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Summary of significant accounting policies - Schedule of Property and Equipment Estimated Useful Life (Detail)
12 Months Ended
Dec. 31, 2017
Servers and network equipment [Member]  
Property, Plant and Equipment [Line Items]  
Estimated useful lives 5
Residual rate 5.00%
Computer equipment [Member]  
Property, Plant and Equipment [Line Items]  
Estimated useful lives 5
Residual rate 5.00%
Furniture, fittings and office equipment [Member]  
Property, Plant and Equipment [Line Items]  
Estimated useful lives 5
Residual rate 5.00%
Motor vehicles [Member]  
Property, Plant and Equipment [Line Items]  
Estimated useful lives 5
Residual rate 5.00%
Leasehold improvements [Member]  
Property, Plant and Equipment [Line Items]  
Estimated useful lives shorter of lease term or 3 years
Residual rate 0.00%