Investments - Marketable Debt Securities (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 27, 2014	Sep. 28, 2013	Sep. 27, 2014	Sep. 28, 2013	Dec. 28, 2013
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sale and Maturity of Available-for-sale Securities	$ 14,600,000	$ 8,500,000	$ 38,100,000	$ 31,800,000
Investments of marketable debt securities [Abstract]
Amortized cost	98,679,000		98,679,000		86,770,000
Unrealized gains	53,000		53,000		51,000
Unrealized losses	(39,000)		(39,000)		(30,000)
Fair value	98,693,000		98,693,000		86,791,000
Amortized Cost [Abstract]
Marketable debt securities - current (due in less than one year)	55,290,000		55,290,000		52,122,000
Marketable debt securities - non-current (due in one to two years)	43,389,000		43,389,000		34,648,000
Marketable debt securities Total Amortized Cost	98,679,000		98,679,000		86,770,000
Fair Value [Abstract]
Marketable debt securities - current (due in less than one year)	55,317,000		55,317,000		52,159,000
Marketable debt securities - non-current (due in one to two years)	43,376,000		43,376,000		34,632,000
Fair value	98,693,000		98,693,000		86,791,000
U.S. Treasury securities [Member]
Investments of marketable debt securities [Abstract]
Amortized cost	32,494,000		32,494,000		23,975,000
Unrealized gains	21,000		21,000		15,000
Unrealized losses	(3,000)		(3,000)		(1,000)
Fair value	32,512,000		32,512,000		23,989,000
Amortized Cost [Abstract]
Marketable debt securities Total Amortized Cost	32,494,000		32,494,000		23,975,000
Fair Value [Abstract]
Fair value	32,512,000		32,512,000		23,989,000
Corporate bonds [Member]
Investments of marketable debt securities [Abstract]
Amortized cost	32,996,000		32,996,000		35,804,000
Unrealized gains	4,000		4,000		3,000
Unrealized losses	(22,000)		(22,000)		(23,000)
Fair value	32,978,000		32,978,000		35,784,000
Amortized Cost [Abstract]
Marketable debt securities Total Amortized Cost	32,996,000		32,996,000		35,804,000
Fair Value [Abstract]
Fair value	32,978,000		32,978,000		35,784,000
U.S. Agency bonds [Member]
Investments of marketable debt securities [Abstract]
Amortized cost	19,568,000		19,568,000		19,517,000
Unrealized gains	8,000		8,000		10,000
Unrealized losses	(13,000)		(13,000)		(4,000)
Fair value	19,563,000		19,563,000		19,523,000
Amortized Cost [Abstract]
Marketable debt securities Total Amortized Cost	19,568,000		19,568,000		19,517,000
Fair Value [Abstract]
Fair value	19,563,000		19,563,000		19,523,000
Municipal bonds [Member]
Investments of marketable debt securities [Abstract]
Amortized cost	8,625,000		8,625,000		7,474,000
Unrealized gains	20,000		20,000		23,000
Unrealized losses	0		0		(2,000)
Fair value	8,645,000		8,645,000		7,495,000
Amortized Cost [Abstract]
Marketable debt securities Total Amortized Cost	8,625,000		8,625,000		7,474,000
Fair Value [Abstract]
Fair value	8,645,000		8,645,000		7,495,000
Commercial Paper [Member]
Investments of marketable debt securities [Abstract]
Amortized cost	4,996,000		4,996,000
Unrealized gains	0		0
Unrealized losses	(1,000)		(1,000)
Fair value	4,995,000		4,995,000
Amortized Cost [Abstract]
Marketable debt securities Total Amortized Cost	4,996,000		4,996,000
Fair Value [Abstract]
Fair value	$ 4,995,000		$ 4,995,000