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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022
Goodwill [Line Items] |  |
Beginning Balance | $ 9,489.4 | $ 8,666.2
Goodwill acquired during the year | 1,789.2 | 709.9
Goodwill adjustments related to appraisals and other acquisition adjustments | 19.9 | 426.9
Foreign currency translation adjustments during the year | 177.1 | (313.6)
Ending Balance | 11,475.6 | 9,489.4
Brokerage [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 9,358.1 | 8,544.6
Goodwill acquired during the year | 1,667.4 | 693.9
Goodwill adjustments related to appraisals and other acquisition adjustments | 20.0 | 428.3
Foreign currency translation adjustments during the year | 172.3 | (308.7)
Ending Balance | 11,217.8 | 9,358.1
Risk Management [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 112.2 | 100.9
Goodwill acquired during the year | 121.8 | 16.0
Goodwill adjustments related to appraisals and other acquisition adjustments | (0.1) | (1.6)
Foreign currency translation adjustments during the year | 4.9 | (3.1)
Ending Balance | 238.8 | 112.2
Corporate [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 19.1 | 20.7
Goodwill adjustments related to appraisals and other acquisition adjustments |  | 0.2
Foreign currency translation adjustments during the year | (0.1) | (1.8)
Ending Balance | $ 19.0 | $ 19.1

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