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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Goodwill [Line Items]    
Beginning Balance $ 3,767.8 $ 3,662.9
Goodwill acquired during the year 304.5 225.4
Goodwill adjustments related to appraisals and other acquisition adjustments 14.7 3.4
Foreign currency translation adjustments during the year 110.9 (123.9)
Ending Balance 4,197.9 3,767.8
Brokerage [Member]    
Goodwill [Line Items]    
Beginning Balance 3,736.9 3,635.6
Goodwill acquired during the year 290.4 222.6
Goodwill adjustments related to appraisals and other acquisition adjustments 14.7 1.8
Foreign currency translation adjustments during the year 110.3 (123.1)
Ending Balance 4,152.3 3,736.9
Risk Management [Member]    
Goodwill [Line Items]    
Beginning Balance 28.1 27.3
Goodwill acquired during the year 14.1  
Goodwill adjustments related to appraisals and other acquisition adjustments   1.6
Foreign currency translation adjustments during the year 0.4 (0.8)
Ending Balance 42.6 28.1
Corporate [Member]    
Goodwill [Line Items]    
Beginning Balance 2.8  
Goodwill acquired during the year   2.8
Foreign currency translation adjustments during the year 0.2  
Ending Balance $ 3.0 $ 2.8