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Fair Value Measurements - Schedule of Financial Instruments Measured at Fair Value (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Fair value measurements, Short term investments $ 138,915 $ 181,960
Fair value measurements, Total 254,449 259,075
Time deposits [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Fair value measurements, Cash equivalent, time deposits with original maturities less than three months 115,534 115,944
Investments in financial instruments [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Fair value measurements, Short term investments 138,915 143,131
Significant other observable inputs (Level 2) [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Fair value measurements, Total 254,449 259,075
Significant other observable inputs (Level 2) [Member] | Time deposits [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Fair value measurements, Cash equivalent, time deposits with original maturities less than three months 115,534 115,944
Significant other observable inputs (Level 2) [Member] | Investments in financial instruments [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Fair value measurements, Short term investments $ 138,915 $ 143,131