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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Goodwill [Line Items]    
Total goodwill $ 6,627.5 $ 6,127.0
Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 3,633.7 3,433.0
United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 1,621.8 1,343.4
Canada [Member]    
Goodwill [Line Items]    
Total goodwill 523.0 492.9
Australia [Member]    
Goodwill [Line Items]    
Total goodwill 461.0 473.6
New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 231.1 232.6
Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 156.9 151.5
Brokerage [Member]    
Goodwill [Line Items]    
Total goodwill 6,522.8 6,053.6
Brokerage [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 3,568.9 3,399.8
Brokerage [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 1,606.4 1,328.3
Brokerage [Member] | Canada [Member]    
Goodwill [Line Items]    
Total goodwill 523.0 492.9
Brokerage [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill 450.0 462.1
Brokerage [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 220.5 221.9
Brokerage [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 154.0 148.6
Risk Management [Member]    
Goodwill [Line Items]    
Total goodwill 101.8 70.5
Risk Management [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 64.8 33.2
Risk Management [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 15.4 15.1
Risk Management [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill 11.0 11.5
Risk Management [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 10.6 10.7
Corporate [Member]    
Goodwill [Line Items]    
Total goodwill 2.9 2.9
Corporate [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill $ 2.9 $ 2.9