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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Mar. 31, 2017
Dec. 31, 2016
Goodwill [Line Items]    
Total goodwill - net $ 3,896.9 $ 3,767.8
Unites States [Member]    
Goodwill [Line Items]    
Total goodwill - net 2,225.1 2,138.5
United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill - net 672.1 666.5
Canada [Member]    
Goodwill [Line Items]    
Total goodwill - net 293.8 292.2
Australia [Member]    
Goodwill [Line Items]    
Total goodwill - net 410.9 382.7
New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill - net 208.8 205.3
Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill - net 86.2 82.6
Brokerage [Member]    
Goodwill [Line Items]    
Total goodwill - net 3,865.9 3,736.9
Brokerage [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill - net 2,201.6 2,115.0
Brokerage [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill - net 667.8 662.2
Brokerage [Member] | Canada [Member]    
Goodwill [Line Items]    
Total goodwill - net 293.8 292.2
Brokerage [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill - net 410.9 382.7
Brokerage [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill - net 208.5 205.0
Brokerage [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill - net 83.3 79.8
Risk Management [Member]    
Goodwill [Line Items]    
Total goodwill - net 28.1 28.1
Risk Management [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill - net 23.5 23.5
Risk Management [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill - net 4.3 4.3
Risk Management [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill - net 0.3 0.3
Corporate [Member]    
Goodwill [Line Items]    
Total goodwill - net 2.9 2.8
Corporate [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill - net $ 2.9 $ 2.8