Investments - Marketable Debt Securities (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 28, 2014	Jun. 29, 2013	Jun. 28, 2014	Jun. 29, 2013	Dec. 28, 2013
Schedule of Available-for-sale Securities [Line Items]
Proceeds from maturity of marketable debt securities	$ 13,500,000	$ 17,500,000	$ 23,500,000	$ 23,300,000
Investments of marketable debt securities [Abstract]
Amortized cost	83,582,000		83,582,000		86,770,000
Unrealized gains	80,000		80,000		51,000
Unrealized losses	(9,000)		(9,000)		(30,000)
Fair value	83,653,000		83,653,000		86,791,000
Amortized Cost [Abstract]
Marketable debt securities - current (due in less than one year)	45,791,000		45,791,000		52,122,000
Marketable debt securities - non-current (due in one to two years)	37,791,000		37,791,000		34,648,000
Marketable debt securities Total Amortized Cost	83,582,000		83,582,000		86,770,000
Fair Value [Abstract]
Marketable debt securities - current (due in less than one year)	45,831,000		45,831,000		52,159,000
Marketable debt securities - non-current (due in one to two years)	37,822,000		37,822,000		34,632,000
Marketable Debt Securities Total Fair value	83,653,000		83,653,000		86,791,000
U.S. Treasury securities [Member]
Investments of marketable debt securities [Abstract]
Amortized cost	23,978,000		23,978,000		23,975,000
Unrealized gains	24,000		24,000		15,000
Unrealized losses	(3,000)		(3,000)		(1,000)
Fair value	23,999,000		23,999,000		23,989,000
Corporate bonds [Member]
Investments of marketable debt securities [Abstract]
Amortized cost	30,476,000		30,476,000		35,804,000
Unrealized gains	17,000		17,000		3,000
Unrealized losses	(2,000)		(2,000)		(23,000)
Fair value	30,491,000		30,491,000		35,784,000
U.S. Agency bonds [Member]
Investments of marketable debt securities [Abstract]
Amortized cost	19,505,000		19,505,000		19,517,000
Unrealized gains	12,000		12,000		10,000
Unrealized losses	(4,000)		(4,000)		(4,000)
Fair value	19,513,000		19,513,000		19,523,000
Municipal bonds [Member]
Investments of marketable debt securities [Abstract]
Amortized cost	9,623,000		9,623,000		7,474,000
Unrealized gains	27,000		27,000		23,000
Unrealized losses	0		0		(2,000)
Fair value	$ 9,650,000		$ 9,650,000		$ 7,495,000