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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

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