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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Sep. 30, 2020
Dec. 31, 2019
Goodwill [Line Items]    
Total goodwill $ 5,832.2 $ 5,618.5
Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 3,300.2 3,196.9
United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 1,274.3 1,190.7
Canada [Member]    
Goodwill [Line Items]    
Total goodwill 450.3 454.4
Australia [Member]    
Goodwill [Line Items]    
Total goodwill 444.6 427.0
New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 216.7 218.1
Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 146.1 131.4
Brokerage [Member]    
Goodwill [Line Items]    
Total goodwill 5,761.4 5,548.9
Brokerage [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 3,267.0 3,163.8
Brokerage [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 1,260.4 1,177.8
Brokerage [Member] | Canada [Member]    
Goodwill [Line Items]    
Total goodwill 450.3 454.4
Brokerage [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill 433.8 416.5
Brokerage [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 206.7 208.0
Brokerage [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 143.2 128.4
Risk Management [Member]    
Goodwill [Line Items]    
Total goodwill 67.9 66.6
Risk Management [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 33.2 33.1
Risk Management [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 13.9 12.9
Risk Management [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill 10.8 10.5
Risk Management [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 10.0 10.1
Corporate [Member]    
Goodwill [Line Items]    
Total goodwill 2.9 3.0
Corporate [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill $ 2.9 $ 3.0