Cash, Cash Equivalents and Investments (Details) (USD $)	6 Months Ended
	Jun. 28, 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	56,959,000	45,544,000
Cash on hand, Fair Value	56,959,000	45,544,000
Cash and cash equivalents, Cost	93,290,000	95,800,000
Cash and cash equivalents, Fair Value	93,290,000	95,800,000
Cash Equivalents:
Available-for-sale securities
Available-for-sale securities, Cost	36,331,000	50,256,000
Available-for-sale securities, Fair Value	36,331,000	50,256,000
Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	235,307,000	179,365,000
Available-for-sale securities, Gross Unrealized Losses	(22,000)	(15,000)
Available-for-sale securities, Gross Unrealized Gains	360,000	243,000
Available-for-sale securities, Fair Value	235,645,000	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	(1,432,000)	(1,793,000)
Available-for-sale securities, Fair Value	10,993,000	10,632,000
Money market funds | Cash Equivalents:
Available-for-sale securities
Available-for-sale securities, Cost	16,670,000	39,538,000
Available-for-sale securities, Fair Value	16,670,000	39,538,000
Commercial paper | Cash Equivalents:
Available-for-sale securities
Available-for-sale securities, Cost	13,446,000	2,499,000
Available-for-sale securities, Fair Value	13,446,000	2,499,000
Commercial paper | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	6,542,000	3,748,000
Available-for-sale securities, Fair Value	6,542,000	3,748,000
Certificates of deposit | Cash Equivalents:
Available-for-sale securities
Available-for-sale securities, Cost	5,315,000	7,768,000
Available-for-sale securities, Fair Value	5,315,000	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	900,000	451,000
Available-for-sale securities, Fair Value	900,000	451,000
Municipal bonds | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	157,600,000	119,289,000
Available-for-sale securities, Gross Unrealized Losses	(2,000)	(11,000)
Available-for-sale securities, Gross Unrealized Gains	290,000	182,000
Available-for-sale securities, Fair Value	157,888,000	119,460,000
Corporate bonds | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	32,470,000	17,788,000
Available-for-sale securities, Gross Unrealized Losses	(15,000)	(4,000)
Available-for-sale securities, Gross Unrealized Gains	67,000	60,000
Available-for-sale securities, Fair Value	32,522,000	17,844,000
Variable-rate demand notes | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	29,645,000	38,025,000
Available-for-sale securities, Fair Value	29,645,000	38,025,000
Asset-backed securities | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	4,410,000	515,000
Available-for-sale securities, Gross Unrealized Losses	(3,000)
Available-for-sale securities, Gross Unrealized Gains	2,000	1,000
Available-for-sale securities, Fair Value	4,409,000	516,000
International government bonds | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	2,539,000
Available-for-sale securities, Gross Unrealized Losses	(1,000)
Available-for-sale securities, Gross Unrealized Gains	1,000
Available-for-sale securities, Fair Value	2,539,000
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
U.S. government agency | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	601,000
Available-for-sale securities, Gross Unrealized Losses	(1,000)
Available-for-sale securities, Fair Value	600,000
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	(1,432,000)	(1,793,000)
Available-for-sale securities, Fair Value	$ 10,993,000	$ 10,632,000