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Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Dec. 31, 2023
Segment Reporting Information [Line Items]      
Total revenues $ 3,256.7 $ 2,706.1  
Earnings before income taxes 772.7 605.8  
Identifiable assets at March 31, 2024 and 2023 60,975.9 45,094.4 $ 51,615.8
Operating Segments [Member] | Brokerage [Member]      
Segment Reporting Information [Line Items]      
Total revenues 2,864.9 2,375.2  
Earnings before income taxes 876.1 690.9  
Identifiable assets at March 31, 2024 and 2023 56,371.6 40,919.9  
Operating Segments [Member] | Risk Management [Member]      
Segment Reporting Information [Line Items]      
Total revenues 391.4 330.8  
Earnings before income taxes 53.2 45.5  
Identifiable assets at March 31, 2024 and 2023 1,673.5 1,126.7  
Operating Segments [Member] | Corporate [Member]      
Segment Reporting Information [Line Items]      
Total revenues 0.4 0.1  
Earnings before income taxes (156.6) (130.6)  
Identifiable assets at March 31, 2024 and 2023 $ 2,930.8 $ 3,047.8