Balance Sheet Components - Cash Equivalents and Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Cash Equivalents and Available-for-sale Securities
Total cash equivalents and available-for-sale securities, Amortized Cost	$ 135,260	$ 153,910
Total cash equivalents and available-for-sale securities, Gross Unrealized Losses	(80)	(132)
Total cash equivalents and available-for-sale securities, Fair Value	135,180	153,778
Corporate Bonds
Cash Equivalents and Available-for-sale Securities
Total cash equivalents and available-for-sale securities, Amortized Cost	6,031	6,254
Total cash equivalents and available-for-sale securities, Gross Unrealized Losses	(11)	(7)
Total cash equivalents and available-for-sale securities, Fair Value	6,020	6,247
Governmental Bonds
Cash Equivalents and Available-for-sale Securities
Total cash equivalents and available-for-sale securities, Amortized Cost	39,261	40,428
Total cash equivalents and available-for-sale securities, Gross Unrealized Losses	(69)	(125)
Total cash equivalents and available-for-sale securities, Fair Value	39,192	40,303
Money Market Funds
Cash Equivalents and Available-for-sale Securities
Total cash equivalents and available-for-sale securities, Amortized Cost	27,582	48,704
Total cash equivalents and available-for-sale securities, Fair Value	27,582	48,704
Commercial Paper
Cash Equivalents and Available-for-sale Securities
Total cash equivalents and available-for-sale securities, Amortized Cost	62,386	58,524
Total cash equivalents and available-for-sale securities, Fair Value	$ 62,386	$ 58,524