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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Sep. 30, 2019
Dec. 31, 2018
Goodwill [Line Items]    
Total goodwill $ 5,314.8 $ 4,625.6
Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 2,983.0 2,744.9
United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 1,150.9 762.9
Canada [Member]    
Goodwill [Line Items]    
Total goodwill 445.4 378.6
Australia [Member]    
Goodwill [Line Items]    
Total goodwill 414.3 406.6
New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 206.0 219.8
Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 115.2 112.8
Brokerage [Member]    
Goodwill [Line Items]    
Total goodwill 5,246.8 4,573.6
Brokerage [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 2,950.0 2,715.3
Brokerage [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 1,138.6 753.7
Brokerage [Member] | Canada [Member]    
Goodwill [Line Items]    
Total goodwill 445.4 378.6
Brokerage [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill 404.2 406.3
Brokerage [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 196.5 209.6
Brokerage [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 112.1 110.1
Risk Management [Member]    
Goodwill [Line Items]    
Total goodwill 64.9 49.3
Risk Management [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 33.0 29.6
Risk Management [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 12.3 9.2
Risk Management [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill 10.1 0.3
Risk Management [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 9.5 10.2
Corporate [Member]    
Goodwill [Line Items]    
Total goodwill 3.1 2.7
Corporate [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill $ 3.1 $ 2.7