Fair Values on Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair value of financial assets and liabilities
Fair Value	$ 72,659	$ 94,885
Total financial assets	72,659	94,885
Money market funds
Fair value of financial assets and liabilities
Fair Value		6,167
Total financial assets		6,167
United States government agency securities
Fair value of financial assets and liabilities
Fair Value	11,030	15,948
Total financial assets	11,030	15,948
United States government agency securities | Short-term Investments
Fair value of financial assets and liabilities
Fair Value	11,030	15,948
Total financial assets	11,030	15,948
Corporate debt securities
Fair value of financial assets and liabilities
Fair Value	61,629	72,770
Total financial assets	61,629	72,770
Level 1
Fair value of financial assets and liabilities
Fair Value		6,167
Total financial assets		6,167
Level 1 | Money market funds
Fair value of financial assets and liabilities
Fair Value		6,167
Total financial assets		6,167
Level 2
Fair value of financial assets and liabilities
Fair Value	72,659	88,718
Total financial assets	72,659	88,718
Level 2 | United States government agency securities | Short-term Investments
Fair value of financial assets and liabilities
Fair Value	11,030	15,948
Total financial assets	11,030	15,948
Level 2 | Corporate debt securities
Fair value of financial assets and liabilities
Fair Value	61,629	72,770
Total financial assets	$ 61,629	$ 72,770