Balance Sheet Components - Marketable Securities (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 297,056,000	$ 300,362,000
Unrealized Gains	157,000	114,000
Unrealized Losses	(538,000)	(566,000)
Fair Value	296,675,000	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	$ 29,844,000	5,962,000
Unrealized Gains	39,000	0
Unrealized Losses	0	0
Fair Value	29,883,000	5,962,000
U.S. government notes
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	98,591,000	110,945,000
Unrealized Gains	0	5,000
Unrealized Losses	(214,000)	(194,000)
Fair Value	98,377,000	110,756,000
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	168,621,000	183,455,000
Unrealized Gains	118,000	109,000
Unrealized Losses	(324,000)	(372,000)
Fair Value	$ 168,415,000	$ 183,192,000