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Basis of Presentation and Summary of Significant Accounting Policies - Gross Billings Based on Billing Address of Clients or Client Affiliates (Detail) - Accounting Standards Update 2014-09 - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Disaggregation Of Revenue [Line Items]        
Gross Billings $ 568,744 $ 396,504 $ 1,524,697 $ 989,843
US        
Disaggregation Of Revenue [Line Items]        
Gross Billings 479,489 337,134 1,282,178 843,557
International        
Disaggregation Of Revenue [Line Items]        
Gross Billings $ 89,255 $ 59,370 $ 242,519 $ 146,286