Fair Values on Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair value of financial assets and liabilities
Total financial assets	$ 103,260	$ 50,771
Money market funds
Fair value of financial assets and liabilities
Total financial assets	6,591	3,758
United States government agency securities
Fair value of financial assets and liabilities
Total financial assets	20,903	11,228
United States government agency securities | Short-term Investments
Fair value of financial assets and liabilities
Total financial assets	20,903	11,228
Corporate debt securities
Fair value of financial assets and liabilities
Total financial assets	75,766	35,785
Level 1
Fair value of financial assets and liabilities
Total financial assets	6,591	3,758
Level 1 | Money market funds
Fair value of financial assets and liabilities
Total financial assets	6,591	3,758
Level 2
Fair value of financial assets and liabilities
Total financial assets	96,669	47,013
Level 2 | United States government agency securities | Short-term Investments
Fair value of financial assets and liabilities
Total financial assets	20,903	11,228
Level 2 | Corporate debt securities
Fair value of financial assets and liabilities
Total financial assets	$ 75,766	$ 35,785