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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Goodwill [Line Items]      
Total goodwill - net $ 9,489.4 $ 8,666.2 $ 6,127.0
Unites States [Member]      
Goodwill [Line Items]      
Total goodwill - net 5,140.5 4,473.9  
United Kingdom [Member]      
Goodwill [Line Items]      
Total goodwill - net 2,197.9 2,213.7  
Canada [Member]      
Goodwill [Line Items]      
Total goodwill - net 569.7 572.9  
Australia [Member]      
Goodwill [Line Items]      
Total goodwill - net 477.8 498.1  
New Zealand [Member]      
Goodwill [Line Items]      
Total goodwill - net 213.3 227.8  
Other Foreign [Member]      
Goodwill [Line Items]      
Total goodwill - net 890.2 679.8  
Brokerage [Member]      
Goodwill [Line Items]      
Total goodwill - net 9,358.1 8,544.6 6,053.6
Brokerage [Member] | Unites States [Member]      
Goodwill [Line Items]      
Total goodwill - net 5,065.7 4,409.1  
Brokerage [Member] | United Kingdom [Member]      
Goodwill [Line Items]      
Total goodwill - net 2,180.2 2,198.7  
Brokerage [Member] | Canada [Member]      
Goodwill [Line Items]      
Total goodwill - net 569.7 572.9  
Brokerage [Member] | Australia [Member]      
Goodwill [Line Items]      
Total goodwill - net 467.6 487.2  
Brokerage [Member] | New Zealand [Member]      
Goodwill [Line Items]      
Total goodwill - net 203.8 217.6  
Brokerage [Member] | Other Foreign [Member]      
Goodwill [Line Items]      
Total goodwill - net 871.1 659.1  
Risk Management [Member]      
Goodwill [Line Items]      
Total goodwill - net 112.2 100.9 70.5
Risk Management [Member] | Unites States [Member]      
Goodwill [Line Items]      
Total goodwill - net 74.8 64.8  
Risk Management [Member] | United Kingdom [Member]      
Goodwill [Line Items]      
Total goodwill - net 17.7 15.0  
Risk Management [Member] | Australia [Member]      
Goodwill [Line Items]      
Total goodwill - net 10.2 10.9  
Risk Management [Member] | New Zealand [Member]      
Goodwill [Line Items]      
Total goodwill - net 9.5 10.2  
Corporate [Member]      
Goodwill [Line Items]      
Total goodwill - net 19.1 20.7 $ 2.9
Corporate [Member] | Other Foreign [Member]      
Goodwill [Line Items]      
Total goodwill - net $ 19.1 $ 20.7