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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail)
$ in Millions
 6 Months Ended
Jun. 30, 2023
USD ($)
Goodwill [Line Items]  
Beginning Balance $ 9,489.4
Goodwill acquired during the period 858.7
Goodwill true-ups due to appraisals and other acquisition adjustments 10.0
Foreign currency translation adjustments during the period 155.6
Ending Balance 10,513.7
Brokerage [Member]  
Goodwill [Line Items]  
Beginning Balance 9,358.1
Goodwill acquired during the period 858.7
Goodwill true-ups due to appraisals and other acquisition adjustments 10.1
Foreign currency translation adjustments during the period 154.5
Ending Balance 10,381.4
Risk Management [Member]  
Goodwill [Line Items]  
Beginning Balance 112.2
Goodwill true-ups due to appraisals and other acquisition adjustments (0.1)
Foreign currency translation adjustments during the period 0.9
Ending Balance 113.0
Corporate [Member]  
Goodwill [Line Items]  
Beginning Balance 19.1
Foreign currency translation adjustments during the period 0.2
Ending Balance $ 19.3