Financial Instruments - Amortized to fair value (Details) - USD ($)	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Oct. 01, 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	280,681,000	$ 301,320,000	$ 284,220,000	$ 535,846,000
Available-for-sale debt securities, gross unrealized gains	54,000	32,000
Available-for-sale debt securities, gross unrealized losses	(118,000)	(219,000)
Cash, cash equivalents, and available-for-sale securities, amortized cost	659,293,000	706,200,000
Cash, cash equivalents, available-for-sale securities, fair value	659,229,000	706,013,000
Total	378,548,000	404,693,000
U.S. government agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	25,795,000	0
Available-for-sale debt securities, amortized cost	50,703,000	86,533,000
Available-for-sale debt securities, gross unrealized gains	4,000	8,000
Available-for-sale debt securities, gross unrealized losses	0	(15,000)
Cash, cash equivalents, available-for-sale securities, fair value	50,707,000	86,526,000
Total	24,912,000	86,526,000
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	22,255,000	72,119,000
Available-for-sale debt securities, amortized cost	375,959,000	390,466,000
Available-for-sale debt securities, gross unrealized gains	50,000	24,000
Available-for-sale debt securities, gross unrealized losses	(118,000)	(204,000)
Cash, cash equivalents, available-for-sale securities, fair value	375,891,000	390,286,000
Total	353,636,000	318,167,000
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	102,841,000	179,076,000
Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	$ 129,790,000	$ 50,125,000