Financial Instruments - Amortized to fair value (Details) - USD ($)	6 Months Ended
	Jul. 01, 2017	Dec. 31, 2016	Jul. 02, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Impairment losses recorded on available-for-sale securities	$ 0
Cash and cash equivalents	318,708,000	$ 301,320,000	$ 416,142,000	$ 535,846,000
Available-for-sale debt securities, gross unrealized gains	65,000	32,000
Available-for-sale debt securities, gross unrealized losses	(140,000)	(219,000)
Cash, cash equivalents, and available-for-sale securities, amortized cost	675,873,000	706,200,000
Cash, cash equivalents, available-for-sale securities, fair value	675,798,000	706,013,000
Total	357,090,000	404,693,000
U.S. government agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	21,926,000	0
Available-for-sale debt securities, amortized cost	39,974,000	86,533,000
Available-for-sale debt securities, gross unrealized gains	1,000	8,000
Available-for-sale debt securities, gross unrealized losses	(14,000)	(15,000)
Cash, cash equivalents, available-for-sale securities, fair value	39,961,000	86,526,000
Total	18,035,000	86,526,000
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	13,991,000	72,119,000
Available-for-sale debt securities, amortized cost	353,108,000	390,466,000
Available-for-sale debt securities, gross unrealized gains	64,000	24,000
Available-for-sale debt securities, gross unrealized losses	(126,000)	(204,000)
Cash, cash equivalents, available-for-sale securities, fair value	353,046,000	390,286,000
Total	339,055,000	318,167,000
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	121,029,000	179,076,000
Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	$ 161,762,000	$ 50,125,000