Fair Value Measurements - Financial Instruments Measured at Fair Value on Recurring Basis (Detail) - Recurring [Member] - Money market funds [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	$ 62,428	$ 85,911
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	$ 62,428	$ 85,911