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Schedules of Concentration of Risk Based on Accounts Receivable (Detail) - Customer Concentration Risk - Accounts Receivable
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Concentration Risk [Line Items]    
Concentration risk, percentage 29.00% 23.00%
Advertiser A    
Concentration Risk [Line Items]    
Concentration risk, percentage   15.00%
Advertiser B    
Concentration Risk [Line Items]    
Concentration risk, percentage 35.00% 31.00%