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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Mar. 31, 2024
Dec. 31, 2023
Goodwill [Line Items]    
Total goodwill $ 11,532.1 $ 11,475.6
Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 6,500.0 6,379.3
United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 2,511.7 2,511.9
Canada [Member]    
Goodwill [Line Items]    
Total goodwill 607.8 623.7
Australia [Member]    
Goodwill [Line Items]    
Total goodwill 623.7 650.5
New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 202.5 213.8
Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 1,086.4 1,096.4
Brokerage [Member]    
Goodwill [Line Items]    
Total goodwill 11,281.0 11,217.8
Risk Management [Member]    
Goodwill [Line Items]    
Total goodwill 232.1 238.8
Corporate [Member]    
Goodwill [Line Items]    
Total goodwill 19.0 19.0
Operating Segments [Member] | Brokerage [Member]    
Goodwill [Line Items]    
Total goodwill 11,281.0 11,217.8
Operating Segments [Member] | Brokerage [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 6,425.2 6,304.5
Operating Segments [Member] | Brokerage [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 2,493.3 2,493.4
Operating Segments [Member] | Brokerage [Member] | Canada [Member]    
Goodwill [Line Items]    
Total goodwill 607.8 623.7
Operating Segments [Member] | Brokerage [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill 493.8 514.6
Operating Segments [Member] | Brokerage [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 193.5 204.2
Operating Segments [Member] | Brokerage [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 1,067.4 1,077.4
Operating Segments [Member] | Risk Management [Member]    
Goodwill [Line Items]    
Total goodwill 232.1 238.8
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 18.4 18.5
Operating Segments [Member] | Risk Management [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill 129.9 135.9
Operating Segments [Member] | Risk Management [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 9.0 9.6
Operating Segments [Member] | Corporate [Member]    
Goodwill [Line Items]    
Total goodwill 19.0 19.0
Operating Segments [Member] | Corporate [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill $ 19.0 $ 19.0