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Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Segment Reporting Information [Line Items]    
Revenues before reimbursements $ 3,218.1 $ 2,672.9
Reimbursements 38.6 33.2
Total revenues 3,256.7 2,706.1
Brokerage [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 2,864.9 2,375.2
Total revenues 2,864.9 2,375.2
Risk Management [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 352.8 297.6
Reimbursements 38.6 33.2
Total revenues 391.4 330.8
Corporate [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 0.4 0.1
Total revenues 0.4 0.1
Commissions [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 1,993.6 1,747.4
Commissions [Member] | Brokerage [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 1,993.6 1,747.4
Broker Fees [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 951.2 705.7
Broker Fees [Member] | Brokerage [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 606.7 412.7
Broker Fees [Member] | Risk Management [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 344.5 293.0
Supplemental Revenue Member [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 93.9 81.6
Supplemental Revenue Member [Member] | Brokerage [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 93.9 81.6
Contingent Revenue [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 86.0 71.8
Contingent Revenue [Member] | Brokerage [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 86.0 71.8
Interest Income Premium Finance Revenues and Other Income [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 93.4 66.3
Interest Income Premium Finance Revenues and Other Income [Member] | Brokerage [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 84.7 61.7
Interest Income Premium Finance Revenues and Other Income [Member] | Risk Management [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 8.3 4.6
Interest Income Premium Finance Revenues and Other Income [Member] | Corporate [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 0.4  
Investment Income [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements $ 93.4 66.4
Other Net Revenue [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements   0.1
Other Net Revenue [Member] | Corporate [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements   $ 0.1