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	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Cash Cash Equivalents And Investments [Line Items]
Amortized Cost	$ 58,152	$ 100,551
Gross Unrealized Gains		9
Gross Unrealized Losses	(36)	(19)
Fair Value	58,116	100,541
Cash
Schedule Of Cash Cash Equivalents And Investments [Line Items]
Amortized Cost	7,892	10,667
Fair Value	7,892	10,667
Cash Equivalents | Money Market Funds
Schedule Of Cash Cash Equivalents And Investments [Line Items]
Amortized Cost	10,393	14,065
Fair Value	10,393	14,065
Cash Equivalents | U.S. Government Agency Securities
Schedule Of Cash Cash Equivalents And Investments [Line Items]
Amortized Cost		4,996
Gross Unrealized Gains		1
Fair Value		4,997
Cash Equivalents | Commercial Paper
Schedule Of Cash Cash Equivalents And Investments [Line Items]
Amortized Cost	1,000	2,948
Fair Value	1,000	2,948
Cash Equivalents | Corporate Notes and Bonds
Schedule Of Cash Cash Equivalents And Investments [Line Items]
Amortized Cost		959
Fair Value		959
Short-term Investments | U.S. Government Agency Securities
Schedule Of Cash Cash Equivalents And Investments [Line Items]
Amortized Cost	13,979	36,535
Gross Unrealized Gains		6
Gross Unrealized Losses	(9)	(10)
Fair Value	13,970	36,531
Short-term Investments | Commercial Paper
Schedule Of Cash Cash Equivalents And Investments [Line Items]
Amortized Cost	8,972	11,122
Fair Value	8,972	11,122
Short-term Investments | Corporate Notes and Bonds
Schedule Of Cash Cash Equivalents And Investments [Line Items]
Amortized Cost	7,514	19,259
Gross Unrealized Gains		2
Gross Unrealized Losses	(11)	(9)
Fair Value	7,503	$ 19,252
Short-term Investments | Treasury Bills
Schedule Of Cash Cash Equivalents And Investments [Line Items]
Amortized Cost	8,402
Gross Unrealized Losses	(16)
Fair Value	$ 8,386