Fair Value of Financial Instruments - Fair Value of Cash Equivalents and Investments by Level within Fair Value Hierarchy (Details) - Fair Value, Measurements, Recurring Basis - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets:
Money market funds	$ 2,035	$ 5,383
Corporate debt securities and commercial paper	23,613	22,098
Total assets measured at fair value	25,648	27,481
Level I
Assets:
Money market funds	2,035	5,383
Total assets measured at fair value	2,035	5,383
Level II
Assets:
Corporate debt securities and commercial paper	23,613	22,098
Total assets measured at fair value	$ 23,613	$ 22,098