Balance Sheet Components - Unrealized Gains and Losses and Fair Value of Investments (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 677,952,000	$ 300,362,000
Unrealized Gains	22,000	114,000
Unrealized Losses	(1,611,000)	(566,000)
Fair Value	676,363,000	299,910,000
Other-than-temporary losses realized	$ 0	0
Maximum maturity of marketable securities	2 years
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 11,924,000	5,962,000
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	11,924,000	5,962,000
U.S. government notes
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	137,025,000	110,945,000
Unrealized Gains	0	5,000
Unrealized Losses	(378,000)	(194,000)
Fair Value	136,647,000	110,756,000
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	313,080,000	183,455,000
Unrealized Gains	20,000	109,000
Unrealized Losses	(616,000)	(372,000)
Fair Value	312,484,000	$ 183,192,000
Agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	215,923,000
Unrealized Gains	2,000
Unrealized Losses	(617,000)
Fair Value	$ 215,308,000