Marketable Securities - Available for Sale (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 30, 2018
Schedule of Available-for-sale Securities [Line Items]
Net unrealized loss in other comprehensive income	$ 800,000	$ 2,800,000
OTTI loss recognized	0
Short-term AFS securities
Amortized cost, short-term AFS securities	388,843,000	372,650,000
Gross unrealized gains, short-term AFS	42,000	1,000
Gross unrealized losses, short-term AFS	(808,000)	(1,350,000)
Fair value, short-term AFS securities	388,077,000	371,301,000
Long-term AFS securities
Amortized cost, long-term AFS securities	185,088,000	215,053,000
Gross unrealized gains, long-term AFS	407,000	52,000
Gross unrealized losses, long-term AFS	(412,000)	(1,547,000)
Fair value, long-term AFS securities	185,083,000	213,558,000
Corporate debt securities
Short-term AFS securities
Amortized cost, short-term AFS securities	141,399,000	140,631,000
Gross unrealized gains, short-term AFS	25,000	1,000
Gross unrealized losses, short-term AFS	(279,000)	(464,000)
Fair value, short-term AFS securities	141,145,000	140,168,000
Long-term AFS securities
Amortized cost, long-term AFS securities	107,085,000	130,612,000
Gross unrealized gains, long-term AFS	372,000	44,000
Gross unrealized losses, long-term AFS	(165,000)	(1,032,000)
Fair value, long-term AFS securities	107,292,000	129,624,000
U.S. Treasury securities
Short-term AFS securities
Amortized cost, short-term AFS securities	121,241,000	107,717,000
Gross unrealized gains, short-term AFS	9,000	0
Gross unrealized losses, short-term AFS	(183,000)	(232,000)
Fair value, short-term AFS securities	121,067,000	107,485,000
Long-term AFS securities
Amortized cost, long-term AFS securities	38,688,000	47,079,000
Gross unrealized gains, long-term AFS	28,000	5,000
Gross unrealized losses, long-term AFS	(155,000)	(347,000)
Fair value, long-term AFS securities	38,561,000	46,737,000
U.S. governmental agency securities
Short-term AFS securities
Amortized cost, short-term AFS securities	88,157,000	92,628,000
Gross unrealized gains, short-term AFS	8,000	0
Gross unrealized losses, short-term AFS	(346,000)	(654,000)
Fair value, short-term AFS securities	87,819,000	91,974,000
Long-term AFS securities
Amortized cost, long-term AFS securities	39,315,000	37,362,000
Gross unrealized gains, long-term AFS	7,000	3,000
Gross unrealized losses, long-term AFS	(92,000)	(168,000)
Fair value, long-term AFS securities	39,230,000	37,197,000
Certificates of deposit
Short-term AFS securities
Amortized cost, short-term AFS securities	21,688,000	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	21,688,000	23,497,000
Commercial paper
Short-term AFS securities
Amortized cost, short-term AFS securities	16,358,000	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	$ 16,358,000	$ 8,177,000