash, Cash Equivalents and Investments
Auction-rate securities guaranteed by the U.S. government	$ 10,400,000
Auction-rate securities privately insured	2,000,000
Auction-rate securities having credit rating 'AA'	6,000,000
Auction-rate securities having credit rating 'A'	2,000,000
Auction-rate securities having credit rating 'BBB'	4,400,000
Other-than-temporary impairment losses	0
Cash on hand, Cost	63,284,000	45,544,000
Cash on hand, Fair Value	63,284,000	45,544,000
Cash and cash equivalents, Cost	111,705,000	95,800,000
Cash and cash equivalents, Fair Value	111,705,000	95,800,000
Available-for-sale securities
Available-for-sale securities, Cost	277,575,000
Cash Equivalents:
Available-for-sale securities
Available-for-sale securities, Cost	48,421,000	50,256,000
Available-for-sale securities, Fair Value	48,421,000	50,256,000
Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	216,729,000	179,365,000
Available-for-sale securities, Gross Unrealized Losses	(44,000)	(15,000)
Available-for-sale securities, Gross Unrealized Gains	288,000	243,000
Available-for-sale securities, Fair Value	216,973,000	179,593,000
Long-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	12,425,000	12,425,000
Available-for-sale securities, Gross Unrealized Losses	(920,000)	(1,793,000)
Available-for-sale securities, Fair Value	11,505,000	10,632,000
Money market funds | Cash Equivalents:
Available-for-sale securities
Available-for-sale securities, Cost	28,036,000	39,538,000
Available-for-sale securities, Fair Value	28,036,000	39,538,000
Commercial paper | Cash Equivalents:
Available-for-sale securities
Available-for-sale securities, Cost	14,147,000	2,499,000
Available-for-sale securities, Fair Value	14,147,000	2,499,000
Commercial paper | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	11,439,000	3,748,000
Available-for-sale securities, Fair Value	11,439,000	3,748,000
Certificates of deposit | Cash Equivalents:
Available-for-sale securities
Available-for-sale securities, Cost	5,331,000	7,768,000
Available-for-sale securities, Fair Value	5,331,000	7,768,000
Certificates of deposit | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	850,000
Available-for-sale securities, Fair Value	850,000
Municipal bonds | Cash Equivalents:
Available-for-sale securities
Available-for-sale securities, Cost	907,000	451,000
Available-for-sale securities, Fair Value	907,000	451,000
Municipal bonds | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	143,212,000	119,289,000
Available-for-sale securities, Gross Unrealized Losses	(3,000)	(11,000)
Available-for-sale securities, Gross Unrealized Gains	240,000	182,000
Available-for-sale securities, Fair Value	143,449,000	119,460,000
Corporate bonds | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	36,000,000	17,788,000
Available-for-sale securities, Gross Unrealized Losses	(33,000)	(4,000)
Available-for-sale securities, Gross Unrealized Gains	47,000	60,000
Available-for-sale securities, Fair Value	36,014,000	17,844,000
Variable-rate demand notes | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	15,755,000	38,025,000
Available-for-sale securities, Fair Value	15,755,000	38,025,000
Asset-backed securities | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	5,689,000	515,000
Available-for-sale securities, Gross Unrealized Losses	(3,000)
Available-for-sale securities, Gross Unrealized Gains	1,000	1,000
Available-for-sale securities, Fair Value	5,687,000	516,000
International government bonds | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	2,533,000
Available-for-sale securities, Gross Unrealized Losses	(5,000)
Available-for-sale securities, Fair Value	2,528,000
U.S. government bonds | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	650,000
Available-for-sale securities, Fair Value	650,000
US government agency | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	601,000
Available-for-sale securities, Fair Value	601,000
Auction rate securities | Long-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	12,425,000	12,425,000
Available-for-sale securities, Gross Unrealized Losses	(920,000)	(1,793,000)
Available-for-sale securities, Fair Value	$ 11,505,000	$ 10,632,000