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Cash and cash equivalents - Additional Information (Detail) - USD ($)
1 Months Ended
Jan. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Cash and Cash Equivalents [Line Items]      
Time Deposits, at Carrying Value   $ 115,534,000 $ 115,944,000
Web Game Business [Member] | Subsequent Event [Member]      
Cash and Cash Equivalents [Line Items]      
Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax $ 1,400,000