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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Goodwill [Line Items]    
Total goodwill - net $ 4,236.3 $ 4,164.8 [1]
Unites States [Member]    
Goodwill [Line Items]    
Total goodwill - net 2,341.0 2,306.7
United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill - net 783.6 745.7
Canada [Member]    
Goodwill [Line Items]    
Total goodwill - net 367.8 374.0
Australia [Member]    
Goodwill [Line Items]    
Total goodwill - net 414.1 416.6
New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill - net 224.9 218.9
Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill - net 104.9 102.9
Brokerage [Member]    
Goodwill [Line Items]    
Total goodwill - net 4,189.8 4,119.2
Brokerage [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill - net 2,315.9 2,280.9
Brokerage [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill - net 776.0 738.5
Brokerage [Member] | Canada [Member]    
Goodwill [Line Items]    
Total goodwill - net 367.8 374.0
Brokerage [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill - net 414.1 416.6
Brokerage [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill - net 214.0 209.3
Brokerage [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill - net 102.0 99.9
Risk Management [Member]    
Goodwill [Line Items]    
Total goodwill - net 43.6 42.6
Risk Management [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill - net 25.1 25.8
Risk Management [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill - net 7.6 7.2
Risk Management [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill - net 10.9 9.6
Corporate [Member]    
Goodwill [Line Items]    
Total goodwill - net 2.9 3.0
Corporate [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill - net $ 2.9 $ 3.0
[1] See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to ASC 606.