Major Category of Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring $ in Thousands	Mar. 31, 2017 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	$ 97,472
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	85,292
U.S. Government and agency securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	402
Corporate and other debt securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	11,778
Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	85,694
Quoted prices in active markets for identical assets (Level 1) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	85,292
Quoted prices in active markets for identical assets (Level 1) | U.S. Government and agency securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	402
Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	11,778
Significant other observable inputs (Level 2) | Corporate and other debt securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	$ 11,778