Balance Sheet Components (Schedule of Available-for-Sale Short-Term Investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Cost	$ 133,391	$ 124,978
Unrealized Gains	0	0
Unrealized Losses	(75)	(146)
Estimated Fair Value	133,316	124,832
U.S. treasuries
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Cost	133,239	124,816
Unrealized Gains	0	0
Unrealized Losses	(75)	(146)
Estimated Fair Value	133,164	124,670
Certificates of deposit
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Cost	152	162
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	$ 152	$ 162