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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Goodwill [Line Items]    
Total goodwill $ 4,800.9 $ 4,625.6
Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 2,824.1 2,744.9
United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 793.8 762.9
Canada [Member]    
Goodwill [Line Items]    
Total goodwill 437.4 378.6
Australia [Member]    
Goodwill [Line Items]    
Total goodwill 409.5 406.6
New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 223.6 219.8
Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 112.5 112.8
Brokerage [Member]    
Goodwill [Line Items]    
Total goodwill 4,748.4 4,573.6
Brokerage [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 2,794.5 2,715.3
Brokerage [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 784.2 753.7
Brokerage [Member] | Canada [Member]    
Goodwill [Line Items]    
Total goodwill 437.4 378.6
Brokerage [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill 409.2 406.3
Brokerage [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 213.3 209.6
Brokerage [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 109.8 110.1
Risk Management [Member]    
Goodwill [Line Items]    
Total goodwill 49.8 49.3
Risk Management [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 29.6 29.6
Risk Management [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 9.6 9.2
Risk Management [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill 0.3 0.3
Risk Management [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 10.3 10.2
Corporate [Member]    
Goodwill [Line Items]    
Total goodwill 2.7 2.7
Corporate [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill $ 2.7 $ 2.7