Cash, Cash Equivalents and Investments - Summary of Amortized Cost, Gross Unrealized Gains and Losses, and Estimated Fair Market Value of Cash and Cash Equivalents and Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule Of Cash Cash Equivalents And Investments [Line Items]
Amortized Cost	$ 52,843	$ 58,152
Gross Unrealized Losses	(36)	(36)
Fair Value	52,807	58,116
Cash
Schedule Of Cash Cash Equivalents And Investments [Line Items]
Amortized Cost	2,492	7,892
Fair Value	2,492	7,892
Cash Equivalents | Money Market Funds
Schedule Of Cash Cash Equivalents And Investments [Line Items]
Amortized Cost	13,008	10,393
Fair Value	13,008	10,393
Cash Equivalents | Commercial Paper
Schedule Of Cash Cash Equivalents And Investments [Line Items]
Amortized Cost		1,000
Fair Value		1,000
Short-term Investments | U.S. Government Agency Securities
Schedule Of Cash Cash Equivalents And Investments [Line Items]
Amortized Cost	9,971	13,979
Gross Unrealized Losses	(14)	(9)
Fair Value	9,957	13,970
Short-term Investments | Corporate Notes and Bonds
Schedule Of Cash Cash Equivalents And Investments [Line Items]
Amortized Cost	9,448	7,514
Gross Unrealized Losses	(10)	(11)
Fair Value	9,438	7,503
Short-term Investments | Commercial Paper
Schedule Of Cash Cash Equivalents And Investments [Line Items]
Amortized Cost	6,978	8,972
Fair Value	6,978	8,972
Short-term Investments | Treasury Bills
Schedule Of Cash Cash Equivalents And Investments [Line Items]
Amortized Cost	10,946	8,402
Gross Unrealized Losses	(12)	(16)
Fair Value	$ 10,934	$ 8,386