Marketable Securities - Available for Sale (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 30, 2018
Debt Securities, Available-for-sale [Line Items]
Net unrealized loss in other comprehensive income	$ 600,000	$ 2,800,000
OTTI loss recognized	0
Short-term AFS securities
Amortized cost, short-term AFS securities	427,925,000	372,650,000
Gross unrealized gains, short-term AFS	219,000	1,000
Gross unrealized losses, short-term AFS	(347,000)	(1,350,000)
Fair value, short-term AFS securities	427,797,000	371,301,000
Long-term AFS securities
Amortized cost, long-term AFS securities	162,138,000	215,053,000
Gross unrealized gains, long-term AFS	867,000	52,000
Gross unrealized losses, long-term AFS	(94,000)	(1,547,000)
Fair value, long-term AFS securities	162,911,000	213,558,000
Corporate debt securities
Short-term AFS securities
Amortized cost, short-term AFS securities	120,936,000	140,631,000
Gross unrealized gains, short-term AFS	117,000	1,000
Gross unrealized losses, short-term AFS	(116,000)	(464,000)
Fair value, short-term AFS securities	120,937,000	140,168,000
Long-term AFS securities
Amortized cost, long-term AFS securities	103,793,000	130,612,000
Gross unrealized gains, long-term AFS	748,000	44,000
Gross unrealized losses, long-term AFS	(64,000)	(1,032,000)
Fair value, long-term AFS securities	104,477,000	129,624,000
U.S. Treasury securities
Short-term AFS securities
Amortized cost, short-term AFS securities	144,518,000	107,717,000
Gross unrealized gains, short-term AFS	73,000	0
Gross unrealized losses, short-term AFS	(111,000)	(232,000)
Fair value, short-term AFS securities	144,480,000	107,485,000
Long-term AFS securities
Amortized cost, long-term AFS securities	27,098,000	47,079,000
Gross unrealized gains, long-term AFS	81,000	5,000
Gross unrealized losses, long-term AFS	(22,000)	(347,000)
Fair value, long-term AFS securities	27,157,000	46,737,000
U.S. governmental agency securities
Short-term AFS securities
Amortized cost, short-term AFS securities	108,911,000	92,628,000
Gross unrealized gains, short-term AFS	29,000	0
Gross unrealized losses, short-term AFS	(120,000)	(654,000)
Fair value, short-term AFS securities	108,820,000	91,974,000
Long-term AFS securities
Amortized cost, long-term AFS securities	31,247,000	37,362,000
Gross unrealized gains, long-term AFS	38,000	3,000
Gross unrealized losses, long-term AFS	(8,000)	(168,000)
Fair value, long-term AFS securities	31,277,000	37,197,000
Certificates of deposit
Short-term AFS securities
Amortized cost, short-term AFS securities	18,497,000	23,497,000
Gross unrealized gains, short-term AFS	0	0
Gross unrealized losses, short-term AFS	0	0
Fair value, short-term AFS securities	18,497,000	23,497,000
Commercial paper
Short-term AFS securities
Amortized cost, short-term AFS securities	35,063,000	8,177,000
Gross unrealized gains, short-term AFS	0	0
Gross unrealized losses, short-term AFS	0	0
Fair value, short-term AFS securities	$ 35,063,000	$ 8,177,000