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Short-term investments - Schedule of Short-Term Investments (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Schedule of Investments [Line Items]    
Short-term investments $ 138,915 $ 181,960
Time deposits [Member]    
Schedule of Investments [Line Items]    
Short-term investments 118,779 38,829
Investments in financial instruments [Member]    
Schedule of Investments [Line Items]    
Short-term investments [1] $ 20,136 $ 143,131
[1] the investments were issued by commercial banks in the PRC with a variable interest rate indexed to performance of underlying assets. Since these investments’ maturity dates are within one year, they are classified as short-term investments.