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Geographic Information - Gross Billings, Based on Billing Address of Clients or Client Affiliates (Detail) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Revenues From External Customers And Long Lived Assets [Line Items]        
Gross Billings $ 688,359 $ 749,829 $ 1,440,114 $ 1,306,261
U.S.        
Revenues From External Customers And Long Lived Assets [Line Items]        
Gross Billings 592,490 642,168 1,244,688 1,112,706
International        
Revenues From External Customers And Long Lived Assets [Line Items]        
Gross Billings $ 95,869 $ 107,661 $ 195,426 $ 193,555