Fair Values on Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair value of financial assets and liabilities
Total financial assets	$ 58,059	$ 96,163
Money market funds
Fair value of financial assets and liabilities
Total financial assets	8,991	59,302
United States government agency securities
Fair value of financial assets and liabilities
Total financial assets	8,029
United States government agency securities | Short-term Investments
Fair value of financial assets and liabilities
Total financial assets	8,029
Corporate debt securities
Fair value of financial assets and liabilities
Total financial assets	37,087	25,698
Marketable equity securities
Fair value of financial assets and liabilities
Total financial assets	3,952	11,163
Level 1
Fair value of financial assets and liabilities
Total financial assets	12,943	70,465
Level 1 | Money market funds
Fair value of financial assets and liabilities
Total financial assets	8,991	59,302
Level 1 | Marketable equity securities
Fair value of financial assets and liabilities
Total financial assets	3,952	11,163
Level 2
Fair value of financial assets and liabilities
Total financial assets	45,116	25,698
Level 2 | United States government agency securities | Short-term Investments
Fair value of financial assets and liabilities
Total financial assets	8,029
Level 2 | Corporate debt securities
Fair value of financial assets and liabilities
Total financial assets	$ 37,087	$ 25,698