Balance Sheet Components - Marketable Securities (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) security	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 489,188,000	$ 300,362,000
Unrealized Gains	77,000	114,000
Unrealized Losses	(630,000)	(566,000)
Fair Value	488,635,000	299,910,000
Other-than-temporary losses	$ 0	0
Securities in an unrealized loss position, greater than or equal to one year | security	0
Maximum maturity of marketable securities	2 years
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 11,879,000	5,962,000
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	11,879,000	5,962,000
U.S. government notes
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	104,955,000	110,945,000
Unrealized Gains	0	5,000
Unrealized Losses	(233,000)	(194,000)
Fair Value	104,722,000	110,756,000
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	372,354,000	183,455,000
Unrealized Gains	77,000	109,000
Unrealized Losses	(397,000)	(372,000)
Fair Value	$ 372,034,000	$ 183,192,000