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Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Dec. 31, 2023
Segment Reporting Information [Line Items]          
Total revenues $ 2,806.8 $ 2,492.0 $ 8,838.9 $ 7,640.0  
Earnings before income taxes 403.3 364.1 1,541.6 1,267.3  
Identifiable assets at September 30, 2024 and 2023 57,242.7 53,203.9 57,242.7 53,203.9 $ 51,615.8
Operating Segments [Member] | Brokerage [Member]          
Segment Reporting Information [Line Items]          
Total revenues 2,396.4 2,122.1 7,637.6 6,585.7  
Earnings before income taxes 511.9 454.0 1,834.3 1,535.4  
Identifiable assets at September 30, 2024 and 2023 52,472.6 49,325.8 52,472.6 49,325.8  
Operating Segments [Member] | Risk Management [Member]          
Segment Reporting Information [Line Items]          
Total revenues 410.0 369.4 1,199.4 1,053.8  
Earnings before income taxes 60.7 56.4 179.3 151.8  
Identifiable assets at September 30, 2024 and 2023 1,785.6 1,275.9 1,785.6 1,275.9  
Operating Segments [Member] | Corporate [Member]          
Segment Reporting Information [Line Items]          
Total revenues 0.4 0.5 1.9 0.5  
Earnings before income taxes (169.3) (146.3) (472.0) (419.9)  
Identifiable assets at September 30, 2024 and 2023 $ 2,984.5 $ 2,602.2 $ 2,984.5 $ 2,602.2