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Gross Billings, Based on Billing Address of Clients or Client Affiliates (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Segment Reporting Information [Line Items]      
Gross Billings $ 1,491,742 $ 990,561 $ 529,975
UNITED STATES      
Segment Reporting Information [Line Items]      
Gross Billings 1,270,116 868,877 477,585
International      
Segment Reporting Information [Line Items]      
Gross Billings $ 221,626 $ 121,684 $ 52,390