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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Mar. 31, 2021
Dec. 31, 2020
Goodwill [Line Items]    
Total goodwill $ 6,510.9 $ 6,127.0
Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 3,535.6 3,433.0
United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 1,601.0 1,343.4
Canada [Member]    
Goodwill [Line Items]    
Total goodwill 515.3 492.9
Australia [Member]    
Goodwill [Line Items]    
Total goodwill 478.9 473.6
New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 230.8 232.6
Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 149.3 151.5
Brokerage [Member]    
Goodwill [Line Items]    
Total goodwill 6,418.1 6,053.6
Brokerage [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 3,483.4 3,399.8
Brokerage [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 1,585.5 1,328.3
Brokerage [Member] | Canada [Member]    
Goodwill [Line Items]    
Total goodwill 515.3 492.9
Brokerage [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill 467.3 462.1
Brokerage [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 220.2 221.9
Brokerage [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 146.4 148.6
Risk Management [Member]    
Goodwill [Line Items]    
Total goodwill 89.9 70.5
Risk Management [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 52.2 33.2
Risk Management [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 15.5 15.1
Risk Management [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill 11.6 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