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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2024 | Dec. 31, 2023
Goodwill [Line Items] |  |
Beginning Balance | $ 11,475.6 | $ 9,489.4
Goodwill acquired during the year | 837.2 | 1,789.2
Goodwill adjustments related to appraisals and other acquisition adjustments | 200.2 | 19.9
Goodwill written-off related to sales of business | (5.8) |
Foreign currency translation adjustments during the year | (237.0) | 177.1
Ending Balance | 12,270.2 | 11,475.6
Brokerage [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 11,217.8 | 9,358.1
Goodwill acquired during the year | 829.7 | 1,667.4
Goodwill adjustments related to appraisals and other acquisition adjustments | 98.7 | 20.0
Goodwill written-off related to sales of business | (5.8) |
Foreign currency translation adjustments during the year | (217.0) | 172.3
Ending Balance | 11,923.4 | 11,217.8
Risk Management [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 238.8 | 112.2
Goodwill acquired during the year | 7.5 | 121.8
Goodwill adjustments related to appraisals and other acquisition adjustments | 101.5 | (0.1)
Foreign currency translation adjustments during the year | (19.5) | 4.9
Ending Balance | 328.3 | 238.8
Corporate [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 19.0 | 19.1
Goodwill adjustments related to appraisals and other acquisition adjustments | 0.0 |
Foreign currency translation adjustments during the year | (0.5) | (0.1)
Ending Balance | $ 18.5 | $ 19.0

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