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Fair Value Measurements - Financial Instruments Measured at Fair Value on Recurring Basis (Detail) - Recurring [Member] - Money market funds [Member] - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Assets, fair value $ 85,911 $ 132,647
Level 1 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Assets, fair value $ 85,911 $ 132,647