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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Goodwill [Line Items]    
Total goodwill - net $ 4,160.4 $ 3,767.8
Unites States [Member]    
Goodwill [Line Items]    
Total goodwill - net 2,311.3 2,138.5
United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill - net 745.6 666.5
Canada [Member]    
Goodwill [Line Items]    
Total goodwill - net 345.9 292.2
Australia [Member]    
Goodwill [Line Items]    
Total goodwill - net 427.8 382.7
New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill - net 226.7 205.3
Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill - net 103.1 82.6
Brokerage [Member]    
Goodwill [Line Items]    
Total goodwill - net 4,114.5 3,736.9
Brokerage [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill - net 2,285.5 2,115.0
Brokerage [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill - net 738.3 662.2
Brokerage [Member] | Canada [Member]    
Goodwill [Line Items]    
Total goodwill - net 345.9 292.2
Brokerage [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill - net 427.8 382.7
Brokerage [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill - net 216.8 205.0
Brokerage [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill - net 100.2 79.8
Risk Management [Member]    
Goodwill [Line Items]    
Total goodwill - net 43.0 28.1
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 7.3 4.3
Risk Management [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill - net 9.9 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