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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Goodwill [Line Items]    
Beginning Balance $ 8,666.2 $ 6,127.0
Goodwill acquired during the year 709.9 2,561.5
Goodwill adjustments related to appraisals and other acquisition adjustments 426.9 50.8
Goodwill written-off related to sales of business   (1.0)
Foreign currency translation adjustments during the year (313.6) (72.1)
Ending Balance 9,489.4 8,666.2
Brokerage [Member]    
Goodwill [Line Items]    
Beginning Balance 8,544.6 6,053.6
Goodwill acquired during the year 693.9 2,511.8
Goodwill adjustments related to appraisals and other acquisition adjustments 428.3 50.8
Goodwill written-off related to sales of business   (1.0)
Foreign currency translation adjustments during the year (308.7) (70.6)
Ending Balance 9,358.1 8,544.6
Risk Management [Member]    
Goodwill [Line Items]    
Beginning Balance 100.9 70.5
Goodwill acquired during the year 16.0 31.6
Goodwill adjustments related to appraisals and other acquisition adjustments (1.6)  
Foreign currency translation adjustments during the year (3.1) (1.2)
Ending Balance 112.2 100.9
Corporate [Member]    
Goodwill [Line Items]    
Beginning Balance 20.7 2.9
Goodwill acquired during the year   18.1
Goodwill adjustments related to appraisals and other acquisition adjustments 0.2  
Foreign currency translation adjustments during the year (1.8) (0.3)
Ending Balance $ 19.1 $ 20.7