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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Jun. 30, 2023
Dec. 31, 2022
Goodwill [Line Items]    
Total goodwill $ 10,513.7 $ 9,489.4
Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 5,780.5 5,140.5
United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 2,406.4 2,197.9
Canada [Member]    
Goodwill [Line Items]    
Total goodwill 592.8 569.7
Australia [Member]    
Goodwill [Line Items]    
Total goodwill 509.5 477.8
New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 208.9 213.3
Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 1,015.6 890.2
Brokerage [Member]    
Goodwill [Line Items]    
Total goodwill 10,381.4 9,358.1
Risk Management [Member]    
Goodwill [Line Items]    
Total goodwill 113.0 112.2
Corporate [Member]    
Goodwill [Line Items]    
Total goodwill 19.3 19.1
Operating Segments [Member] | Brokerage [Member]    
Goodwill [Line Items]    
Total goodwill 10,381.4 9,358.1
Operating Segments [Member] | Brokerage [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 5,705.7 5,065.7
Operating Segments [Member] | Brokerage [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 2,387.9 2,180.2
Operating Segments [Member] | Brokerage [Member] | Canada [Member]    
Goodwill [Line Items]    
Total goodwill 592.8 569.7
Operating Segments [Member] | Brokerage [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill 499.2 467.6
Operating Segments [Member] | Brokerage [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 199.5 203.8
Operating Segments [Member] | Brokerage [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 996.3 871.1
Operating Segments [Member] | Risk Management [Member]    
Goodwill [Line Items]    
Total goodwill 113.0 112.2
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 18.5 17.7
Operating Segments [Member] | Risk Management [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill 10.3 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 19.3 19.1
Operating Segments [Member] | Corporate [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill $ 19.3 $ 19.1