Marketable Securities - Available for Sale (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Apr. 01, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Net unrealized loss in other comprehensive income	$ 3,900	$ 2,500
Short-term AFS securities
Amortized cost, short-term AFS securities	300,642	309,029
Gross unrealized gains, short-term AFS	1
Gross unrealized losses, short-term AFS	(809)	(440)
Fair value, short-term AFS securities	299,834	308,589
Long-term AFS securities
Amortized cost, long-term AFS securities	238,747	243,516
Gross unrealized gains, long-term AFS	3	0
Gross unrealized losses, long-term AFS	(3,109)	(2,105)
Fair value, long-term AFS securities	235,641	241,411
Corporate debt securities
Short-term AFS securities
Amortized cost, short-term AFS securities	147,292	150,334
Gross unrealized gains, short-term AFS	0
Gross unrealized losses, short-term AFS	(460)	(227)
Fair value, short-term AFS securities	146,832	150,107
Long-term AFS securities
Amortized cost, long-term AFS securities	99,699	92,687
Gross unrealized gains, long-term AFS	2	0
Gross unrealized losses, long-term AFS	(1,326)	(683)
Fair value, long-term AFS securities	98,375	92,004
U.S Treasury securities
Short-term AFS securities
Amortized cost, short-term AFS securities	71,460	70,985
Gross unrealized gains, short-term AFS	1
Gross unrealized losses, short-term AFS	(62)	(34)
Fair value, short-term AFS securities	71,399	70,951
Long-term AFS securities
Amortized cost, long-term AFS securities	52,522	53,031
Gross unrealized gains, long-term AFS	0	0
Gross unrealized losses, long-term AFS	(670)	(403)
Fair value, long-term AFS securities	51,852	52,628
U.S. governmental agency securities
Short-term AFS securities
Amortized cost, short-term AFS securities	60,240	45,819
Gross unrealized gains, short-term AFS	0
Gross unrealized losses, short-term AFS	(287)	(179)
Fair value, short-term AFS securities	59,953	45,640
Long-term AFS securities
Amortized cost, long-term AFS securities	86,526	97,798
Gross unrealized gains, long-term AFS	1	0
Gross unrealized losses, long-term AFS	(1,113)	(1,019)
Fair value, long-term AFS securities	85,414	96,779
Commercial paper
Short-term AFS securities
Amortized cost, short-term AFS securities	15,450	32,591
Gross unrealized gains, short-term AFS	0	0
Gross unrealized losses, short-term AFS	0	0
Fair value, short-term AFS securities	15,450	32,591
Certificates of deposit
Short-term AFS securities
Amortized cost, short-term AFS securities	6,200	9,300
Gross unrealized gains, short-term AFS	0
Gross unrealized losses, short-term AFS	0	0
Fair value, short-term AFS securities	$ 6,200	$ 9,300