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	Jun. 30, 2017	Dec. 31, 2016
Assets:
Money market funds	$ 2,289	$ 2,669
Corporate debt securities and commercial paper	20,523	22,030
Total assets measured at fair value	22,812	24,699
Level I
Assets:
Money market funds	2,289	2,669
Total assets measured at fair value	2,289	2,669
Level II
Assets:
Corporate debt securities and commercial paper	20,523	22,030
Total assets measured at fair value	$ 20,523	$ 22,030