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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Jun. 30, 2020
Dec. 31, 2019
Goodwill [Line Items]    
Total goodwill $ 5,756.4 $ 5,618.5
Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 3,284.0 3,196.9
United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 1,250.6 1,190.7
Canada [Member]    
Goodwill [Line Items]    
Total goodwill 439.2 454.4
Australia [Member]    
Goodwill [Line Items]    
Total goodwill 427.6 427.0
New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 211.6 218.1
Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 143.4 131.4
Brokerage [Member]    
Goodwill [Line Items]    
Total goodwill 5,686.7 5,548.9
Brokerage [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 3,250.9 3,163.8
Brokerage [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 1,237.1 1,177.8
Brokerage [Member] | Canada [Member]    
Goodwill [Line Items]    
Total goodwill 439.2 454.4
Brokerage [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill 417.1 416.5
Brokerage [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 201.8 208.0
Brokerage [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 140.6 128.4
Risk Management [Member]    
Goodwill [Line Items]    
Total goodwill 66.9 66.6
Risk Management [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 33.1 33.1
Risk Management [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 13.5 12.9
Risk Management [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill 10.5 10.5
Risk Management [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 9.8 10.1
Corporate [Member]    
Goodwill [Line Items]    
Total goodwill 2.8 3.0
Corporate [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill $ 2.8 $ 3.0