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Geographic Information - Gross Billings Percentage, Based on Billing Address of Clients or Client Affiliates (Detail)
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Revenues From External Customers And Long Lived Assets [Line Items]        
Gross Billings 100.00% 100.00% 100.00% 100.00%
U.S.        
Revenues From External Customers And Long Lived Assets [Line Items]        
Gross Billings 87.00% 86.00% 86.00% 86.00%
International        
Revenues From External Customers And Long Lived Assets [Line Items]        
Gross Billings 13.00% 14.00% 14.00% 14.00%