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Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2023
Mar. 31, 2022
Dec. 31, 2022
Segment Reporting Information [Line Items]      
Total revenues $ 2,706.1 $ 2,435.3  
Earnings before income taxes 605.8 537.7  
Identifiable assets at March 31, 2023 and 2022 45,094.4 42,897.5 $ 38,358.4
Operating Segments [Member] | Brokerage [Member]      
Segment Reporting Information [Line Items]      
Total revenues 2,375.2 2,122.6  
Earnings before income taxes 690.9 618.4  
Identifiable assets at March 31, 2023 and 2022 40,919.9 39,237.3  
Operating Segments [Member] | Risk Management [Member]      
Segment Reporting Information [Line Items]      
Total revenues 330.8 289.9  
Earnings before income taxes 45.5 32.3  
Identifiable assets at March 31, 2023 and 2022 1,126.7 1,047.2  
Operating Segments [Member] | Corporate [Member]      
Segment Reporting Information [Line Items]      
Total revenues 0.1 22.8  
Earnings before income taxes (130.6) (113.0)  
Identifiable assets at March 31, 2023 and 2022 $ 3,047.8 $ 2,613.0