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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Mar. 31, 2023
Dec. 31, 2022
Goodwill [Line Items]    
Total goodwill $ 9,703.2 $ 9,489.4
Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 5,202.7 5,140.5
United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 2,260.6 2,197.9
Canada [Member]    
Goodwill [Line Items]    
Total goodwill 566.4 569.7
Australia [Member]    
Goodwill [Line Items]    
Total goodwill 478.0 477.8
New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 210.4 213.3
Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 985.1 890.2
Brokerage [Member]    
Goodwill [Line Items]    
Total goodwill 9,571.8 9,358.1
Risk Management [Member]    
Goodwill [Line Items]    
Total goodwill 112.2 112.2
Corporate [Member]    
Goodwill [Line Items]    
Total goodwill 19.2 19.1
Operating Segments [Member] | Brokerage [Member]    
Goodwill [Line Items]    
Total goodwill 9,571.8 9,358.1
Operating Segments [Member] | Brokerage [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 5,127.9 5,065.7
Operating Segments [Member] | Brokerage [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 2,242.7 2,180.2
Operating Segments [Member] | Brokerage [Member] | Canada [Member]    
Goodwill [Line Items]    
Total goodwill 566.4 569.7
Operating Segments [Member] | Brokerage [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill 467.9 467.6
Operating Segments [Member] | Brokerage [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 201.0 203.8
Operating Segments [Member] | Brokerage [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 965.9 871.1
Operating Segments [Member] | Risk Management [Member]    
Goodwill [Line Items]    
Total goodwill 112.2 112.2
Operating Segments [Member] | Risk Management [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 74.8 74.8
Operating Segments [Member] | Risk Management [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 17.9 17.7
Operating Segments [Member] | Risk Management [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill 10.1 10.2
Operating Segments [Member] | Risk Management [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 9.4 9.5
Operating Segments [Member] | Corporate [Member]    
Goodwill [Line Items]    
Total goodwill 19.2 19.1
Operating Segments [Member] | Corporate [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill $ 19.2 $ 19.1