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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail)
$ in Millions
 3 Months Ended
Mar. 31, 2017
USD ($)
Goodwill [Line Items]  
Beginning Balance $ 3,767.8
Goodwill acquired during the period 91.9
Goodwill adjustments due to appraisals and other acquisition adjustments 1.6
Foreign currency translation adjustments during the period 35.6
Balance as of March 31, 2017 3,896.9
Brokerage [Member]  
Goodwill [Line Items]  
Beginning Balance 3,736.9
Goodwill acquired during the period 91.9
Goodwill adjustments due to appraisals and other acquisition adjustments 1.6
Foreign currency translation adjustments during the period 35.5
Balance as of March 31, 2017 3,865.9
Risk Management [Member]  
Goodwill [Line Items]  
Beginning Balance 28.1
Balance as of March 31, 2017 28.1
Corporate [Member]  
Goodwill [Line Items]  
Beginning Balance 2.8
Foreign currency translation adjustments during the period 0.1
Balance as of March 31, 2017 $ 2.9