Marketable Securities - Available for Sale (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jul. 01, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Net unrealized loss in other comprehensive income	$ 3,600,000	$ 2,500,000
OTTI loss recognized	0
Short-term AFS securities
Amortized cost, short-term AFS securities	294,534,000	309,029,000
Gross unrealized gains, short-term AFS	1,000
Gross unrealized losses, short-term AFS	(744,000)	(440,000)
Fair value, short-term AFS securities	293,791,000	308,589,000
Long-term AFS securities
Amortized cost, long-term AFS securities	251,747,000	243,516,000
Gross unrealized gains, long-term AFS	7,000	0
Gross unrealized losses, long-term AFS	(2,906,000)	(2,105,000)
Fair value, long-term AFS securities	248,848,000	241,411,000
Corporate debt securities
Short-term AFS securities
Amortized cost, short-term AFS securities	120,937,000	150,334,000
Gross unrealized gains, short-term AFS	1,000
Gross unrealized losses, short-term AFS	(453,000)	(227,000)
Fair value, short-term AFS securities	120,485,000	150,107,000
Long-term AFS securities
Amortized cost, long-term AFS securities	117,341,000	92,687,000
Gross unrealized gains, long-term AFS	7,000	0
Gross unrealized losses, long-term AFS	(1,123,000)	(683,000)
Fair value, long-term AFS securities	116,225,000	92,004,000
U.S. Treasury securities
Short-term AFS securities
Amortized cost, short-term AFS securities	101,952,000	70,985,000
Gross unrealized gains, short-term AFS	0
Gross unrealized losses, short-term AFS	(142,000)	(34,000)
Fair value, short-term AFS securities	101,810,000	70,951,000
Long-term AFS securities
Amortized cost, long-term AFS securities	42,582,000	53,031,000
Gross unrealized gains, long-term AFS	0	0
Gross unrealized losses, long-term AFS	(637,000)	(403,000)
Fair value, long-term AFS securities	41,945,000	52,628,000
U.S. governmental agency securities
Short-term AFS securities
Amortized cost, short-term AFS securities	50,932,000	45,819,000
Gross unrealized gains, short-term AFS	0
Gross unrealized losses, short-term AFS	(149,000)	(179,000)
Fair value, short-term AFS securities	50,783,000	45,640,000
Long-term AFS securities
Amortized cost, long-term AFS securities	91,824,000	97,798,000
Gross unrealized gains, long-term AFS	0	0
Gross unrealized losses, long-term AFS	(1,146,000)	(1,019,000)
Fair value, long-term AFS securities	90,678,000	96,779,000
Commercial paper
Short-term AFS securities
Amortized cost, short-term AFS securities	17,520,000	32,591,000
Gross unrealized gains, short-term AFS	0	0
Gross unrealized losses, short-term AFS	0	0
Fair value, short-term AFS securities	17,520,000	32,591,000
Certificates of deposit
Short-term AFS securities
Amortized cost, short-term AFS securities	3,193,000	9,300,000
Gross unrealized gains, short-term AFS	0
Gross unrealized losses, short-term AFS	0	0
Fair value, short-term AFS securities	$ 3,193,000	$ 9,300,000