Quarterly report pursuant to Section 13 or 15(d)

Revenue from Contracts with Customers - Summary of Revenue from Contracts (Detail)

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Revenue from Contracts with Customers - Summary of Revenue from Contracts (Detail) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Disaggregation of Revenue [Line Items]        
Net commissions and fees $ 389,846 $ 246,065 $ 701,190 $ 453,150
Wholesale brokerage [Member]        
Disaggregation of Revenue [Line Items]        
Net commissions and fees 255,959 172,118 447,083 306,222
Binding authorities [Member]        
Disaggregation of Revenue [Line Items]        
Net commissions and fees 53,596 31,561 108,641 65,707
Underwriting management [Member]        
Disaggregation of Revenue [Line Items]        
Net commissions and fees $ 80,291 $ 42,386 $ 145,466 $ 81,221