Investments - Marketable Debt Securities (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Oct. 03, 2015	Sep. 27, 2014	Oct. 03, 2015	Sep. 27, 2014	Jan. 03, 2015
Investments of marketable debt securities [Abstract]
Marketable debt securities Total Amortized Cost	$ 41,812		$ 41,812		$ 114,105
Unrealized gains	19		19		20
Unrealized losses	(7)		(7)		(75)
Fair value	41,824		41,824		114,050
Amortized Cost [Abstract]
Marketable debt securities - current (due in less than one year)	33,231		33,231		69,607
Marketable debt securities - non-current (due in one to two years)	8,581		8,581		44,498
Marketable debt securities Total Amortized Cost	41,812		41,812		114,105
Fair Value [Abstract]
Marketable debt securities - current (due in less than one year)	33,243		33,243		69,609
Marketable debt securities - non-current (due in one to two years)	8,581		8,581		44,441
Fair value	41,824		41,824		114,050
Proceeds from marketable debt securities	59,200	$ 14,600	101,000	$ 38,100
U.S. Treasury securities [Member]
Investments of marketable debt securities [Abstract]
Marketable debt securities Total Amortized Cost					32,507
Unrealized gains					12
Unrealized losses					(23)
Fair value					32,496
Amortized Cost [Abstract]
Marketable debt securities Total Amortized Cost					32,507
Fair Value [Abstract]
Fair value					32,496
Corporate bonds [Member]
Investments of marketable debt securities [Abstract]
Marketable debt securities Total Amortized Cost	17,612		17,612		35,409
Unrealized gains	4		4		2
Unrealized losses	(6)		(6)		(36)
Fair value	17,610		17,610		35,375
Amortized Cost [Abstract]
Marketable debt securities Total Amortized Cost	17,612		17,612		35,409
Fair Value [Abstract]
Fair value	17,610		17,610		35,375
U.S. Agency bonds [Member]
Investments of marketable debt securities [Abstract]
Marketable debt securities Total Amortized Cost	4,998		4,998		22,545
Unrealized gains	13		13		4
Unrealized losses	0		0		(10)
Fair value	5,011		5,011		22,539
Amortized Cost [Abstract]
Marketable debt securities Total Amortized Cost	4,998		4,998		22,545
Fair Value [Abstract]
Fair value	5,011		5,011		22,539
Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Investments of marketable debt securities [Abstract]
Marketable debt securities Total Amortized Cost	9,993		9,993		12,487
Unrealized gains	0		0		0
Unrealized losses	0		0		(1)
Fair value	9,993		9,993		12,486
Amortized Cost [Abstract]
Marketable debt securities Total Amortized Cost	9,993		9,993		12,487
Fair Value [Abstract]
Fair value	9,993		9,993		12,486
Municipal bonds [Member]
Investments of marketable debt securities [Abstract]
Marketable debt securities Total Amortized Cost	9,209		9,209		11,157
Unrealized gains	2		2		2
Unrealized losses	(1)		(1)		(5)
Fair value	9,210		9,210		11,154
Amortized Cost [Abstract]
Marketable debt securities Total Amortized Cost	9,209		9,209		11,157
Fair Value [Abstract]
Fair value	$ 9,210		$ 9,210		$ 11,154