Financial Instruments - Amortized to fair value (Details) - USD ($) $ in Thousands	Mar. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	$ 410,813	$ 473,956
Debt Securities, Trading and Available-for-sale	233,383	249,493
Available-for-sale debt securities, amortized cost	644,160	723,580
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	52	1
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(16)	(132)
Total	644,196	723,449
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	113,563	148,110
Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	225,733	273,546
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	62,629	42,562
Debt Securities, Trading and Available-for-sale	161,463	186,391
Available-for-sale debt securities, amortized cost	224,068	229,040
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	32	0
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(8)	(87)
Total	224,092	228,953
U.S. government agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	8,888	9,738
Debt Securities, Trading and Available-for-sale	71,920	63,102
Available-for-sale debt securities, amortized cost	80,796	72,884
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	20	1
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(8)	(45)
Total	$ 80,808	$ 72,840