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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail)
$ in Millions
 6 Months Ended
Jun. 30, 2017
USD ($)
Goodwill [Line Items]  
Beginning Balance $ 3,767.8
Goodwill acquired during the period 154.4
Goodwill adjustments due to appraisals and other acquisition adjustments 14.7
Foreign currency translation adjustments during the period 52.1
Balance as of June 30, 2017 3,989.0
Brokerage [Member]  
Goodwill [Line Items]  
Beginning Balance 3,736.9
Goodwill acquired during the period 142.7
Goodwill adjustments due to appraisals and other acquisition adjustments 14.7
Foreign currency translation adjustments during the period 51.7
Balance as of June 30, 2017 3,946.0
Risk Management [Member]  
Goodwill [Line Items]  
Beginning Balance 28.1
Goodwill acquired during the period 11.7
Foreign currency translation adjustments during the period 0.3
Balance as of June 30, 2017 40.1
Corporate [Member]  
Goodwill [Line Items]  
Beginning Balance 2.8
Foreign currency translation adjustments during the period 0.1
Balance as of June 30, 2017 $ 2.9