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Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2023
Mar. 31, 2022
Segment Reporting Information [Line Items]    
Revenues before reimbursements $ 2,672.9 $ 2,404.5
Reimbursements 33.2 30.8
Total revenues 2,706.1 2,435.3
Brokerage [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 2,375.2 2,122.6
Total revenues 2,375.2 2,122.6
Risk Management [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 297.6 259.1
Reimbursements 33.2 30.8
Total revenues 330.8 289.9
Corporate [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 0.1 22.8
Total revenues 0.1 22.8
Commissions [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 1,747.4 1,565.3
Commissions [Member] | Brokerage [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 1,747.4 1,565.3
Broker Fees [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 705.7 650.9
Broker Fees [Member] | Brokerage [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 412.7 391.9
Broker Fees [Member] | Risk Management [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 293.0 259.0
Supplemental Revenue Member [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 81.6 74.3
Supplemental Revenue Member [Member] | Brokerage [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 81.6 74.3
Contingent Revenue [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 71.8 71.6
Contingent Revenue [Member] | Brokerage [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 71.8 71.6
Investment Income [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 66.0 18.2
Investment Income [Member] | Brokerage [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 61.5 18.1
Investment Income [Member] | Risk Management [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 4.5 0.1
Net gains on divestitures [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 0.3 1.4
Net gains on divestitures [Member] | Brokerage [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 0.2 1.4
Net gains on divestitures [Member] | Risk Management [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 0.1  
Clean Coal Activities [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements   22.7
Clean Coal Activities [Member] | Corporate [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements   22.7
Other Net Revenue [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements 0.1 0.1
Other Net Revenue [Member] | Corporate [Member] | Operating Segments [Member]    
Segment Reporting Information [Line Items]    
Revenues before reimbursements $ 0.1 $ 0.1