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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Sep. 30, 2022
Dec. 31, 2021
Goodwill [Line Items]    
Total goodwill $ 8,837.2 $ 8,666.2
Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 4,712.8 4,473.9
United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 2,056.6 2,213.7
Canada [Member]    
Goodwill [Line Items]    
Total goodwill 581.0 572.9
Australia [Member]    
Goodwill [Line Items]    
Total goodwill 459.5 498.1
New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 198.5 227.8
Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 828.8 679.8
Brokerage [Member]    
Goodwill [Line Items]    
Total goodwill 8,707.1 8,544.6
Brokerage [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 4,638.0 4,409.1
Brokerage [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 2,040.1 2,198.7
Brokerage [Member] | Canada [Member]    
Goodwill [Line Items]    
Total goodwill 581.0 572.9
Brokerage [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill 449.4 487.2
Brokerage [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 189.6 217.6
Brokerage [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 809.0 659.1
Risk Management [Member]    
Goodwill [Line Items]    
Total goodwill 110.3 100.9
Risk Management [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 74.8 64.8
Risk Management [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 16.5 15.0
Risk Management [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill 10.1 10.9
Risk Management [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 8.9 10.2
Corporate [Member]    
Goodwill [Line Items]    
Total goodwill 19.8 20.7
Corporate [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill $ 19.8 $ 20.7