Investments - Marketable Debt Securities (Details) (USD $)	3 Months Ended
	Apr. 04, 2015	Mar. 29, 2014	Jan. 03, 2015
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sale and Maturity of Available-for-sale Securities	$ 16,200,000	$ 10,000,000
Investments of marketable debt securities [Abstract]
Marketable debt securities Total Amortized Cost	115,847,000		114,105,000
Unrealized gains	74,000		20,000
Unrealized losses	(12,000)		(75,000)
Fair value	115,909,000		114,050,000
Amortized Cost [Abstract]
Marketable debt securities - current (due in less than one year)	75,019,000		69,607,000
Marketable debt securities - non-current (due in one to two years)	40,828,000		44,498,000
Marketable debt securities Total Amortized Cost	115,847,000		114,105,000
Fair Value [Abstract]
Marketable debt securities - current (due in less than one year)	75,035,000		69,609,000
Marketable debt securities - non-current (due in one to two years)	40,874,000		44,441,000
Fair value	115,909,000		114,050,000
U.S. Treasury securities [Member]
Investments of marketable debt securities [Abstract]
Marketable debt securities Total Amortized Cost	32,506,000		32,507,000
Unrealized gains	25,000		12,000
Unrealized losses	0		(23,000)
Fair value	32,531,000		32,496,000
Amortized Cost [Abstract]
Marketable debt securities Total Amortized Cost	32,506,000		32,507,000
Fair Value [Abstract]
Fair value	32,531,000		32,496,000
Corporate bonds [Member]
Investments of marketable debt securities [Abstract]
Marketable debt securities Total Amortized Cost	32,791,000		35,409,000
Unrealized gains	19,000		2,000
Unrealized losses	(9,000)		(36,000)
Fair value	32,801,000		35,375,000
Amortized Cost [Abstract]
Marketable debt securities Total Amortized Cost	32,791,000		35,409,000
Fair Value [Abstract]
Fair value	32,801,000		35,375,000
U.S. Agency bonds [Member]
Investments of marketable debt securities [Abstract]
Marketable debt securities Total Amortized Cost	22,528,000		22,545,000
Unrealized gains	27,000		4,000
Unrealized losses	0		(10,000)
Fair value	22,555,000		22,539,000
Amortized Cost [Abstract]
Marketable debt securities Total Amortized Cost	22,528,000		22,545,000
Fair Value [Abstract]
Fair value	22,555,000		22,539,000
Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Investments of marketable debt securities [Abstract]
Marketable debt securities Total Amortized Cost	14,987,000		12,487,000
Unrealized gains	0		0
Unrealized losses	0		(1,000)
Fair value	14,987,000		12,486,000
Amortized Cost [Abstract]
Marketable debt securities Total Amortized Cost	14,987,000		12,487,000
Fair Value [Abstract]
Fair value	14,987,000		12,486,000
Municipal bonds [Member]
Investments of marketable debt securities [Abstract]
Marketable debt securities Total Amortized Cost	13,035,000		11,157,000
Unrealized gains	3,000		2,000
Unrealized losses	(3,000)		(5,000)
Fair value	13,035,000		11,154,000
Amortized Cost [Abstract]
Marketable debt securities Total Amortized Cost	13,035,000		11,157,000
Fair Value [Abstract]
Fair value	$ 13,035,000		$ 11,154,000