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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

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- Definition Disclosure of gross transaction volume for revenues which are reported net. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=84158767&loc=d3e18780-107790 + Details
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