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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail)
$ in Millions
 6 Months Ended
Jun. 30, 2022
USD ($)
Goodwill [Line Items]  
Beginning Balance $ 8,666.2
Goodwill acquired during the period 213.6
Goodwill true-ups due to appraisals and other acquisition adjustments 408.0
Foreign currency translation adjustments during the period (251.4)
Ending Balance 9,036.4
Brokerage [Member]  
Goodwill [Line Items]  
Beginning Balance 8,544.6
Goodwill acquired during the period 207.6
Goodwill true-ups due to appraisals and other acquisition adjustments 407.8
Foreign currency translation adjustments during the period (248.2)
Ending Balance 8,911.8
Risk Management [Member]  
Goodwill [Line Items]  
Beginning Balance 100.9
Goodwill acquired during the period 6.0
Foreign currency translation adjustments during the period (2.6)
Ending Balance 104.3
Corporate [Member]  
Goodwill [Line Items]  
Beginning Balance 20.7
Goodwill true-ups due to appraisals and other acquisition adjustments 0.2
Foreign currency translation adjustments during the period (0.6)
Ending Balance $ 20.3