Balance Sheet Components - Cash Equivalents and Available-for-sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Cash Equivalents and Available-for-sale Securities
Total cash equivalents and available-for-sale securities, Amortized Cost	$ 139,208	$ 153,910
Total cash equivalents and available-for-sale securities, Gross Unrealized Losses	(180)	(132)
Total cash equivalents and available-for-sale securities, Fair Value	139,028	153,778
Corporate Bonds Member]
Cash Equivalents and Available-for-sale Securities
Total cash equivalents and available-for-sale securities, Amortized Cost	6,792	6,254
Total cash equivalents and available-for-sale securities, Gross Unrealized Losses	(27)	(7)
Total cash equivalents and available-for-sale securities, Fair Value	6,765	6,247
Governmental Bonds [Member]
Cash Equivalents and Available-for-sale Securities
Total cash equivalents and available-for-sale securities, Amortized Cost	61,436	40,428
Total cash equivalents and available-for-sale securities, Gross Unrealized Losses	(153)	(125)
Total cash equivalents and available-for-sale securities, Fair Value	61,283	40,303
Money Market Funds [Member]
Cash Equivalents and Available-for-sale Securities
Total cash equivalents and available-for-sale securities, Amortized Cost	68,088	48,704
Total cash equivalents and available-for-sale securities, Fair Value	68,088	48,704
Commercial Paper [Member]
Cash Equivalents and Available-for-sale Securities
Total cash equivalents and available-for-sale securities, Amortized Cost	2,892	58,524
Total cash equivalents and available-for-sale securities, Fair Value	$ 2,892	$ 58,524