Fair Value Measurements - Financial Instruments Measured at Fair Value on Recurring Basis (Detail) - Money market funds [Member] - Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	$ 167,721	$ 160,125
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	$ 167,721	$ 160,125