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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Goodwill [Line Items]    
Total goodwill - net $ 4,403.9 $ 4,164.8 [1]
Unites States [Member]    
Goodwill [Line Items]    
Total goodwill - net 2,570.4 2,306.7
United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill - net 739.2 745.7
Canada [Member]    
Goodwill [Line Items]    
Total goodwill - net 360.0 374.0
Australia [Member]    
Goodwill [Line Items]    
Total goodwill - net 400.7 416.6
New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill - net 214.9 218.9
Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill - net 118.7 102.9
Brokerage [Member]    
Goodwill [Line Items]    
Total goodwill - net 4,358.5 4,119.2
Brokerage [Member] | Unites States [Member]    
Goodwill [Line Items]    
Total goodwill - net 2,545.3 2,280.9
Brokerage [Member] | United Kingdom [Member]    
Goodwill [Line Items]    
Total goodwill - net 732.1 738.5
Brokerage [Member] | Canada [Member]    
Goodwill [Line Items]    
Total goodwill - net 360.0 374.0
Brokerage [Member] | Australia [Member]    
Goodwill [Line Items]    
Total goodwill - net 400.7 416.6
Brokerage [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill - net 204.5 209.3
Brokerage [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill - net 115.9 99.9
Risk Management [Member]    
Goodwill [Line Items]    
Total goodwill - net 42.6 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 7.1 7.2
Risk Management [Member] | New Zealand [Member]    
Goodwill [Line Items]    
Total goodwill - net 10.4 9.6
Corporate [Member]    
Goodwill [Line Items]    
Total goodwill - net 2.8 3.0
Corporate [Member] | Other Foreign [Member]    
Goodwill [Line Items]    
Total goodwill - net $ 2.8 $ 3.0
[1] See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to ASC 606.