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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Goodwill [Line Items]      
Total goodwill - net $ 4,197.9 $ 3,767.8 $ 3,662.9
Unites States [Member]      
Goodwill [Line Items]      
Total goodwill - net 2,340.0 2,138.5  
United Kingdom [Member]      
Goodwill [Line Items]      
Total goodwill - net 745.7 666.5  
Canada [Member]      
Goodwill [Line Items]      
Total goodwill - net 375.3 292.2  
Australia [Member]      
Goodwill [Line Items]      
Total goodwill - net 416.6 382.7  
New Zealand [Member]      
Goodwill [Line Items]      
Total goodwill - net 218.9 205.3  
Other Foreign [Member]      
Goodwill [Line Items]      
Total goodwill - net 101.4 82.6  
Brokerage [Member]      
Goodwill [Line Items]      
Total goodwill - net 4,152.3 3,736.9 3,635.6
Brokerage [Member] | Unites States [Member]      
Goodwill [Line Items]      
Total goodwill - net 2,314.2 2,115.0  
Brokerage [Member] | United Kingdom [Member]      
Goodwill [Line Items]      
Total goodwill - net 738.5 662.2  
Brokerage [Member] | Canada [Member]      
Goodwill [Line Items]      
Total goodwill - net 375.3 292.2  
Brokerage [Member] | Australia [Member]      
Goodwill [Line Items]      
Total goodwill - net 416.6 382.7  
Brokerage [Member] | New Zealand [Member]      
Goodwill [Line Items]      
Total goodwill - net 209.3 205.0  
Brokerage [Member] | Other Foreign [Member]      
Goodwill [Line Items]      
Total goodwill - net 98.4 79.8  
Risk Management [Member]      
Goodwill [Line Items]      
Total goodwill - net 42.6 28.1 $ 27.3
Risk Management [Member] | Unites States [Member]      
Goodwill [Line Items]      
Total goodwill - net 25.8 23.5  
Risk Management [Member] | United Kingdom [Member]      
Goodwill [Line Items]      
Total goodwill - net 7.2 4.3  
Risk Management [Member] | New Zealand [Member]      
Goodwill [Line Items]      
Total goodwill - net 9.6 0.3  
Corporate [Member]      
Goodwill [Line Items]      
Total goodwill - net 3.0 2.8  
Corporate [Member] | Other Foreign [Member]      
Goodwill [Line Items]      
Total goodwill - net $ 3.0 $ 2.8