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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Millions | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2024 | Dec. 31, 2023
Goodwill [Line Items] |  |
Beginning Balance | $ 11,475.6 |
Goodwill acquired during the period | 341.0 |
Goodwill true-ups due to appraisals and other acquisition adjustments (see Note 3) | 196.4 | $ 256.7
Goodwill written-off related to sales of business and impairment | (5.8) |
Foreign currency translation adjustments during the period | (91.7) |
Ending Balance | 11,915.5 | 11,475.6
Brokerage [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 11,217.8 |
Goodwill acquired during the period | 341.0 |
Goodwill true-ups due to appraisals and other acquisition adjustments (see Note 3) | 94.4 |
Goodwill written-off related to sales of business and impairment | (5.8) |
Foreign currency translation adjustments during the period | (87.7) |
Ending Balance | 11,559.7 | 11,217.8
Risk Management [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 238.8 |
Goodwill true-ups due to appraisals and other acquisition adjustments (see Note 3) | 102.0 |
Foreign currency translation adjustments during the period | (3.9) |
Ending Balance | 336.9 | 238.8
Corporate [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 19.0 |
Foreign currency translation adjustments during the period | (0.1) |
Ending Balance | $ 18.9 | $ 19.0

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