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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail)
$ in Millions
 6 Months Ended
Jun. 30, 2018
USD ($)
Goodwill [Line Items]  
Beginning Balance $ 4,164.8 [1]
Goodwill acquired during the period 294.5
Goodwill adjustments due to appraisals and other acquisition adjustments 0.2
Foreign currency translation adjustments during the period (55.6)
Ending Balance 4,403.9
As Previously Reported [Member]  
Goodwill [Line Items]  
Beginning Balance 4,197.9
Accounting Standards Update 2014-09 [Member]  
Goodwill [Line Items]  
Beginning Balance (33.1)
Brokerage [Member]  
Goodwill [Line Items]  
Beginning Balance 4,119.2
Goodwill acquired during the period 291.8
Goodwill adjustments due to appraisals and other acquisition adjustments 2.5
Foreign currency translation adjustments during the period (55.0)
Ending Balance 4,358.5
Brokerage [Member] | As Previously Reported [Member]  
Goodwill [Line Items]  
Beginning Balance 4,152.3
Brokerage [Member] | Accounting Standards Update 2014-09 [Member]  
Goodwill [Line Items]  
Beginning Balance (33.1)
Risk Management [Member]  
Goodwill [Line Items]  
Beginning Balance 42.6
Goodwill acquired during the period 2.7
Goodwill adjustments due to appraisals and other acquisition adjustments (2.3)
Foreign currency translation adjustments during the period (0.4)
Ending Balance 42.6
Risk Management [Member] | As Previously Reported [Member]  
Goodwill [Line Items]  
Beginning Balance 42.6
Corporate [Member]  
Goodwill [Line Items]  
Beginning Balance 3.0
Foreign currency translation adjustments during the period (0.2)
Ending Balance 2.8
Corporate [Member] | As Previously Reported [Member]  
Goodwill [Line Items]  
Beginning Balance $ 3.0
[1] See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to ASC 606.