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Cost of revenues - Schedule of Cost of Revenues (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Deferred Revenue Arrangement [Line Items]      
Cost of revenues $ 117,876 $ 79,928 $ 59,250
Bandwidth costs [Member]      
Deferred Revenue Arrangement [Line Items]      
Cost of revenues 68,441 55,135 37,218
Payment handling fees [Member]      
Deferred Revenue Arrangement [Line Items]      
Cost of revenues 4,855 6,919 8,909
Depreciation of servers and other equipment [Member]      
Deferred Revenue Arrangement [Line Items]      
Cost of revenues 7,647 5,848 4,873
Cost Of Inventories Sold [Member]      
Deferred Revenue Arrangement [Line Items]      
Cost of revenues 21,485 4,357 2,621
Cost Of Live Video [Member]      
Deferred Revenue Arrangement [Line Items]      
Cost of revenues 12,724 2,505 204
Other Cost Of Reveue [Member]      
Deferred Revenue Arrangement [Line Items]      
Cost of revenues $ 2,724 $ 5,164 $ 5,425