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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) $ in Millions | 9 Months Ended
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Sep. 30, 2020 USD ($)
Goodwill [Line Items] |
Beginning Balance | $ 5,618.5
Goodwill acquired during the period | 200.7
Goodwill adjustments due to appraisals and other acquisition adjustments | 30.5
Foreign currency translation adjustments during the period | (17.5)
Ending Balance | 5,832.2
Brokerage [Member] |
Goodwill [Line Items] |
Beginning Balance | 5,548.9
Goodwill acquired during the period | 200.7
Goodwill adjustments due to appraisals and other acquisition adjustments | 29.2
Foreign currency translation adjustments during the period | (17.4)
Ending Balance | 5,761.4
Risk Management [Member] |
Goodwill [Line Items] |
Beginning Balance | 66.6
Goodwill adjustments due to appraisals and other acquisition adjustments | 1.3
Ending Balance | 67.9
Corporate [Member] |
Goodwill [Line Items] |
Beginning Balance | 3.0
Foreign currency translation adjustments during the period | (0.1)
Ending Balance | $ 2.9

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