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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Goodwill [Line Items]      
Total goodwill - net $ 3,662.9 $ 3,449.6 $ 2,145.2
Unites States [Member]      
Goodwill [Line Items]      
Total goodwill - net 1,970.4 1,672.8  
United Kingdom [Member]      
Goodwill [Line Items]      
Total goodwill - net 782.8 820.6  
Canada [Member]      
Goodwill [Line Items]      
Total goodwill - net 282.6 318.5  
Australia [Member]      
Goodwill [Line Items]      
Total goodwill - net 380.1 336.8  
New Zealand [Member]      
Goodwill [Line Items]      
Total goodwill - net 204.5 258.7  
Other Foreign [Member]      
Goodwill [Line Items]      
Total goodwill - net 42.5 42.2  
Brokerage [Member]      
Goodwill [Line Items]      
Total goodwill - net 3,635.6 3,427.5 2,122.9
Brokerage [Member] | Unites States [Member]      
Goodwill [Line Items]      
Total goodwill - net 1,946.9 1,652.6  
Brokerage [Member] | United Kingdom [Member]      
Goodwill [Line Items]      
Total goodwill - net 779.3 818.7  
Brokerage [Member] | Canada [Member]      
Goodwill [Line Items]      
Total goodwill - net 282.6 318.5  
Brokerage [Member] | Australia [Member]      
Goodwill [Line Items]      
Total goodwill - net 380.1 336.8  
Brokerage [Member] | New Zealand [Member]      
Goodwill [Line Items]      
Total goodwill - net 204.2 258.7  
Brokerage [Member] | Other Foreign [Member]      
Goodwill [Line Items]      
Total goodwill - net 42.5 42.2  
Risk Management [Member]      
Goodwill [Line Items]      
Total goodwill - net 27.3 22.1 $ 22.3
Risk Management [Member] | Unites States [Member]      
Goodwill [Line Items]      
Total goodwill - net 23.5 20.2  
Risk Management [Member] | United Kingdom [Member]      
Goodwill [Line Items]      
Total goodwill - net 3.5 1.9  
Risk Management [Member] | Canada [Member]      
Goodwill [Line Items]      
Total goodwill - net 0.0    
Risk Management [Member] | Australia [Member]      
Goodwill [Line Items]      
Total goodwill - net 0.0    
Risk Management [Member] | New Zealand [Member]      
Goodwill [Line Items]      
Total goodwill - net 0.3    
Corporate [Member]      
Goodwill [Line Items]      
Total goodwill - net 0.0 $ 0.0  
Corporate [Member] | Unites States [Member]      
Goodwill [Line Items]      
Total goodwill - net 0.0    
Corporate [Member] | United Kingdom [Member]      
Goodwill [Line Items]      
Total goodwill - net 0.0    
Corporate [Member] | Canada [Member]      
Goodwill [Line Items]      
Total goodwill - net 0.0    
Corporate [Member] | Australia [Member]      
Goodwill [Line Items]      
Total goodwill - net 0.0    
Corporate [Member] | New Zealand [Member]      
Goodwill [Line Items]      
Total goodwill - net 0.0    
Corporate [Member] | Other Foreign [Member]      
Goodwill [Line Items]      
Total goodwill - net $ 0.0