Fair Values on Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair value of financial assets and liabilities
Total financial assets	$ 75,571	$ 94,885
Money market funds
Fair value of financial assets and liabilities
Total financial assets	4,145	6,167
United States government agency securities
Fair value of financial assets and liabilities
Total financial assets	7,477	15,948
United States government agency securities | Short-term Investments
Fair value of financial assets and liabilities
Total financial assets	7,477	15,948
Corporate debt securities
Fair value of financial assets and liabilities
Total financial assets	63,949	72,770
Level 1
Fair value of financial assets and liabilities
Total financial assets	4,145	6,167
Level 1 | Money market funds
Fair value of financial assets and liabilities
Total financial assets	4,145	6,167
Level 2
Fair value of financial assets and liabilities
Total financial assets	71,426	88,718
Level 2 | United States government agency securities | Short-term Investments
Fair value of financial assets and liabilities
Total financial assets	7,477	15,948
Level 2 | Corporate debt securities
Fair value of financial assets and liabilities
Total financial assets	$ 63,949	$ 72,770