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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) $ in Millions | 3 Months Ended
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Mar. 31, 2022 USD ($)
Goodwill [Line Items] |
Beginning Balance | $ 8,666.2
Goodwill acquired during the period | 68.9
Goodwill true-ups due to appraisals and other acquisition adjustments | (18.8)
Foreign currency translation adjustments during the period | 11.4
Ending Balance | 8,727.7
Brokerage [Member] |
Goodwill [Line Items] |
Beginning Balance | 8,544.6
Goodwill acquired during the period | 68.9
Goodwill true-ups due to appraisals and other acquisition adjustments | (18.8)
Foreign currency translation adjustments during the period | 11.0
Ending Balance | 8,605.7
Risk Management [Member] |
Goodwill [Line Items] |
Beginning Balance | 100.9
Foreign currency translation adjustments during the period | 0.5
Ending Balance | 101.4
Corporate [Member] |
Goodwill [Line Items] |
Beginning Balance | 20.7
Foreign currency translation adjustments during the period | (0.1)
Ending Balance | $ 20.6

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