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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Jun. 30, 2019
Dec. 31, 2018
Goodwill [Line Items]    
Total goodwill $ 5,241.9 $ 4,625.6
Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 2,891.5 2,744.9
United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 1,153.4 762.9
Canada [Member]    
Goodwill [Line Items]    
Total goodwill 446.5 378.6
Australia [Member]    
Goodwill [Line Items]    
Total goodwill 416.0 406.6
New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 218.0 219.8
Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 116.5 112.8
Brokerage [Member]    
Goodwill [Line Items]    
Total goodwill 5,190.0 4,573.6
Brokerage [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill 2,861.9 2,715.3
Brokerage [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill 1,144.6 753.7
Brokerage [Member] | Canada [Member]    
Goodwill [Line Items]    
Total goodwill 446.5 378.6
Brokerage [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill 415.7 406.3
Brokerage [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill 207.9 209.6
Brokerage [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill 113.4 110.1
Risk Management [Member]    
Goodwill [Line Items]    
Total goodwill 48.8 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 8.8 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 10.1 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