Fair Value of Assets and Liabilities - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) (Fair Value, measurements, recurring [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total Assets	$ 56,970	$ 34,515
Money market funds [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total Assets	47,181	25,201
Short-term investments - commercial paper [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total Assets	999	1,997
Short-term investments - corporate bonds [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total Assets	8,790	1,128
Long-term investments - corporate bonds [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total Assets		6,189
Quoted prices in active markets for identical assets or liabilities (Level 1) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total Assets	47,181	25,201
Quoted prices in active markets for identical assets or liabilities (Level 1) [Member] | Money market funds [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total Assets	47,181	25,201
Significant other observable inputs (Level 2) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total Assets	9,789	9,314
Significant other observable inputs (Level 2) [Member] | Short-term investments - commercial paper [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total Assets	999	1,997
Significant other observable inputs (Level 2) [Member] | Short-term investments - corporate bonds [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total Assets	8,790	1,128
Significant other observable inputs (Level 2) [Member] | Long-term investments - corporate bonds [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total Assets		$ 6,189