Cash, Cash Equivalents and Investments (Details) (USD $)	9 Months Ended
	Sep. 28, 2013	Dec. 29, 2012
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	39,600,000	56,690,000
Cash on hand, Fair Value	39,600,000	56,690,000
Cash and cash equivalents, Cost	116,539,000	105,424,000
Cash and cash equivalents, Gross Unrealized Gains		2,000
Cash and cash equivalents, Fair Value	116,539,000	105,426,000
Cash Equivalents:
Available-for-sale securities
Available-for-sale securities, Cost	76,939,000	48,734,000
Available-for-sale securities, Gross Unrealized Gains		2,000
Available-for-sale securities, Fair Value	76,939,000	48,736,000
Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	153,688,000	176,207,000
Available-for-sale securities, Gross Unrealized Losses	(20,000)	(12,000)
Available-for-sale securities, Gross Unrealized Gains	189,000	370,000
Available-for-sale securities, Fair Value	153,857,000	176,565,000
Long-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	12,425,000	12,525,000
Available-for-sale securities, Gross Unrealized Losses	(1,754,000)	(1,156,000)
Available-for-sale securities, Fair Value	10,671,000	11,369,000
U.S. Treasury bills | Cash Equivalents:
Available-for-sale securities
Available-for-sale securities, Cost		25,049,000
Available-for-sale securities, Gross Unrealized Gains		1,000
Available-for-sale securities, Fair Value		25,050,000
Money market funds | Cash Equivalents:
Available-for-sale securities
Available-for-sale securities, Cost	61,025,000	22,685,000
Available-for-sale securities, Gross Unrealized Gains		1,000
Available-for-sale securities, Fair Value	61,025,000	22,686,000
Commercial paper | Cash Equivalents:
Available-for-sale securities
Available-for-sale securities, Cost	9,349,000
Available-for-sale securities, Fair Value	9,349,000
Commercial paper | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	6,795,000
Available-for-sale securities, Fair Value	6,795,000
Certificates of deposit | Cash Equivalents:
Available-for-sale securities
Available-for-sale securities, Cost	6,565,000
Available-for-sale securities, Fair Value	6,565,000
Corporate bonds | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	19,851,000	59,089,000
Available-for-sale securities, Gross Unrealized Losses	(7,000)	(5,000)
Available-for-sale securities, Gross Unrealized Gains	57,000	267,000
Available-for-sale securities, Fair Value	19,901,000	59,351,000
Municipal bonds | Cash Equivalents:
Available-for-sale securities
Available-for-sale securities, Cost		1,000,000
Available-for-sale securities, Fair Value		1,000,000
Municipal bonds | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	93,405,000	45,646,000
Available-for-sale securities, Gross Unrealized Losses	(13,000)	(7,000)
Available-for-sale securities, Gross Unrealized Gains	131,000	50,000
Available-for-sale securities, Fair Value	93,523,000	45,689,000
Variable-rate demand notes | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	33,225,000	41,785,000
Available-for-sale securities, Fair Value	33,225,000	41,785,000
Asset-backed securities | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	412,000	15,058,000
Available-for-sale securities, Gross Unrealized Gains	1,000	11,000
Available-for-sale securities, Fair Value	413,000	15,069,000
U.S. government bonds | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost		12,638,000
Available-for-sale securities, Gross Unrealized Gains		25,000
Available-for-sale securities, Fair Value		12,663,000
International government bonds | Short-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost		1,991,000
Available-for-sale securities, Gross Unrealized Gains		17,000
Available-for-sale securities, Fair Value		2,008,000
Auction rate securities | Long-term Investments:
Available-for-sale securities
Available-for-sale securities, Cost	12,425,000	12,525,000
Available-for-sale securities, Gross Unrealized Losses	(1,754,000)	(1,156,000)
Available-for-sale securities, Fair Value	$ 10,671,000	$ 11,369,000