Marketable Securities - Available for Sale (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 29, 2019	Dec. 30, 2018
Debt Securities, Available-for-sale [Line Items]
Net unrealized gains (loss) in other comprehensive income	$ 900,000	$ (2,800,000)
OTTI loss recognized	0
Short-term AFS securities
Amortized cost, short-term AFS securities	376,639,000	372,650,000
Gross unrealized gains, short-term AFS	324,000	1,000
Gross unrealized losses, short-term AFS	(100,000)	(1,350,000)
Fair value, short-term AFS securities	376,863,000	371,301,000
Long-term AFS securities
Amortized cost, long-term AFS securities	216,560,000	215,053,000
Gross unrealized gains, long-term AFS	844,000	52,000
Gross unrealized losses, long-term AFS	(139,000)	(1,547,000)
Fair value, long-term AFS securities	217,265,000	213,558,000
Corporate debt securities
Short-term AFS securities
Amortized cost, short-term AFS securities	109,792,000	140,631,000
Gross unrealized gains, short-term AFS	263,000	1,000
Gross unrealized losses, short-term AFS	(36,000)	(464,000)
Fair value, short-term AFS securities	110,019,000	140,168,000
Long-term AFS securities
Amortized cost, long-term AFS securities	93,989,000	130,612,000
Gross unrealized gains, long-term AFS	696,000	44,000
Gross unrealized losses, long-term AFS	(22,000)	(1,032,000)
Fair value, long-term AFS securities	94,663,000	129,624,000
U.S. Treasury securities
Short-term AFS securities
Amortized cost, short-term AFS securities	123,224,000	107,717,000
Gross unrealized gains, short-term AFS	46,000	0
Gross unrealized losses, short-term AFS	(47,000)	(232,000)
Fair value, short-term AFS securities	123,223,000	107,485,000
Long-term AFS securities
Amortized cost, long-term AFS securities	71,313,000	47,079,000
Gross unrealized gains, long-term AFS	121,000	5,000
Gross unrealized losses, long-term AFS	(67,000)	(347,000)
Fair value, long-term AFS securities	71,367,000	46,737,000
U.S. governmental agency securities
Short-term AFS securities
Amortized cost, short-term AFS securities	60,515,000	92,628,000
Gross unrealized gains, short-term AFS	15,000	0
Gross unrealized losses, short-term AFS	(17,000)	(654,000)
Fair value, short-term AFS securities	60,513,000	91,974,000
Long-term AFS securities
Amortized cost, long-term AFS securities	51,258,000	37,362,000
Gross unrealized gains, long-term AFS	27,000	3,000
Gross unrealized losses, long-term AFS	(50,000)	(168,000)
Fair value, long-term AFS securities	51,235,000	37,197,000
Certificates of deposit
Short-term AFS securities
Amortized cost, short-term AFS securities	13,810,000	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	13,810,000	23,497,000
Commercial paper
Short-term AFS securities
Amortized cost, short-term AFS securities	69,298,000	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	$ 69,298,000	$ 8,177,000