Fair Values on Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair value of financial assets and liabilities
Total financial assets	$ 49,248	$ 58,059
Money market funds
Fair value of financial assets and liabilities
Total financial assets	6,603	8,991
United States government agency securities
Fair value of financial assets and liabilities
Total financial assets	14,770	8,029
United States government agency securities | Short-term Investments
Fair value of financial assets and liabilities
Total financial assets	14,770	8,029
Corporate debt securities
Fair value of financial assets and liabilities
Total financial assets	27,875	37,087
Marketable equity securities
Fair value of financial assets and liabilities
Total financial assets		3,952
Level 1
Fair value of financial assets and liabilities
Total financial assets	6,603	12,943
Level 1 | Money market funds
Fair value of financial assets and liabilities
Total financial assets	6,603	8,991
Level 1 | Marketable equity securities
Fair value of financial assets and liabilities
Total financial assets		3,952
Level 2
Fair value of financial assets and liabilities
Total financial assets	42,645	45,116
Level 2 | United States government agency securities | Short-term Investments
Fair value of financial assets and liabilities
Total financial assets	14,770	8,029
Level 2 | Corporate debt securities
Fair value of financial assets and liabilities
Total financial assets	$ 27,875	$ 37,087