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Schedules of Concentration of Risk Based on Revenue (Detail) - Customer Concentration Risk - Revenue
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Concentration Risk [Line Items]    
Concentration risk, percentage 14.00% 20.00%
Advertiser A    
Concentration Risk [Line Items]    
Concentration risk, percentage 27.00% 18.00%
Advertiser B    
Concentration Risk [Line Items]    
Concentration risk, percentage 15.00% 27.00%