Balance Sheet Components - Cash Equivalents and Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Cash Equivalents and Available-for-sale Securities
Total cash equivalents and available-for-sale securities, Amortized Cost	$ 131,911	$ 127,540
Total cash equivalents and available-for-sale securities, Gross Unrealized Gains	40
Total cash equivalents and available-for-sale securities, Gross Unrealized Losses	(4)	(37)
Total cash equivalents and available-for-sale securities, Fair Value	131,947	127,503
Corporate Bonds
Cash Equivalents and Available-for-sale Securities
Total cash equivalents and available-for-sale securities, Amortized Cost	28,083	8,997
Total cash equivalents and available-for-sale securities, Gross Unrealized Gains	23
Total cash equivalents and available-for-sale securities, Gross Unrealized Losses	(1)	(8)
Total cash equivalents and available-for-sale securities, Fair Value	28,105	8,989
Government Bonds
Cash Equivalents and Available-for-sale Securities
Total cash equivalents and available-for-sale securities, Amortized Cost	46,579	33,423
Total cash equivalents and available-for-sale securities, Gross Unrealized Gains	17
Total cash equivalents and available-for-sale securities, Gross Unrealized Losses		(29)
Total cash equivalents and available-for-sale securities, Fair Value	46,596	33,394
Money Market Funds
Cash Equivalents and Available-for-sale Securities
Total cash equivalents and available-for-sale securities, Amortized Cost	16,381	25,390
Total cash equivalents and available-for-sale securities, Fair Value	16,381	25,390
Commercial Paper
Cash Equivalents and Available-for-sale Securities
Total cash equivalents and available-for-sale securities, Amortized Cost	40,868	59,730
Total cash equivalents and available-for-sale securities, Gross Unrealized Losses	(3)
Total cash equivalents and available-for-sale securities, Fair Value	$ 40,865	$ 59,730