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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, 2023
Sep. 30, 2022
Sep. 30, 2023
Sep. 30, 2022
Dec. 31, 2022
Segment Reporting Information [Line Items]          
Total revenues $ 2,492.0 $ 2,044.0 $ 7,640.0 $ 6,522.3  
Earnings before income taxes 364.1 315.0 1,267.3 1,205.0  
Identifiable assets at June 30, 2023 and 2022 53,203.9 38,517.2 53,203.9 38,517.2 $ 38,358.4
Operating Segments [Member] | Brokerage [Member]          
Segment Reporting Information [Line Items]          
Total revenues 2,122.1 1,736.2 6,585.7 5,599.5  
Earnings before income taxes 454.0 370.7 1,535.4 1,400.0  
Identifiable assets at June 30, 2023 and 2022 49,325.8 34,773.9 49,325.8 34,773.9  
Operating Segments [Member] | Risk Management [Member]          
Segment Reporting Information [Line Items]          
Total revenues 369.4 307.5 1,053.8 899.4  
Earnings before income taxes 56.4 36.6 151.8 107.7  
Identifiable assets at June 30, 2023 and 2022 1,275.9 1,128.0 1,275.9 1,128.0  
Operating Segments [Member] | Corporate [Member]          
Segment Reporting Information [Line Items]          
Total revenues 0.5 0.3 0.5 23.4  
Earnings before income taxes (146.3) (92.3) (419.9) (302.7)  
Identifiable assets at June 30, 2023 and 2022 $ 2,602.2 $ 2,615.3 $ 2,602.2 $ 2,615.3