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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Goodwill [Line Items]      
Total goodwill - net $ 5,618.5 $ 4,625.6 $ 4,164.8
Unites States [Member]      
Goodwill [Line Items]      
Total goodwill - net 3,196.9 2,744.9  
United Kingdom [Member]      
Goodwill [Line Items]      
Total goodwill - net 1,190.7 762.9  
Canada [Member]      
Goodwill [Line Items]      
Total goodwill - net 454.4 378.6  
Australia [Member]      
Goodwill [Line Items]      
Total goodwill - net 427.0 406.6  
New Zealand [Member]      
Goodwill [Line Items]      
Total goodwill - net 218.1 219.8  
Other Foreign [Member]      
Goodwill [Line Items]      
Total goodwill - net 131.4 112.8  
Brokerage [Member]      
Goodwill [Line Items]      
Total goodwill - net 5,548.9 4,573.6 4,119.2
Brokerage [Member] | Unites States [Member]      
Goodwill [Line Items]      
Total goodwill - net 3,163.8 2,715.3  
Brokerage [Member] | United Kingdom [Member]      
Goodwill [Line Items]      
Total goodwill - net 1,177.8 753.7  
Brokerage [Member] | Canada [Member]      
Goodwill [Line Items]      
Total goodwill - net 454.4 378.6  
Brokerage [Member] | Australia [Member]      
Goodwill [Line Items]      
Total goodwill - net 416.5 406.3  
Brokerage [Member] | New Zealand [Member]      
Goodwill [Line Items]      
Total goodwill - net 208.0 209.6  
Brokerage [Member] | Other Foreign [Member]      
Goodwill [Line Items]      
Total goodwill - net 128.4 110.1  
Risk Management [Member]      
Goodwill [Line Items]      
Total goodwill - net 66.6 49.3 42.6
Risk Management [Member] | Unites States [Member]      
Goodwill [Line Items]      
Total goodwill - net 33.1 29.6  
Risk Management [Member] | United Kingdom [Member]      
Goodwill [Line Items]      
Total goodwill - net 12.9 9.2  
Risk Management [Member] | Australia [Member]      
Goodwill [Line Items]      
Total goodwill - net 10.5 0.3  
Risk Management [Member] | New Zealand [Member]      
Goodwill [Line Items]      
Total goodwill - net 10.1 10.2  
Corporate [Member]      
Goodwill [Line Items]      
Total goodwill - net 3.0 2.7 $ 3.0
Corporate [Member] | Other Foreign [Member]      
Goodwill [Line Items]      
Total goodwill - net $ 3.0 $ 2.7